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The Application of Statistical Parametric Mapping to Evaluate Differences in Topspin Backhand between Chinese and Polish Female Table Tennis Players

The Application of Statistical Parametric Mapping to Evaluate Differences in Topspin Backhand... Hindawi Applied Bionics and Biomechanics Volume 2021, Article ID 5555874, 11 pages https://doi.org/10.1155/2021/5555874 Research Article The Application of Statistical Parametric Mapping to Evaluate Differences in Topspin Backhand between Chinese and Polish Female Table Tennis Players Ziemowit Bańkosz and Sławomir Winiarski Department of Biomechanics, Faculty of Physical Education and Sports, University School of Physical Education in Wroclaw, Poland Correspondence should be addressed to Sławomir Winiarski; slawomir.winiarski@awf.wroc.pl Received 15 January 2021; Accepted 30 June 2021; Published 15 July 2021 Academic Editor: Ukadike C. Ugbolue Copyright © 2021 Ziemowit Bańkosz and Sławomir Winiarski. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The research is aimed at comparing the kinematics (the movement pattern in the most important joints and accelerations of the playing hand) between female table tennis players coached in Poland (POL) and China (CHIN) during the performance of a topspin backhand stroke (so-called quick topspin). The study involved six female table tennis players at a high sports skill level, playing in Poland’s highest league. Three were national team members of Poland (age: 20:3±1:9), while the other three were players from China (age: 20:0±0:0). Kinematics was measured using MR3 myoMuscle Master Edition system—inertial measurement unit (IMU) system. The participants performed one task of topspin backhand as a response to a topspin ball, repeated 15 times. Statistical parametric mapping (SPM) was calculated using SPM1D in a Python package that offered a high- level interface to SPM1D. The SPM method allowed for the determination of differences between the Chinese and Polish female athletes. The differences found are probably mainly due to differences in the training methodologies caused by different coaching systems. The observed differences include, among others, greater use of the so-called small steps in order to adapt and be ready during the back to ready position and backswing phases, which gives the CHIN players slightly better conditions for preparation for the next plays. The CHIN players’ position compared to that of the POL players favours a quicker transition from the backhand to the forehand play. This difference is probably related to the difference in the dominant playing styles of the groups studied. Despite the differences in movement patterns in both groups, the exact value of playing hand was achieved. This may be a manifestation of the phenomenon of equifinality and compensation. All the differences found are probably mainly due to differences in the training methodologies caused by different coaching systems. 1. Introduction nis players must also adjust their position to the ball using a different kind of footwork, changing kinematics and kinetics As a very complex and multifaceted sport, table tennis is characteristics of body segments [2, 3]. These differences lead characterized by various strokes, legwork techniques, tactical to a large variety and variability of movement in this sport. Issues related to movement variability have recently been solutions and playing styles, and a multitude of solutions for an almost infinite number of game situations. The main quite often addressed in the literature. Traditionally, move- groups of strokes that yield points are topspin strokes, intro- ment variability is considered to reflect the “noise” in the sys- duced to the game in the 1950s by Japanese players [1]. tem of human movements, while learning a given activity Players use many variations of topspin strokes in their game requires decreasing variability as it is perceived as incorrect [4, 5]. Movement variability is also viewed and considered a (e.g., backhand and forehand strokes, differing in strength involved, speed of ball rotation, flight trajectory, ball speed, normal phenomenon, resulting from the diversity and vari- and the moment of hitting the ball) depending on the solu- ability present in the entire biological system used by tion used or the need to adjust to ball parameters. Table ten- humans, and its occurrence is associated with adaptive and 2 Applied Bionics and Biomechanics seems to be an interesting issue. Identification of differences functional processes [6]. Movement variability has been explained using many theories available in the literature, such and, at the same time, similar or perhaps unchangeable ele- as generalized motor program [7, 8], GMP-uncontrolled man- ments of table tennis stroke techniques in athletes coached using different training systems can provide important ifold (UCM) [9], and dynamic systems theory [5]. Assessing the occurrence and scale of movement variability appears to insights into the technique of performing a given stroke. be extremely important in the sports training process. It seems Some differences in the technique may indicate the possibil- to be also critical in the process of improving skills of purpo- ity of using different solutions in the performance of the sive movements and explaining how to control human move- stroke, while the same, similar, and unchanging elements may highlight their importance in table tennis. Therefore, ments. Linear measures have been used in the assessment of variability, such as standard deviation, range, or coefficient the aim of the research was to compare the kinematics of variation. Taking into account discrete (numerical) or serial between female players coached in Europe (Poland) and Asia data, i.e., continuous and changing over time, would improve (China) during the performance of a topspin backhand the assessment of variability. This is because when assessing stroke. In accordance with the findings of other authors and previous studies [12, 13], it was assumed that, despite movement coordination, for instance, the change of the angle in a given joint over time, and comparing the repetitions by the comparable level of players, there are many differences one or many people, a method should be used to compare in the kinematics of topspin backhand between them. The time waveforms rather than just single, selected parameters. greater differences between the players would occur in the Such criteria are met by the statistical parametric mapping joints and segments located farther from the place of the racket contact with the ball (upper body and shoulder joint) (SPM) method. It is the gold standard statistical method ded- icated to numerical signal data analysis. For the one- than in those closer, located in the playing hand (wrist joint). dimensional variables recorded with the motion analysis sys- It can also be assumed that at the key instant of the stroke, tem, the general SPM model can be simplified to the one- which is the moment of maximum acceleration, occurring dimensional model SPM1D. This method and its characteriza- at around the contact between the racket and the ball, the least differences are observed in players’ kinematics. tion were presented in previous studies [10]. The assessment of variability of movement seems to be important in table tennis, which is a very complex sport, 2. Material and Methods where technique and its improvement are the essential ele- ments used to achieve the champion level, with the basis of 2.1. Study Design. It was an observational study with adopted the technique being a stroke and precise hitting the ball with retrospective convenience sampling. The minimal sample the racket. The few available studies on table tennis and the size of our data was determined in the planning stage of the variability of movement have been based on the methods of experiment using the margin of error approach to get results evaluation of standard deviation, correlation, and analysis as accurate as needed (with an assumed 5% margin of error at of variance (ANOVA and least significant difference (LSD)) 95% level of confidence and α level of 0.05). The assumed and presented UCM calculations. Iino et al. emphasized that standard deviation was taken from preliminary studies using the possibility of using different configurations in the evalu- the same population of interest. ated joints to stabilize the vertical angle of the racket in table tennis strokes can be a critical factor in playing performance [11]. A previous study by Bańkosz and Winiarski also evalu- 2.2. Participants. The study involved six female table tennis ated the variability of movement by analyzing the coefficient players at a high sports skill level, playing in the highest lea- of variation of kinematic parameters in selected important gue in Poland (Ekstraklasa table tennis league). Three of moments of the hitting movement [10, 12]. However, the them were national team members of Poland (POL) in the coordination of movements in individual joints was taken category of adult players (age: 20:3±1:9y:), while the other into account to a small extent. The variability of temporal three were players from China (CHIN, age: 20:0±0:0y:), and spatial coordination of movements, the possibility of coached within the Chinese training system (i.e., in China). compensation, and functional variability are significant All of the players had more than 10 years of experience in problems in the coaching practice and in the process of table tennis and presented the offensive style of the game. teaching and improving technique and its monitoring. Mak- One player from China was a left hander. Average body ing the coaches and players aware of the different variants of height was 161:7±4:5cm in the group of Polish players strokes even for a specific solution (e.g., playing with the right and 162:7±4:1cm in the group of Chinese players, whereas strength, speed, and rotation to the same place) seems to be body weight was 59:0±6:9kg and 56:7±6:4kg, respectively. very important and necessary for improving the training pro- Before the study, all participants were informed about the cess. Therefore, copying and imposing a single pattern of per- purpose of the study and the possibility of withdrawing par- forming the movement seem to be a wrong way. Considering ticipation at any stage, without giving a reason. All the partic- the differences between athletes and looking for individual ipants provided informed consent before the research. Pain technical solutions instead would be a better choice [10]. or recent injury was the exclusion criterion for the study par- Interpersonal variation of the sports technique may ticipants. All procedures performed in this study received result, for example, from gender differences, differences in positive approval from the Senate’s Research Bioethics Com- anatomical structure, and differences in sports skill level. mission at the University School of Physical Education in The diversity of techniques due to the training system also Wrocław, Poland (Ethics IRB number 34/2019). Applied Bionics and Biomechanics 3 Head (middle front part) Upper thoracic (below C7) Upper arm (lateral and Lower thoracic (at L1/T12) longitudinal to bone axis) Pelvic (sacrum) Hand (dorsal part) Forearm (posterior and distal) igh (frontal and distal half) Shank (front and medial) Foot (shoe adapter) MR3 myoMuscle Master Newgy Robo-Pong Edition system Robot 2050 Figure 1: Measurement site. 2.3. Laboratory Set-Up. Kinematics was measured using the accurately, and as quick as you can”). After video analysis, MR3 myoMuscle Master Edition system (myoMOTION™, only successful shot considered “on table” and played diago- Noraxon, USA, Figure 1). The myoMOTION system consists nally was recorded for further calculations (missed balls, balls of a set of (1 to 16) sensors using inertial sensor technology. hit out of bounds, and balls hit into the net were excluded). Based on the so-called fusion algorithms, the information The balls were shot by a dedicated table tennis robot (Newgy from a 3D accelerometer, gyroscope, and magnetometer is Robo-Pong Robot 2050, Newgy Industries, Tennessee, USA, used to measure the 3D rotation angles of each sensor in Figure 1) at constant parameters of rotation, speed, direction, absolute space (yaw-pitch-roll, also called orientation or nav- and flight trajectory. The settings of the robot were as follows: igation angles, [12]). Inertial sensors were located on the (i) Rotation type: topspin body of the study participant to record the accelerations, according to the myoMOTION protocol described in the (ii) Speed (determines both speed and spin, where 0 is manual. The accuracy and validity of the inertial measure- the minimum and 30 is the maximum): 18 ment unit (IMU) system in angle determination were the subject of the previous research [14, 15]. (iii) Left position (leftmost position to which the ball is Sensors were attached with elastic straps and self- delivered): 15 adhesive tape. The sensors were placed bilaterally so that (iv) Wing (robot’s head angle indicator): 7.5 the positive x-coordinate on the sensor label corresponded to a superior orientation for the trunk, head, and pelvis (v) Frequency (time interval between balls thrown): (Figure 1). For the limb segment sensors, the positive x 1.4 s -coordinate corresponded to a proximal orientation. For the foot sensor, the x-coordinate was directed distally (to Each player had had three to five familiarization trials before the task. The same racket with the following character- the toes). At the beginning of the measurement, each partic- istics was used for the experiment: blade, Jonyer-H-AN (But- ipant was checked and the system was calibrated according to terfly, Japan); rubber, Tenergy 05, 2.1 mm (Butterfly, Japan); the manufacturer’s recommendations. The recording speed Plastic Andro Speedball 3S 40+ balls (Andro, Germany); and of the piezoelectric sensor was adjusted to the maximal sam- pling rate for a given sensor (100 Hz per sensor) for the whole a Stiga Premium Compact table (Stiga, Sweden). 16-sensor set. Noraxon’s IMU technology mathematically combines and filters incoming source signals on the sensor 2.5. Kinematics. A total of 90 cycles of topspin backhand level and transmits the 4 quaternions of each sensor. We used stroke were studied. Based on the ISB recommendations con- system-built fusion algorithms and Kalman filtering (digital cerning the definitions of the joint coordinate system of var- bandpass finite impulse response filter (FIR)). This mode ious joints for the reporting of human joint motion [16, 17], allowed direct access to all unprocessed raw IMU sensor data. the following angles (measured in degrees) were chosen for both sides and sampled every 0.01 percent of cycle time: 2.4. Experimental Procedures. The participants performed one task of topspin backhand (TBH) as a response to a top- (i) Ankle dorsiflexion/plantar flexion (AFE): rotation spin ball, repeated 15 times. Each player was asked to hit of the foot with respect to the tibia coordinate sys- the ball in the early stage of its flight (so-called quick topspin) tem in the sagittal plane; a negative sign denotes and to reach the marked area in the corner of the table plantar flexion (extension) and positive sign dorsi- flexion (flexion) (30 × 30 cm) diagonally (after instruction: “play diagonally, 4 Applied Bionics and Biomechanics (iv) Elbow flexion-extension (EFE): movement of the (ii) Ankle abduction-adduction: movement of the foot away or towards the midline of the body; a forearm relative to the humerus along the transver- negative sign denotes adduction while positive sign sal axis; negative sign denotes (hyper)extension abduction while positive flexion (iii) Ankle inversion-eversion: rotation of the foot (v) Wrist flexion-extension (WFE): movement of wrist around its long axis; a negative sign denotes ever- relative to the radius along the transversal axis and sion (away from the median plane) while positive measured between upper arm and hand sensors; a sign inversion (towards the median plane) negative sign denotes extension while positive flexion (iv) Knee flexion-extension (KFE): movement of the tibia with respect to the femur coordinate system (vi) Wrist supination-pronation (WSup): movement of in the sagittal plane; a negative sign denotes exten- wrist relative to the radius along the axis and mea- sion and positive flexion sured between the upper arm and hand sensors; pronation is a positive rotation and supination is a (v) Hip flexion-extension (HFE): movement of the negative rotation femur with respect to the pelvis coordinate system in the sagittal plane; a negative sign denotes exten- (vii) Wrist radial abduction-adduction (WRad): move- sion while positive flexion ment of wrist relative to the radius and measured between the upper arm and hand sensors; adduction (vi) Hip abduction-adduction (HAA): movement of the (or ulnar deviation) is negative while abduction (or femur with respect to the pelvis coordinate system radial deviation) is positive in the frontal plane; a negative sign denotes adduc- tion while positive abduction The movement of the playing hand was used to assess specific events of the cycle: (vii) Hip internal-external rotation (HIER): internal or external movement of the femur with respect to (i) Ready position, where the hand is not moving after the pelvis coordinate system in the transversal the previous stroke, just before the swing plane; a negative sign denotes internal while posi- tive external rotation (ii) Backswing, which is the moment when the hand changes direction from backward to forward in the (viii) Lumbar internal-external rotation (LIER): internal sagittal plane after the swing or external movement of the loins in the transversal plane; a negative sign denotes internal while posi- (iii) Accmax, which is the moment of maximum acceler- tive external rotation ation of the hand and the moment when the hand reaches the maximum acceleration (ix) Thoracic internal-external rotation (ThIER): inter- nal or external movement of the thorax relative to (iv) Forward, which is the moment when the hand global coordination system in the transversal plane; changes the direction from forward to backward in a negative sign denotes internal while positive the sagittal plane after the stroke (the end of the cycle external rotation and the beginning of the next cycle) For the upper extremity (playing side), a simplified biome- The phases between defined events were as follows: back chanical model was adopted based on the predominant plane to ready position phase (between the forward and ready posi- of movement as described by Wu et al. [17] with segments of tion), backswing phase (between ready position and back- interest being the thorax, clavicle, scapula, humerus, forearm, swing), hitting phase (between backswing and Accmax), and carpus of the hand. Based on the adopted sequence of and forward end phase (between Accmax and forward). Euler angles, the following angles were computed: The timing of events was analyzed and compared between the POL and CHIN players. (i) Shoulder flexion-extension (ShFE): movement of the humerus relative to the thorax in sagittal plane; 2.6. Statistical Analysis. Statistical calculations were per- negative sign denotes extension while positive formed using Statistica 13.1 (TIBCO Software Inc.). The flexion sample size was estimated using recommendations postu- lated by Kontaxis et al. [18]. The statistical power was suffi- (ii) Shoulder abduction-adduction (ShAA): movement cient to detect the described differences. Power analysis of of the humerus relative to the thorax in the frontal discrete data was performed to estimate the SPM test power. plane; negative sign denotes adduction while posi- For the extracted data and for the significant changes tive abduction (alpha = 0:05), the partial η effect size was found between (iii) Shoulder internal-external rotation (ShIER): move- 0.62 and 0.86. The SPM test was applied to identify the differ- ment of the humerus relative to the thorax in the ences between groups in the movement patterns in individual transversal plane; a negative sign denotes internal joints and changes in the acceleration of the playing hand. (medial) while positive external (lateral) rotation The SPM was calculated using SPM1D in a Python package Applied Bionics and Biomechanics 5 (a) (b) p < 0.001 80 p < 0.001 p = 0.002 α = 0.05 : F = 5.993 0 200 400 600 800 1000 (c) p < 0.001 p < 0.001 p < 0.002 Cycle time (%) 0 20 40 60 80 100 Back to ready position Backswing Hitting End phase phase phase phase Forward Ready position Backswing Accmax Forward Figure 2: SPM procedure. The SPM, like other statistical methods, has assumptions. The assumptions for the SPMftg paired sample t-test include continuous waveforms with an equal sample rate and a number of data points; the sample size (or data set size) should be greater than 5 in each group; each waveform should come from a random sample and be normally distributed over time; the waveforms of interest should be spread similarly between the two groups (homogeneity of variance that is maintained over time). that offers a high-level interface to SPM1D. Angle-time SPM test allowed for the identification of the differences numerical series were averaged over trials and reported between groups in the movement patterns in individual against cycle time (Figure 2(a)). For each participant and joints and changes in the acceleration of the playing hand. selected time-dependent angular numerical data, a two- The basic difference that can be noticed is the time of occur- sample t-test SPMftg function (with alpha = 0:05, non- rence of the beginnings and ends of the individual movement sphericity correction, and assumption of unequal variances) phases. For the POL players, the backswing phase starts was numerically computed to check the level of similarity slightly earlier (about 46% of the cycle duration for POL, between the movements [19, 20]. For each test, a statistical 54% for CHIN players) similarly to the hitting phase (83% parametric map SPMftg (Figure 2(b)) was created by calcu- and 87%, respectively), whereas the average time of the max- lating the conventional univariate t-statistic at each point of imum hand acceleration (Accmax) is very similar for both the gait curve [21–24]. When an SPMftg crossed the groups (about 96% of the cycle duration). The observation assumed threshold, an additional threshold cluster was cre- and description of the way of coordinating the movements ated, indicating a significant difference (a grey area) between when hitting the backhand topspin reveals that the average two compared joint motion patterns in a specific location of movement pattern (changes in joint angles throughout the the gait cycle. In the present study, because of the high num- cycle) is consistent with that described in previous studies ber of statistical analyses, the SPM results are visualized in a [25, 26]. The following movements were observed in the summarised manner. Instead of SPMftg curves, blue bars backswing phase: lower limb flexion, upper body flexion (for- are shown, indicating the significance during the cycle ward bend), adduction and internal rotation in the shoulder (Figure 2(c)). joint, elbow joint flexion, and flexion, pronation, and palmar flexion in the wrist joint. In the hitting phase (with different time of inclusion of individual segments into the movement, 3. Results and Discussion according to the principle of the proximal-to-distal move- The study is aimed at evaluating the differences in movement ment sequence), the following movements were observed: kinematics using the SPM method between two different extension in the lower limb and upper body joints, abduc- groups of female table tennis players. The application of the tion, flexion, and external rotation in the shoulder joint, Elbow flexion-extension (deg) Elbow flexion-extension (deg) Statistical function SPM {t} 6 Applied Bionics and Biomechanics movement, differences also occur at the end of the extension and supination in the elbow joint, and extension, supination, and radial abduction in the wrist joint. forward phase. Greater abduction and external rota- The analysis of the SPM test results allowed for the obser- tion can be also observed in the part of the backswing and hitting phases in the discussed joints in the CHIN vation of the differences in the movement patterns in the individual analyzed joints. female players (Figure 5). It should also be emphasized that there is a period with no differences in the flexion- (1) Ankle joints: the movement pattern in the ankle joints extension movement in a significant part of the back- is characterized by the occurrence of many periods swing and hitting phases (up to the moment of reach- that differ between the two groups studied. The lack ing the maximum acceleration—Accmax) of differences in the flexion-extension movement (6) Elbow joint of the playing limb: the SPM test revealed (dorsiflexion, Figure 3) in the nonplaying side ankle differences in flexion-extension movement at the joint (i.e., throughout the hitting phase) and wave- elbow joints in the major part of the back to ready like changes in ankle joint movement observed with position phase, part of the backswing phase, and the higher frequency in CHIN female athletes (Figure 3) end of the hitting phase (Figure 5). Nevertheless, both are noticeable groups showed elbow flexion in the backswing joint (2) Knee joints: in the flexion-extension movement of in the back to ready position phase (up to circa 70- the knee joints, the wave-like character of the changes 90 deg), maintaining this flexion or very slow exten- in the back to ready position phase and the back- sion during the backswing phase, and quite a rapid swing phase observed in CHIN is noteworthy. Signif- extension during the hitting phase (up to circa 20- icantly, more periods differing between the two 40 deg) groups occur in the right knee joint (Figure 3), in (7) Wrist joint of the playing limb: the fewest periods of which the average flexion range is larger in POL com- differences between the two groups demonstrated by pared to the CHIN group during the entire cycle the SPM test occur in the movement of elbow flexion (3) Hip joints: in hip joint movements, there are more and radial abduction in the wrist joint (Figure 5). periods of differences concerning the right hip joint. Maintaining the elbow flexion up to circa -20 to CHIN players exhibit greater abduction and external -30 deg can be observed in both groups in the back rotation throughout the cycle in the right hip joint. It to ready position and backswing phases, and then, is noteworthy that there were no differences between after the beginning of the hitting phase, quite a rapid the groups in the significant part of the backswing movement towards radial flexion (up to circa -10- phase in the abduction movement in the nonplaying 0 deg) was found. The maximum of radial flexion side hip joint and the part of the backswing and hit- occurs at around Accmax, and there is a brief ting phases in the rotation movement in these joints moment of differences between the groups during (Figure 3). this period. The supination-pronation movement in the described joint differentiates between the two (4) Joints of the upper body: very few differences were groups more. A period of no differences between observed in the flexion-extension movements in the the groups occurs in the back to ready position phase lumbar region, in which flexion can be observed in (from circa 5% to circa 30% of the cycle time) and in the backswing phase and extension was found in circa 91-93% of the cycle time in the hitting phase. the hitting phase (Figure 4). The range of rotation Polish female players are characterized by using a movement was slight (about 5 deg), more pro- greater range of this movement. The supination nounced in CHIN, whereas in the POL group, it movement is rapid during the hitting phase, from was characterized by high variability (high SD value the moment after the beginning of this phase to the throughout the cycle). The movement of the upper moment of Accmax in both groups. In the body (thoracic region) differentiates the two groups extension-flexion movement in the wrist joint, it is the most in the sagittal plane (flexion-extension). In noticeable that there are no differences in the back CHIN players, this movement is used to a greater to ready position phase and before the Accmax extent (about 30-40 deg), from slow flexion in the moment. There is a slow flexion of the limb in the backswing phase, through faster flexion in the initial described joint in both groups during the back to hitting phase, to the extension in the Accmax region ready position and backswing phases, accelerating and later (Figure 4). The rotation of this part of the during the hitting phase. At circa 90% of the cycle, upper body and lateral flexion in the backswing phase the direction of movement changes to the extension and most of the hitting phase does not show differ- (within circa 10 deg in both groups) at a high rate ences between the two groups. These movements until reaching Accmax. The latter short period shows take place in small ranges of several degrees no differences between the groups (5) Shoulder joint of the playing limb: in the shoulder joint of the playing limb, it can be observed that the The observation that comes to mind is the occurrence of differences mainly concern the back to ready position the longest periods of differentiation between the groups phase in all planes (Figure 5). In the flexion-extension studied in the lower limb joints, which indicates their Applied Bionics and Biomechanics 7 Playing side Opposite side 20 20 10 10 0 0 -10 -10 -20 -20 -30 P < 0.001 P < 0.001 P = 0.007 P < 0.001 P < 0.001 -5 -10 -10 -20 -15 -30 -20 -40 P = 0.003 P < 0.001 P = 0.027 P < 0.001 25.0 22.5 20.0 17.5 15.0 12.5 10.0 P < 0.001 P = 0.035 P < 0.001 P < 0.001 P < 0.001 P = 0.002 0.023 0.015 0.011 0.006 0.002 P < 0.001 P < 0.001 P < 0.001 P < 0.001 P = 0.001 P < 0.001 P = 0.024 P < 0.001 P = 0.001 P < 0.001 P = 0.012 P < 0.001 P < 0.001 -10 -20 P < 0.001 P = 0.012 P < 0.001 P = 0.001 Cycle time (%) 0 20406080 100 0 20406080 100 Forward Ready position Backswing Accmax Forward Forward Ready position Backswing Accmax Forward Figure 3: Lower extremity kinematics. Red line: average values of POL; green line: average values of CHIN; grey areas: SD values. Blue bars indicate the significance during the cycle. different use by both groups of female players. Undoubtedly, next stroke and keep the lower limbs in constant readiness. a wave-like movement in the ankle and knee joints is more Therefore, it can be concluded that CHIN players use these pronounced in CHIN players, which reflects the use of the steps more often than POL and perhaps this is due to differ- so-called small steps, mainly in the back to ready position ences in coaching. Differences can be observed in the ankle and backswing phases. These steps are used to adapt to the joints in all planes, and they affect the entire backswing and Ankle inversion-eversion Ankle ab-adduction Ankle dorsi- Hip internal-external Hip ab-adduction Hip flexion-extension Knee flexion-extension rotation (deg) (deg) plantar flexion (deg) rotation (deg) (deg) (deg) (deg) +Flexion +External +Abduction +Flexion +Abduction +Flexion +Eversion +Abduction +Eversion +Flexion +Flexion +External +Abduction +Flexion 8 Applied Bionics and Biomechanics orax–thoracic mov. Loins-lumbar mov. -10 -20 P = 0.045 P < 0.001 P = 0.029 P = 0.007 P < 0.001 P = 0.027 2.5 0.0 -2.5 -5.0 -5 -7.5 -10 -10.0 P < 0.001 P < 0.001 0 -5 -10 -5 -15 P = 0.050 P < 0.001 P = 0.035 P < 0.001 Cycle time (%) 0 20 40 60 80 100 0 20 40 60 80 100 Forward Ready position Backswing Accmax Forward Forward Ready position Backswing Accmax Forward Figure 4: Torso kinematics. Red line: average values of POL; green line: average values of CHIN; grey areas: SD values. Blue bars indicate the significance during the cycle. forward phases. It is noticeable that the directions of move- probably due to the different playing styles prevalent in the ment in the hitting phase are the same in both groups in two groups. In all players, the forward phase is accompanied the ankle joints, and the differences are in the degree values. by the extension of the knee joints within a range of several The nonplaying side ankle joint in both groups in the for- dozen degrees. The above findings provide helpful informa- ward phase shows no differences and the toe-raise movement tion for coaches and players with regard to the backhand top- (decreasing dorsiflexion, transitioning to plantar flexion), in spin technique and its modifications regarding lower limb an approximately 20-degree range. A similar movement, movements. but differentiating between the two groups, can be observed The movement in the hip joint showed long periods of in the right ankle joint. For both joints, the range of motion differences between the groups studied. However, similar is smaller in CHIN player. The direction of this movement movement directions were found in individual phases in in the forward phase indicates the use of upward and forward both groups. The small rotation range of a few degrees in transfer of the center of gravity as an action to support the the hip joints should be emphasized, which, according to many authors, greatly helps generate the stroke force and hitting movement performed by the player. The importance of this movement while performing a stroke has been high racket speed in table tennis [26–29]. It is directly sug- highlighted in the literature [26, 27]. Wang et al. also pointed gested that the range of this movement and its use differenti- out the differences between players at different sport skill ates between players of different sports skill levels. The lower levels in the performance of movements in the joints of the use of rotation in these joints is related to the type of stroke analyzed in this study. It is a topspin backhand played early lower limbs, emphasizing that these movements can be used better by an economical work with simultaneous use of the against a topspin ball, so it is a counterstroke from the group energy generated by the elastic components of the joints of strokes that utilize the energy of the flying ball and there- and muscles (based on the stretch-shortening principle) fore does not require the involvement of great strength of the [28]. Perhaps the differences in the movement in the ankle player. Similar aspects were pointed out by Marsan et al., who evaluated the mechanical energy generated from the hip joint joints shown in this paper are related to this method. As mentioned above, a wave-like movement in CHIN players during different variations of strokes, finding that backhand was reported in flexion-extension movements in the knee drive required the lowest hip mechanical work [30]. joints, indicating the use of small steps in the preparation In the lumbar spine, the least differences were found in phases (back to ready position, backswing). A greater flexion the flexion-extension motion. In the backswing phase, this is a few degrees of flexion, whereas in the hitting phase- angle in the right knee joint was also observed in the POL group throughout the cycle. This is probably due to the trans- extension in both groups. The lateral flexion movement indi- fer of center of gravity to the right leg, emphasized more in cates that the POL players are slightly leaning to the right, the POL group throughout the cycle. It can be assumed that with the body weight shifted to the right lower limb, again this difference allows the CHIN players to switch to forehand indicating a more backhanded position than in the Chinese players. The CHIN players seem to stand more universally, play faster and more flexibly after performing a pivot and is Internal-external Lateral bend (obliquity) Anterior-posterior bend rotation (deg) (deg) (tilt) (deg) +External +Inward +Anterior Applied Bionics and Biomechanics 9 Playing side Opposite side -Extension +Flexion -10 -20 -30 -40 -50 P = 0.012 P < 0.001 P = 0.040 P < 0.001 +Abduction -10 -20 -30 –Adduction 5 P = 0.049 P = 0.031 P < 0.001 P = 0.032 +Pronation 125 60 P = 0.040 P < 0.001 P = 0.022 P < 0.001 +Flexion P = 0.039 P < 0.001 P = 0.011 P < 0.001 +Flexion P = 0.005 P = 0.017 P = 0.028 P = 0.019 P = 0.001 P = 0.045 +Abduction P < 0.001 P = 0.033 P < 0.001 P = 0.046 +External -25 -20 –internal -50 -40 -75 -60 -100 P = 0.050 P < 0.001 P = 0.015 P = 0.001 P < 0.001 Cycle time (%) 0 20 40 60 80 100 0 20 40 60 80 100 Forward Ready position Backswing Accmax Forward Forward Ready position Backswing Accmax Forward Figure 5: Upper extremity kinematics. Red line: average values of POL; green line: average values of CHIN; grey areas: SD values. Blue bars indicate the significance during the cycle. confirms previous observations concerning the small contri- with the ability to transition more easily from the backhand to the forehand playing, as discussed above. The CHIN bution of hip and trunk rotation resulting from the type of players also use a certain amount of rotation in the lumbar stroke assessed. region during the hitting phase in contrast to POL players, Regarding the playing upper hand, the most differences who hardly use any rotation in this body segment. It must were found in the abduction-adduction of the shoulder, be admitted, however, that the SD values in the POL group flexion-extension at the elbow joint, and supination- are high, indicating great variation in the way this segment pronation at the wrist joint. In these three cases, the differ- is used in the topspin backhand stroke. Nevertheless, the ences between the groups concern much of the back to ready small range of rotation (similar in both groups) in body trunk position phase, the beginning of the backswing, and the end Shoulder internal- Shoulder ab-adduction Shoulder flexion- Elbow flexion-extension Wrist pronation- Wrist radial abduction- Wrist flexion-extension external rotation (deg) (deg) extension (deg) (deg) supination (deg) adduction (deg) (deg) 10 Applied Bionics and Biomechanics of the forward phase. Actually, the end of the forward phase (from Accmax to the end of this phase) differentiates between the groups in each movement in the joints of the playing upper limb. It must be admitted, however, that the 40 directions of movements are very similar (the curves of the graphs have a very similar shape), and the differences dem- P = 0.021 P = 0.038 P < 0.001 P < 0.001 P < 0.001 P < 0.001 onstrated in the SPM test may be due to the different times Cycle time (%) beginning the individual phases in the groups. The SPM test 0 20 40 60 80 100 showed no differences in flexion-extension and external- internal rotation in the shoulder joint, in radial abduction- Forward Ready position Backswing Accmax Forward adduction, and flexion-extension at the wrist joint during the second part of the backswing and the beginning of the Figure 6: Hand acceleration. Red line: average values of POL; green hitting phase. Movement coordination in the female players line: average values of CHIN; grey areas: SD values. Blue bars indicate the significance during the cycle. studied is consistent with that reported in the literature [25, 29]. Furthermore, the description of basic movement, pre- sented in our work, can provide more clarity in understand- tion for table tennis coaches and players. The SPM method ing the topspin backhand technique. allowed for the determination of differences between the Chi- The values of hand acceleration and its changes over time nese and Polish female athletes. The observed differences demonstrated in the SPM test differentiate between the include, among others, greater use of the so-called small steps groups studied for most of the cycle and in all phases, with in order to adapt and be ready during the back to ready posi- short exceptions of ca. 20% and 40%, and in the hitting phase, tion and backswing phases, which gives the CHIN players especially after reaching Accmax (Figure 6). slightly better conditions for preparation for the next plays. For most of the back to ready position phase and the The position of the CHIN players compared to that of the backswing phase, the acceleration values are close to 0. After POL players favours a quicker transition from the backhand circa half of the backswing phase, acceleration values increase to the forehand play. This difference is probably related to the until they reach maximum values at the end of the forward difference in the dominant playing styles of the groups stud- phase, which are very similar in both groups (about ied. The differences found are probably mainly due to differ- 90 m/s ). The pattern of acceleration values is then interest- ences in the training methodologies caused by different ing. It is different for both groups in each phase, but it is sim- coaching systems. It can be also concluded that despite the ilar at the Accmax point, and the maximum values obtained indicated differences in movement patterns in both groups, by both groups are also similar. Therefore, it can be con- the same value of Accmax was achieved. This may be a man- cluded that despite the indicated differences in movement ifestation of the phenomenon of variability of movement, as patterns in both groups, the same value of Accmax was well as equifinality and compensation. achieved. This may be a manifestation of the phenomenon of equifinality and compensation, indicated in the literature Data Availability as typical of dynamic systems and variability of movement The supplementary data (containing angle waveforms and [5, 10, 31]. Obviously, it should be added that just achieving accelerations) used to support the findings of this study are the right amount of hand acceleration does not determine the included within the supplementary materials. accuracy of the play; the hitting angle, the direction of move- ment, and other factors are also important [32]. Conflicts of Interest 3.1. Limitations of the Study. 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The Application of Statistical Parametric Mapping to Evaluate Differences in Topspin Backhand between Chinese and Polish Female Table Tennis Players

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Copyright © 2021 Ziemowit Bańkosz and Sławomir Winiarski. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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Hindawi Applied Bionics and Biomechanics Volume 2021, Article ID 5555874, 11 pages https://doi.org/10.1155/2021/5555874 Research Article The Application of Statistical Parametric Mapping to Evaluate Differences in Topspin Backhand between Chinese and Polish Female Table Tennis Players Ziemowit Bańkosz and Sławomir Winiarski Department of Biomechanics, Faculty of Physical Education and Sports, University School of Physical Education in Wroclaw, Poland Correspondence should be addressed to Sławomir Winiarski; slawomir.winiarski@awf.wroc.pl Received 15 January 2021; Accepted 30 June 2021; Published 15 July 2021 Academic Editor: Ukadike C. Ugbolue Copyright © 2021 Ziemowit Bańkosz and Sławomir Winiarski. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The research is aimed at comparing the kinematics (the movement pattern in the most important joints and accelerations of the playing hand) between female table tennis players coached in Poland (POL) and China (CHIN) during the performance of a topspin backhand stroke (so-called quick topspin). The study involved six female table tennis players at a high sports skill level, playing in Poland’s highest league. Three were national team members of Poland (age: 20:3±1:9), while the other three were players from China (age: 20:0±0:0). Kinematics was measured using MR3 myoMuscle Master Edition system—inertial measurement unit (IMU) system. The participants performed one task of topspin backhand as a response to a topspin ball, repeated 15 times. Statistical parametric mapping (SPM) was calculated using SPM1D in a Python package that offered a high- level interface to SPM1D. The SPM method allowed for the determination of differences between the Chinese and Polish female athletes. The differences found are probably mainly due to differences in the training methodologies caused by different coaching systems. The observed differences include, among others, greater use of the so-called small steps in order to adapt and be ready during the back to ready position and backswing phases, which gives the CHIN players slightly better conditions for preparation for the next plays. The CHIN players’ position compared to that of the POL players favours a quicker transition from the backhand to the forehand play. This difference is probably related to the difference in the dominant playing styles of the groups studied. Despite the differences in movement patterns in both groups, the exact value of playing hand was achieved. This may be a manifestation of the phenomenon of equifinality and compensation. All the differences found are probably mainly due to differences in the training methodologies caused by different coaching systems. 1. Introduction nis players must also adjust their position to the ball using a different kind of footwork, changing kinematics and kinetics As a very complex and multifaceted sport, table tennis is characteristics of body segments [2, 3]. These differences lead characterized by various strokes, legwork techniques, tactical to a large variety and variability of movement in this sport. Issues related to movement variability have recently been solutions and playing styles, and a multitude of solutions for an almost infinite number of game situations. The main quite often addressed in the literature. Traditionally, move- groups of strokes that yield points are topspin strokes, intro- ment variability is considered to reflect the “noise” in the sys- duced to the game in the 1950s by Japanese players [1]. tem of human movements, while learning a given activity Players use many variations of topspin strokes in their game requires decreasing variability as it is perceived as incorrect [4, 5]. Movement variability is also viewed and considered a (e.g., backhand and forehand strokes, differing in strength involved, speed of ball rotation, flight trajectory, ball speed, normal phenomenon, resulting from the diversity and vari- and the moment of hitting the ball) depending on the solu- ability present in the entire biological system used by tion used or the need to adjust to ball parameters. Table ten- humans, and its occurrence is associated with adaptive and 2 Applied Bionics and Biomechanics seems to be an interesting issue. Identification of differences functional processes [6]. Movement variability has been explained using many theories available in the literature, such and, at the same time, similar or perhaps unchangeable ele- as generalized motor program [7, 8], GMP-uncontrolled man- ments of table tennis stroke techniques in athletes coached using different training systems can provide important ifold (UCM) [9], and dynamic systems theory [5]. Assessing the occurrence and scale of movement variability appears to insights into the technique of performing a given stroke. be extremely important in the sports training process. It seems Some differences in the technique may indicate the possibil- to be also critical in the process of improving skills of purpo- ity of using different solutions in the performance of the sive movements and explaining how to control human move- stroke, while the same, similar, and unchanging elements may highlight their importance in table tennis. Therefore, ments. Linear measures have been used in the assessment of variability, such as standard deviation, range, or coefficient the aim of the research was to compare the kinematics of variation. Taking into account discrete (numerical) or serial between female players coached in Europe (Poland) and Asia data, i.e., continuous and changing over time, would improve (China) during the performance of a topspin backhand the assessment of variability. This is because when assessing stroke. In accordance with the findings of other authors and previous studies [12, 13], it was assumed that, despite movement coordination, for instance, the change of the angle in a given joint over time, and comparing the repetitions by the comparable level of players, there are many differences one or many people, a method should be used to compare in the kinematics of topspin backhand between them. The time waveforms rather than just single, selected parameters. greater differences between the players would occur in the Such criteria are met by the statistical parametric mapping joints and segments located farther from the place of the racket contact with the ball (upper body and shoulder joint) (SPM) method. It is the gold standard statistical method ded- icated to numerical signal data analysis. For the one- than in those closer, located in the playing hand (wrist joint). dimensional variables recorded with the motion analysis sys- It can also be assumed that at the key instant of the stroke, tem, the general SPM model can be simplified to the one- which is the moment of maximum acceleration, occurring dimensional model SPM1D. This method and its characteriza- at around the contact between the racket and the ball, the least differences are observed in players’ kinematics. tion were presented in previous studies [10]. The assessment of variability of movement seems to be important in table tennis, which is a very complex sport, 2. Material and Methods where technique and its improvement are the essential ele- ments used to achieve the champion level, with the basis of 2.1. Study Design. It was an observational study with adopted the technique being a stroke and precise hitting the ball with retrospective convenience sampling. The minimal sample the racket. The few available studies on table tennis and the size of our data was determined in the planning stage of the variability of movement have been based on the methods of experiment using the margin of error approach to get results evaluation of standard deviation, correlation, and analysis as accurate as needed (with an assumed 5% margin of error at of variance (ANOVA and least significant difference (LSD)) 95% level of confidence and α level of 0.05). The assumed and presented UCM calculations. Iino et al. emphasized that standard deviation was taken from preliminary studies using the possibility of using different configurations in the evalu- the same population of interest. ated joints to stabilize the vertical angle of the racket in table tennis strokes can be a critical factor in playing performance [11]. A previous study by Bańkosz and Winiarski also evalu- 2.2. Participants. The study involved six female table tennis ated the variability of movement by analyzing the coefficient players at a high sports skill level, playing in the highest lea- of variation of kinematic parameters in selected important gue in Poland (Ekstraklasa table tennis league). Three of moments of the hitting movement [10, 12]. However, the them were national team members of Poland (POL) in the coordination of movements in individual joints was taken category of adult players (age: 20:3±1:9y:), while the other into account to a small extent. The variability of temporal three were players from China (CHIN, age: 20:0±0:0y:), and spatial coordination of movements, the possibility of coached within the Chinese training system (i.e., in China). compensation, and functional variability are significant All of the players had more than 10 years of experience in problems in the coaching practice and in the process of table tennis and presented the offensive style of the game. teaching and improving technique and its monitoring. Mak- One player from China was a left hander. Average body ing the coaches and players aware of the different variants of height was 161:7±4:5cm in the group of Polish players strokes even for a specific solution (e.g., playing with the right and 162:7±4:1cm in the group of Chinese players, whereas strength, speed, and rotation to the same place) seems to be body weight was 59:0±6:9kg and 56:7±6:4kg, respectively. very important and necessary for improving the training pro- Before the study, all participants were informed about the cess. Therefore, copying and imposing a single pattern of per- purpose of the study and the possibility of withdrawing par- forming the movement seem to be a wrong way. Considering ticipation at any stage, without giving a reason. All the partic- the differences between athletes and looking for individual ipants provided informed consent before the research. Pain technical solutions instead would be a better choice [10]. or recent injury was the exclusion criterion for the study par- Interpersonal variation of the sports technique may ticipants. All procedures performed in this study received result, for example, from gender differences, differences in positive approval from the Senate’s Research Bioethics Com- anatomical structure, and differences in sports skill level. mission at the University School of Physical Education in The diversity of techniques due to the training system also Wrocław, Poland (Ethics IRB number 34/2019). Applied Bionics and Biomechanics 3 Head (middle front part) Upper thoracic (below C7) Upper arm (lateral and Lower thoracic (at L1/T12) longitudinal to bone axis) Pelvic (sacrum) Hand (dorsal part) Forearm (posterior and distal) igh (frontal and distal half) Shank (front and medial) Foot (shoe adapter) MR3 myoMuscle Master Newgy Robo-Pong Edition system Robot 2050 Figure 1: Measurement site. 2.3. Laboratory Set-Up. Kinematics was measured using the accurately, and as quick as you can”). After video analysis, MR3 myoMuscle Master Edition system (myoMOTION™, only successful shot considered “on table” and played diago- Noraxon, USA, Figure 1). The myoMOTION system consists nally was recorded for further calculations (missed balls, balls of a set of (1 to 16) sensors using inertial sensor technology. hit out of bounds, and balls hit into the net were excluded). Based on the so-called fusion algorithms, the information The balls were shot by a dedicated table tennis robot (Newgy from a 3D accelerometer, gyroscope, and magnetometer is Robo-Pong Robot 2050, Newgy Industries, Tennessee, USA, used to measure the 3D rotation angles of each sensor in Figure 1) at constant parameters of rotation, speed, direction, absolute space (yaw-pitch-roll, also called orientation or nav- and flight trajectory. The settings of the robot were as follows: igation angles, [12]). Inertial sensors were located on the (i) Rotation type: topspin body of the study participant to record the accelerations, according to the myoMOTION protocol described in the (ii) Speed (determines both speed and spin, where 0 is manual. The accuracy and validity of the inertial measure- the minimum and 30 is the maximum): 18 ment unit (IMU) system in angle determination were the subject of the previous research [14, 15]. (iii) Left position (leftmost position to which the ball is Sensors were attached with elastic straps and self- delivered): 15 adhesive tape. The sensors were placed bilaterally so that (iv) Wing (robot’s head angle indicator): 7.5 the positive x-coordinate on the sensor label corresponded to a superior orientation for the trunk, head, and pelvis (v) Frequency (time interval between balls thrown): (Figure 1). For the limb segment sensors, the positive x 1.4 s -coordinate corresponded to a proximal orientation. For the foot sensor, the x-coordinate was directed distally (to Each player had had three to five familiarization trials before the task. The same racket with the following character- the toes). At the beginning of the measurement, each partic- istics was used for the experiment: blade, Jonyer-H-AN (But- ipant was checked and the system was calibrated according to terfly, Japan); rubber, Tenergy 05, 2.1 mm (Butterfly, Japan); the manufacturer’s recommendations. The recording speed Plastic Andro Speedball 3S 40+ balls (Andro, Germany); and of the piezoelectric sensor was adjusted to the maximal sam- pling rate for a given sensor (100 Hz per sensor) for the whole a Stiga Premium Compact table (Stiga, Sweden). 16-sensor set. Noraxon’s IMU technology mathematically combines and filters incoming source signals on the sensor 2.5. Kinematics. A total of 90 cycles of topspin backhand level and transmits the 4 quaternions of each sensor. We used stroke were studied. Based on the ISB recommendations con- system-built fusion algorithms and Kalman filtering (digital cerning the definitions of the joint coordinate system of var- bandpass finite impulse response filter (FIR)). This mode ious joints for the reporting of human joint motion [16, 17], allowed direct access to all unprocessed raw IMU sensor data. the following angles (measured in degrees) were chosen for both sides and sampled every 0.01 percent of cycle time: 2.4. Experimental Procedures. The participants performed one task of topspin backhand (TBH) as a response to a top- (i) Ankle dorsiflexion/plantar flexion (AFE): rotation spin ball, repeated 15 times. Each player was asked to hit of the foot with respect to the tibia coordinate sys- the ball in the early stage of its flight (so-called quick topspin) tem in the sagittal plane; a negative sign denotes and to reach the marked area in the corner of the table plantar flexion (extension) and positive sign dorsi- flexion (flexion) (30 × 30 cm) diagonally (after instruction: “play diagonally, 4 Applied Bionics and Biomechanics (iv) Elbow flexion-extension (EFE): movement of the (ii) Ankle abduction-adduction: movement of the foot away or towards the midline of the body; a forearm relative to the humerus along the transver- negative sign denotes adduction while positive sign sal axis; negative sign denotes (hyper)extension abduction while positive flexion (iii) Ankle inversion-eversion: rotation of the foot (v) Wrist flexion-extension (WFE): movement of wrist around its long axis; a negative sign denotes ever- relative to the radius along the transversal axis and sion (away from the median plane) while positive measured between upper arm and hand sensors; a sign inversion (towards the median plane) negative sign denotes extension while positive flexion (iv) Knee flexion-extension (KFE): movement of the tibia with respect to the femur coordinate system (vi) Wrist supination-pronation (WSup): movement of in the sagittal plane; a negative sign denotes exten- wrist relative to the radius along the axis and mea- sion and positive flexion sured between the upper arm and hand sensors; pronation is a positive rotation and supination is a (v) Hip flexion-extension (HFE): movement of the negative rotation femur with respect to the pelvis coordinate system in the sagittal plane; a negative sign denotes exten- (vii) Wrist radial abduction-adduction (WRad): move- sion while positive flexion ment of wrist relative to the radius and measured between the upper arm and hand sensors; adduction (vi) Hip abduction-adduction (HAA): movement of the (or ulnar deviation) is negative while abduction (or femur with respect to the pelvis coordinate system radial deviation) is positive in the frontal plane; a negative sign denotes adduc- tion while positive abduction The movement of the playing hand was used to assess specific events of the cycle: (vii) Hip internal-external rotation (HIER): internal or external movement of the femur with respect to (i) Ready position, where the hand is not moving after the pelvis coordinate system in the transversal the previous stroke, just before the swing plane; a negative sign denotes internal while posi- tive external rotation (ii) Backswing, which is the moment when the hand changes direction from backward to forward in the (viii) Lumbar internal-external rotation (LIER): internal sagittal plane after the swing or external movement of the loins in the transversal plane; a negative sign denotes internal while posi- (iii) Accmax, which is the moment of maximum acceler- tive external rotation ation of the hand and the moment when the hand reaches the maximum acceleration (ix) Thoracic internal-external rotation (ThIER): inter- nal or external movement of the thorax relative to (iv) Forward, which is the moment when the hand global coordination system in the transversal plane; changes the direction from forward to backward in a negative sign denotes internal while positive the sagittal plane after the stroke (the end of the cycle external rotation and the beginning of the next cycle) For the upper extremity (playing side), a simplified biome- The phases between defined events were as follows: back chanical model was adopted based on the predominant plane to ready position phase (between the forward and ready posi- of movement as described by Wu et al. [17] with segments of tion), backswing phase (between ready position and back- interest being the thorax, clavicle, scapula, humerus, forearm, swing), hitting phase (between backswing and Accmax), and carpus of the hand. Based on the adopted sequence of and forward end phase (between Accmax and forward). Euler angles, the following angles were computed: The timing of events was analyzed and compared between the POL and CHIN players. (i) Shoulder flexion-extension (ShFE): movement of the humerus relative to the thorax in sagittal plane; 2.6. Statistical Analysis. Statistical calculations were per- negative sign denotes extension while positive formed using Statistica 13.1 (TIBCO Software Inc.). The flexion sample size was estimated using recommendations postu- lated by Kontaxis et al. [18]. The statistical power was suffi- (ii) Shoulder abduction-adduction (ShAA): movement cient to detect the described differences. Power analysis of of the humerus relative to the thorax in the frontal discrete data was performed to estimate the SPM test power. plane; negative sign denotes adduction while posi- For the extracted data and for the significant changes tive abduction (alpha = 0:05), the partial η effect size was found between (iii) Shoulder internal-external rotation (ShIER): move- 0.62 and 0.86. The SPM test was applied to identify the differ- ment of the humerus relative to the thorax in the ences between groups in the movement patterns in individual transversal plane; a negative sign denotes internal joints and changes in the acceleration of the playing hand. (medial) while positive external (lateral) rotation The SPM was calculated using SPM1D in a Python package Applied Bionics and Biomechanics 5 (a) (b) p < 0.001 80 p < 0.001 p = 0.002 α = 0.05 : F = 5.993 0 200 400 600 800 1000 (c) p < 0.001 p < 0.001 p < 0.002 Cycle time (%) 0 20 40 60 80 100 Back to ready position Backswing Hitting End phase phase phase phase Forward Ready position Backswing Accmax Forward Figure 2: SPM procedure. The SPM, like other statistical methods, has assumptions. The assumptions for the SPMftg paired sample t-test include continuous waveforms with an equal sample rate and a number of data points; the sample size (or data set size) should be greater than 5 in each group; each waveform should come from a random sample and be normally distributed over time; the waveforms of interest should be spread similarly between the two groups (homogeneity of variance that is maintained over time). that offers a high-level interface to SPM1D. Angle-time SPM test allowed for the identification of the differences numerical series were averaged over trials and reported between groups in the movement patterns in individual against cycle time (Figure 2(a)). For each participant and joints and changes in the acceleration of the playing hand. selected time-dependent angular numerical data, a two- The basic difference that can be noticed is the time of occur- sample t-test SPMftg function (with alpha = 0:05, non- rence of the beginnings and ends of the individual movement sphericity correction, and assumption of unequal variances) phases. For the POL players, the backswing phase starts was numerically computed to check the level of similarity slightly earlier (about 46% of the cycle duration for POL, between the movements [19, 20]. For each test, a statistical 54% for CHIN players) similarly to the hitting phase (83% parametric map SPMftg (Figure 2(b)) was created by calcu- and 87%, respectively), whereas the average time of the max- lating the conventional univariate t-statistic at each point of imum hand acceleration (Accmax) is very similar for both the gait curve [21–24]. When an SPMftg crossed the groups (about 96% of the cycle duration). The observation assumed threshold, an additional threshold cluster was cre- and description of the way of coordinating the movements ated, indicating a significant difference (a grey area) between when hitting the backhand topspin reveals that the average two compared joint motion patterns in a specific location of movement pattern (changes in joint angles throughout the the gait cycle. In the present study, because of the high num- cycle) is consistent with that described in previous studies ber of statistical analyses, the SPM results are visualized in a [25, 26]. The following movements were observed in the summarised manner. Instead of SPMftg curves, blue bars backswing phase: lower limb flexion, upper body flexion (for- are shown, indicating the significance during the cycle ward bend), adduction and internal rotation in the shoulder (Figure 2(c)). joint, elbow joint flexion, and flexion, pronation, and palmar flexion in the wrist joint. In the hitting phase (with different time of inclusion of individual segments into the movement, 3. Results and Discussion according to the principle of the proximal-to-distal move- The study is aimed at evaluating the differences in movement ment sequence), the following movements were observed: kinematics using the SPM method between two different extension in the lower limb and upper body joints, abduc- groups of female table tennis players. The application of the tion, flexion, and external rotation in the shoulder joint, Elbow flexion-extension (deg) Elbow flexion-extension (deg) Statistical function SPM {t} 6 Applied Bionics and Biomechanics movement, differences also occur at the end of the extension and supination in the elbow joint, and extension, supination, and radial abduction in the wrist joint. forward phase. Greater abduction and external rota- The analysis of the SPM test results allowed for the obser- tion can be also observed in the part of the backswing and hitting phases in the discussed joints in the CHIN vation of the differences in the movement patterns in the individual analyzed joints. female players (Figure 5). It should also be emphasized that there is a period with no differences in the flexion- (1) Ankle joints: the movement pattern in the ankle joints extension movement in a significant part of the back- is characterized by the occurrence of many periods swing and hitting phases (up to the moment of reach- that differ between the two groups studied. The lack ing the maximum acceleration—Accmax) of differences in the flexion-extension movement (6) Elbow joint of the playing limb: the SPM test revealed (dorsiflexion, Figure 3) in the nonplaying side ankle differences in flexion-extension movement at the joint (i.e., throughout the hitting phase) and wave- elbow joints in the major part of the back to ready like changes in ankle joint movement observed with position phase, part of the backswing phase, and the higher frequency in CHIN female athletes (Figure 3) end of the hitting phase (Figure 5). Nevertheless, both are noticeable groups showed elbow flexion in the backswing joint (2) Knee joints: in the flexion-extension movement of in the back to ready position phase (up to circa 70- the knee joints, the wave-like character of the changes 90 deg), maintaining this flexion or very slow exten- in the back to ready position phase and the back- sion during the backswing phase, and quite a rapid swing phase observed in CHIN is noteworthy. Signif- extension during the hitting phase (up to circa 20- icantly, more periods differing between the two 40 deg) groups occur in the right knee joint (Figure 3), in (7) Wrist joint of the playing limb: the fewest periods of which the average flexion range is larger in POL com- differences between the two groups demonstrated by pared to the CHIN group during the entire cycle the SPM test occur in the movement of elbow flexion (3) Hip joints: in hip joint movements, there are more and radial abduction in the wrist joint (Figure 5). periods of differences concerning the right hip joint. Maintaining the elbow flexion up to circa -20 to CHIN players exhibit greater abduction and external -30 deg can be observed in both groups in the back rotation throughout the cycle in the right hip joint. It to ready position and backswing phases, and then, is noteworthy that there were no differences between after the beginning of the hitting phase, quite a rapid the groups in the significant part of the backswing movement towards radial flexion (up to circa -10- phase in the abduction movement in the nonplaying 0 deg) was found. The maximum of radial flexion side hip joint and the part of the backswing and hit- occurs at around Accmax, and there is a brief ting phases in the rotation movement in these joints moment of differences between the groups during (Figure 3). this period. The supination-pronation movement in the described joint differentiates between the two (4) Joints of the upper body: very few differences were groups more. A period of no differences between observed in the flexion-extension movements in the the groups occurs in the back to ready position phase lumbar region, in which flexion can be observed in (from circa 5% to circa 30% of the cycle time) and in the backswing phase and extension was found in circa 91-93% of the cycle time in the hitting phase. the hitting phase (Figure 4). The range of rotation Polish female players are characterized by using a movement was slight (about 5 deg), more pro- greater range of this movement. The supination nounced in CHIN, whereas in the POL group, it movement is rapid during the hitting phase, from was characterized by high variability (high SD value the moment after the beginning of this phase to the throughout the cycle). The movement of the upper moment of Accmax in both groups. In the body (thoracic region) differentiates the two groups extension-flexion movement in the wrist joint, it is the most in the sagittal plane (flexion-extension). In noticeable that there are no differences in the back CHIN players, this movement is used to a greater to ready position phase and before the Accmax extent (about 30-40 deg), from slow flexion in the moment. There is a slow flexion of the limb in the backswing phase, through faster flexion in the initial described joint in both groups during the back to hitting phase, to the extension in the Accmax region ready position and backswing phases, accelerating and later (Figure 4). The rotation of this part of the during the hitting phase. At circa 90% of the cycle, upper body and lateral flexion in the backswing phase the direction of movement changes to the extension and most of the hitting phase does not show differ- (within circa 10 deg in both groups) at a high rate ences between the two groups. These movements until reaching Accmax. The latter short period shows take place in small ranges of several degrees no differences between the groups (5) Shoulder joint of the playing limb: in the shoulder joint of the playing limb, it can be observed that the The observation that comes to mind is the occurrence of differences mainly concern the back to ready position the longest periods of differentiation between the groups phase in all planes (Figure 5). In the flexion-extension studied in the lower limb joints, which indicates their Applied Bionics and Biomechanics 7 Playing side Opposite side 20 20 10 10 0 0 -10 -10 -20 -20 -30 P < 0.001 P < 0.001 P = 0.007 P < 0.001 P < 0.001 -5 -10 -10 -20 -15 -30 -20 -40 P = 0.003 P < 0.001 P = 0.027 P < 0.001 25.0 22.5 20.0 17.5 15.0 12.5 10.0 P < 0.001 P = 0.035 P < 0.001 P < 0.001 P < 0.001 P = 0.002 0.023 0.015 0.011 0.006 0.002 P < 0.001 P < 0.001 P < 0.001 P < 0.001 P = 0.001 P < 0.001 P = 0.024 P < 0.001 P = 0.001 P < 0.001 P = 0.012 P < 0.001 P < 0.001 -10 -20 P < 0.001 P = 0.012 P < 0.001 P = 0.001 Cycle time (%) 0 20406080 100 0 20406080 100 Forward Ready position Backswing Accmax Forward Forward Ready position Backswing Accmax Forward Figure 3: Lower extremity kinematics. Red line: average values of POL; green line: average values of CHIN; grey areas: SD values. Blue bars indicate the significance during the cycle. different use by both groups of female players. Undoubtedly, next stroke and keep the lower limbs in constant readiness. a wave-like movement in the ankle and knee joints is more Therefore, it can be concluded that CHIN players use these pronounced in CHIN players, which reflects the use of the steps more often than POL and perhaps this is due to differ- so-called small steps, mainly in the back to ready position ences in coaching. Differences can be observed in the ankle and backswing phases. These steps are used to adapt to the joints in all planes, and they affect the entire backswing and Ankle inversion-eversion Ankle ab-adduction Ankle dorsi- Hip internal-external Hip ab-adduction Hip flexion-extension Knee flexion-extension rotation (deg) (deg) plantar flexion (deg) rotation (deg) (deg) (deg) (deg) +Flexion +External +Abduction +Flexion +Abduction +Flexion +Eversion +Abduction +Eversion +Flexion +Flexion +External +Abduction +Flexion 8 Applied Bionics and Biomechanics orax–thoracic mov. Loins-lumbar mov. -10 -20 P = 0.045 P < 0.001 P = 0.029 P = 0.007 P < 0.001 P = 0.027 2.5 0.0 -2.5 -5.0 -5 -7.5 -10 -10.0 P < 0.001 P < 0.001 0 -5 -10 -5 -15 P = 0.050 P < 0.001 P = 0.035 P < 0.001 Cycle time (%) 0 20 40 60 80 100 0 20 40 60 80 100 Forward Ready position Backswing Accmax Forward Forward Ready position Backswing Accmax Forward Figure 4: Torso kinematics. Red line: average values of POL; green line: average values of CHIN; grey areas: SD values. Blue bars indicate the significance during the cycle. forward phases. It is noticeable that the directions of move- probably due to the different playing styles prevalent in the ment in the hitting phase are the same in both groups in two groups. In all players, the forward phase is accompanied the ankle joints, and the differences are in the degree values. by the extension of the knee joints within a range of several The nonplaying side ankle joint in both groups in the for- dozen degrees. The above findings provide helpful informa- ward phase shows no differences and the toe-raise movement tion for coaches and players with regard to the backhand top- (decreasing dorsiflexion, transitioning to plantar flexion), in spin technique and its modifications regarding lower limb an approximately 20-degree range. A similar movement, movements. but differentiating between the two groups, can be observed The movement in the hip joint showed long periods of in the right ankle joint. For both joints, the range of motion differences between the groups studied. However, similar is smaller in CHIN player. The direction of this movement movement directions were found in individual phases in in the forward phase indicates the use of upward and forward both groups. The small rotation range of a few degrees in transfer of the center of gravity as an action to support the the hip joints should be emphasized, which, according to many authors, greatly helps generate the stroke force and hitting movement performed by the player. The importance of this movement while performing a stroke has been high racket speed in table tennis [26–29]. It is directly sug- highlighted in the literature [26, 27]. Wang et al. also pointed gested that the range of this movement and its use differenti- out the differences between players at different sport skill ates between players of different sports skill levels. The lower levels in the performance of movements in the joints of the use of rotation in these joints is related to the type of stroke analyzed in this study. It is a topspin backhand played early lower limbs, emphasizing that these movements can be used better by an economical work with simultaneous use of the against a topspin ball, so it is a counterstroke from the group energy generated by the elastic components of the joints of strokes that utilize the energy of the flying ball and there- and muscles (based on the stretch-shortening principle) fore does not require the involvement of great strength of the [28]. Perhaps the differences in the movement in the ankle player. Similar aspects were pointed out by Marsan et al., who evaluated the mechanical energy generated from the hip joint joints shown in this paper are related to this method. As mentioned above, a wave-like movement in CHIN players during different variations of strokes, finding that backhand was reported in flexion-extension movements in the knee drive required the lowest hip mechanical work [30]. joints, indicating the use of small steps in the preparation In the lumbar spine, the least differences were found in phases (back to ready position, backswing). A greater flexion the flexion-extension motion. In the backswing phase, this is a few degrees of flexion, whereas in the hitting phase- angle in the right knee joint was also observed in the POL group throughout the cycle. This is probably due to the trans- extension in both groups. The lateral flexion movement indi- fer of center of gravity to the right leg, emphasized more in cates that the POL players are slightly leaning to the right, the POL group throughout the cycle. It can be assumed that with the body weight shifted to the right lower limb, again this difference allows the CHIN players to switch to forehand indicating a more backhanded position than in the Chinese players. The CHIN players seem to stand more universally, play faster and more flexibly after performing a pivot and is Internal-external Lateral bend (obliquity) Anterior-posterior bend rotation (deg) (deg) (tilt) (deg) +External +Inward +Anterior Applied Bionics and Biomechanics 9 Playing side Opposite side -Extension +Flexion -10 -20 -30 -40 -50 P = 0.012 P < 0.001 P = 0.040 P < 0.001 +Abduction -10 -20 -30 –Adduction 5 P = 0.049 P = 0.031 P < 0.001 P = 0.032 +Pronation 125 60 P = 0.040 P < 0.001 P = 0.022 P < 0.001 +Flexion P = 0.039 P < 0.001 P = 0.011 P < 0.001 +Flexion P = 0.005 P = 0.017 P = 0.028 P = 0.019 P = 0.001 P = 0.045 +Abduction P < 0.001 P = 0.033 P < 0.001 P = 0.046 +External -25 -20 –internal -50 -40 -75 -60 -100 P = 0.050 P < 0.001 P = 0.015 P = 0.001 P < 0.001 Cycle time (%) 0 20 40 60 80 100 0 20 40 60 80 100 Forward Ready position Backswing Accmax Forward Forward Ready position Backswing Accmax Forward Figure 5: Upper extremity kinematics. Red line: average values of POL; green line: average values of CHIN; grey areas: SD values. Blue bars indicate the significance during the cycle. confirms previous observations concerning the small contri- with the ability to transition more easily from the backhand to the forehand playing, as discussed above. The CHIN bution of hip and trunk rotation resulting from the type of players also use a certain amount of rotation in the lumbar stroke assessed. region during the hitting phase in contrast to POL players, Regarding the playing upper hand, the most differences who hardly use any rotation in this body segment. It must were found in the abduction-adduction of the shoulder, be admitted, however, that the SD values in the POL group flexion-extension at the elbow joint, and supination- are high, indicating great variation in the way this segment pronation at the wrist joint. In these three cases, the differ- is used in the topspin backhand stroke. Nevertheless, the ences between the groups concern much of the back to ready small range of rotation (similar in both groups) in body trunk position phase, the beginning of the backswing, and the end Shoulder internal- Shoulder ab-adduction Shoulder flexion- Elbow flexion-extension Wrist pronation- Wrist radial abduction- Wrist flexion-extension external rotation (deg) (deg) extension (deg) (deg) supination (deg) adduction (deg) (deg) 10 Applied Bionics and Biomechanics of the forward phase. Actually, the end of the forward phase (from Accmax to the end of this phase) differentiates between the groups in each movement in the joints of the playing upper limb. It must be admitted, however, that the 40 directions of movements are very similar (the curves of the graphs have a very similar shape), and the differences dem- P = 0.021 P = 0.038 P < 0.001 P < 0.001 P < 0.001 P < 0.001 onstrated in the SPM test may be due to the different times Cycle time (%) beginning the individual phases in the groups. The SPM test 0 20 40 60 80 100 showed no differences in flexion-extension and external- internal rotation in the shoulder joint, in radial abduction- Forward Ready position Backswing Accmax Forward adduction, and flexion-extension at the wrist joint during the second part of the backswing and the beginning of the Figure 6: Hand acceleration. Red line: average values of POL; green hitting phase. Movement coordination in the female players line: average values of CHIN; grey areas: SD values. Blue bars indicate the significance during the cycle. studied is consistent with that reported in the literature [25, 29]. Furthermore, the description of basic movement, pre- sented in our work, can provide more clarity in understand- tion for table tennis coaches and players. The SPM method ing the topspin backhand technique. allowed for the determination of differences between the Chi- The values of hand acceleration and its changes over time nese and Polish female athletes. The observed differences demonstrated in the SPM test differentiate between the include, among others, greater use of the so-called small steps groups studied for most of the cycle and in all phases, with in order to adapt and be ready during the back to ready posi- short exceptions of ca. 20% and 40%, and in the hitting phase, tion and backswing phases, which gives the CHIN players especially after reaching Accmax (Figure 6). slightly better conditions for preparation for the next plays. For most of the back to ready position phase and the The position of the CHIN players compared to that of the backswing phase, the acceleration values are close to 0. After POL players favours a quicker transition from the backhand circa half of the backswing phase, acceleration values increase to the forehand play. This difference is probably related to the until they reach maximum values at the end of the forward difference in the dominant playing styles of the groups stud- phase, which are very similar in both groups (about ied. The differences found are probably mainly due to differ- 90 m/s ). The pattern of acceleration values is then interest- ences in the training methodologies caused by different ing. It is different for both groups in each phase, but it is sim- coaching systems. It can be also concluded that despite the ilar at the Accmax point, and the maximum values obtained indicated differences in movement patterns in both groups, by both groups are also similar. Therefore, it can be con- the same value of Accmax was achieved. This may be a man- cluded that despite the indicated differences in movement ifestation of the phenomenon of variability of movement, as patterns in both groups, the same value of Accmax was well as equifinality and compensation. achieved. This may be a manifestation of the phenomenon of equifinality and compensation, indicated in the literature Data Availability as typical of dynamic systems and variability of movement The supplementary data (containing angle waveforms and [5, 10, 31]. Obviously, it should be added that just achieving accelerations) used to support the findings of this study are the right amount of hand acceleration does not determine the included within the supplementary materials. accuracy of the play; the hitting angle, the direction of move- ment, and other factors are also important [32]. Conflicts of Interest 3.1. Limitations of the Study. 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Published: Jul 15, 2021

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