Hindawi Applied Bionics and Biomechanics Volume 2021, Article ID 5551199, 9 pages https://doi.org/10.1155/2021/5551199 Research Article Influences of Different Drop Height Training on Lower Extremity Kinematics and Stiffness during Repetitive Drop Jump 1 1 2 2 2 2 I-Lin Wang , Yi-Ming Chen , Ke-Ke Zhang , Yu-Ge Li , Yu Su , Chou Wu , 3,4 and Chun-Sheng Ho College of Physical Education, Hubei Normal University, Huangshi 435002, China Graduate Institute, Jilin Sport University, Changchun, 130022 Jilin, China Division of Physical Medicine and Rehabilitation, Lo-Hsu Medical Foundation, Inc., Lotung Poh-Ai Hospital, Yilan 26546, Taiwan Department of Physical Therapy, College of Medical and Health Science, Asia University, Taichung 41354, Taiwan Correspondence should be addressed to Yi-Ming Chen; firstname.lastname@example.org and Chun-Sheng Ho; email@example.com Received 22 January 2021; Revised 7 February 2021; Accepted 22 February 2021; Published 4 March 2021 Academic Editor: Donato Romano Copyright © 2021 I-Lin Wang et al. 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. Drop jump (DJ) is often used as a plyometric exercise to improve jumping performance. Training from improper drop heights and for improper durations lead to unfavorable biomechanical changes in the lower extremities when landing, which result in reduced training eﬀects and even lower extremity injuries. Purpose. To study the eﬀects of repeated DJ training at drop heights of 30 cm, 40 cm, and 50 cm (drop jump height (DJH) 30, DJH40, and DJH50) on lower extremity kinematics and kinetics. The 1st, 50th, 100th, 150th, and 200th DJs (DJ1, DJs50, DJs100, DJs150, and DJs200) were recorded by using a BTS motion capture system and force platform. The MATLAB software was used to compare the kinematic and stiﬀness data of DJ1, DJs50, DJs100, DJs150, and DJs200 with one-way ANOVA repeated measure. If there were signiﬁcant diﬀerences, the LSD method was used for post hoc comparisons. Methods. Twenty healthy male Division III athlete volunteers were selected as subjects, and 200 drop jumps (DJs200) were performed from DJH30, DJH40, and DJH50. Results. The jumping height (JH), contact time (CT), and GRF increased with drop height, and the stiﬀness of the legs and ankle at DJH30 was higher than that at DJH40 and DJH50 (p <0:05). Conclusion. Within DJs200, training at DJH50 yield the high impact easily leads to lower extremity injury; training at DJH30 can increase the stiﬀnesses of the legs and ankle joints, thus eﬀectively utilizing the SSC beneﬁts to store and release elastic energy, reducing the risk of lower extremity musculoskeletal injury. Therefore, coaches can choose diﬀerent drop heights and training quantities for each person to better prevent lower extremity injury. extremity injury . Therefore, too high of a drop height 1. Introduction leads to excessive GRFs and lower extremity injury. The pre- Plyometric exercise is a type of exercise in which the muscle activation phase characteristics in DJ training systematically is stretched directly before it is explosively contracted during change with the drop height; therefore, lower extremity the stretch-shortening cycle (SSC) to generate high levels of strength can be increased during DJ training . Choosing force and power, and the DJ is often performed during plyo- the appropriate drop height during the DJ task can not only metric training . DJ training with maximum vertical help improve performance by inducing the best training- jumps performed immediately from a platform can be used related physiological adaptations but also help avoid injury to improve jumping performance . Eccentric contraction to the muscle tendon and bone caused by overload on the during the downward phase immediately followed by a rapid lower extremities during jumps from high heights [5, 6]. Compared to the optimal drop height, a lower drop height concentric contraction during the takeoﬀ is needed to com- plete DJ action with the participant of the SSC. However, results in interlimb asymmetry in strength , and a higher an excessive landing force with decreased physiological drop height does not enable small adjustments in muscle absorption causes large knee valgus, which leads to lower activity, which can even decrease rather than increase 2 Applied Bionics and Biomechanics studies have shown that ankle stiﬀness decreases gradually power output [4, 8]. Therefore, the optimal drop height may need to be selected according to the jumping ability with increasing drop height, resulting in smaller SSC beneﬁts of each participant . . A drop height that is too high causes the DJ to over- stretch the muscles during landing, decreasing lower extrem- In competitions, athletes jump and land multiple times to complete multiple high-intensity short-duration sprints, ity stiﬀness, which easily induces the neuroprotective cuts, and spins , and all of these movements require a inhibition process and reduces Hoﬀman reﬂex activity [24, high level of reactive strength and force-generating capabili- 25]. An appropriate level of joint stiﬀness can eﬀectively trig- ties of the lower extremities . High athletic performance ger the SSC mechanism to enhance the training eﬀect, while repeated DJs induce muscle fatigue and changes in stiﬀness requires an adequate level of lower extremity strength , and DJ training yields adequate training eﬀects . Athletes and the landing strategy, which limits jumping performance usually perform plyometric jumps to improve their explosive . Therefore, the stiﬀness of lower extremity joints is jump performance, and the DJ is the most common plyomet- aﬀected by both drop height and training volume. The opti- ric exercise . DJ training can improve muscle power by mal drop height and training volume can eﬀectively trigger the SSC mechanism to yield an appropriate stiﬀness, reduce SSC mechanism, thereby enhancing athletic and vertical jumping performance [1, 14, 15]. Therefore, most athletes the risk of lower extremity injury, and enhance jumping can use strength training with repeated jumps to improve performance during repeated DJ training. the strength and biomechanical characteristics of the lower In summary, drop height and training volume aﬀect DJ extremities. In addition, DJ training improves the ability of landing performance, and changes in these factors yield tendons and muscles to store and release elastic energy diﬀerent training eﬀects. An improper height and training within the landing phase of jumps , thus increasing the volume lead to lower extremity musculoskeletal injuries. In lower extremity strength and allowing individuals to jump this study, it was hypothesized that JH, CT, RSI, and GRF from higher heights. Jumping height can predict muscle increase as the drop height and training volume increase strength in the lower extremities, so various vertical jumps and that stiﬀness decreases as the drop height increases. With are often included in a standard test of athletic performance the optimal drop height and training volume, the SSC mech- . However, repeated DJ training from diﬀerent platforms anism may more eﬀectively increase muscle spindle sensitiv- is likely to lead to neuromuscular fatigue and changes in ity, enhance endurance, improve athletic performance, and lower extremity dynamics, which can lead to injury. Landing prevent lower extremity injury during DJ training. The main strategies, including strategies of bouncing and absorbing objectives of this study were to explore the training eﬀects of energy, aﬀect athletic performance , and improvements highly repetitive DJs from DJH30, DJH40, and DJH50 on the in the stretch reﬂex may lead to higher takeoﬀ speeds . kinematics and stiﬀness of the lower extremities. The characteristics of preactivation can change with the drop height, and DJs from platforms higher than the optimal 2. Methods height do not allow the individual to eﬀectively adjust muscle preactivation to adapt to the landing impact ; moreover, 2.1. Participants. The subjects involved in the study were 20 SSC fatigue after exercise leads to decreased stretch-reﬂex healthy male Division III athlete volunteers (age = 21:5±0:9 sensitivity and muscle injury during DJ training . When years old, height = 174:6±4:7 cm, weight = 67:9±7:9 kg) repeated DJs performed with more extended lower extremi- from Jilin Sport University. None of the subjects had a history ties, the individuals induced muscle fatigue will increase of muscle or bone issues in the lower extremity or neurological ankle plantar ﬂexion to absorb the impact forces as the disease within 2 years. The procedure and possible risks were compensation for increased knee extension and the change explained to subjects, and they signed written consent forms in landing strategy under this circumstance results in poor before the study began. The study was approved by the jumping performance . A study showed that muscle per- regional ethics committee, and all subjects signed informed formance changed after DJs100 was repeated, and knee joint consent forms (JLSU-IRB2020004). extensor fatigue caused the jump height to decrease by 26 ±14% . Therefore, repeated DJ training is likely to 2.2. Protocol. Before the study, the subjects performed a stan- induce lower extremity fatigue and muscle injury, and jumps dard dynamic warm-up for the major muscle groups of the from the optimal drop height and the optimal training lower limbs (running on a treadmill at a speed of 8 km/h volume for DJs can prevent the risk of poor performance for 10 minutes). During the study, the subjects wore standard and injury caused by excessive fatigue and help individuals shoes provided by the laboratory to control for diﬀerences in complete reinforcement training. the absorption characteristics of the soles of shoes. Three DJ The risk of lower extremity injury may be related to experiments (DJH30, DJH40, DJH50) were conducted in a changes in joint stiﬀness; excessive joint stiﬀness increases random order over 3 separate days, with 4 days of rest the risk of bone injury, while insuﬃcient joint stiﬀness may between each dropping height experiment. Before data col- lead to joint instability and soft tissue injury . DJ training lection, the subjects were required to practice the jump ﬁve from diﬀerent drop heights is widely used to assess the risk of times to ensure that their hands were on their waist and their lower extremity injury , so drop height is a key factor feet were on the two force plates during the experiment. The aﬀecting joint stiﬀness during landing. The optimal drop subjects were encouraged to jump with maximum eﬀort height can regulate the lower extremity stiﬀness, leading to within the shortest ground contact time . The data for the best SSC to enhance jumping performance . Previous DJ1, DJs50, DJs100, DJs150, and DJs200 were recorded, Applied Bionics and Biomechanics 3 and a 10-second break was provided between jumps. The K was calculated by the following formula: joint framework for the proposed methodology is shown in Figure 1. ΔM ht joint K BW ∗ = , ð2Þ joint rad Δθ 2.3. Data Collection. Twenty-one reﬂective markers (19 mm joint in diameter) were attached to anatomical landmarks on the legs and pelvis to deﬁne a seven-segment rigid link model where the change in joint moment between the instant of of the lower extremities, according to the Helen Hayes peak joint ﬂexion and ground contact is deﬁned as ΔM joint marker set . The three-dimensional (3D) trajectories of and the angular displacement between the maximum joint the reﬂective markers on the participants were collected with ﬂexion and the contact position is Δθ . joint 10 cameras (BTS DX400, BTS Bioengineering, Milano, Italy) at a sampling frequency of 200 Hz. Two force plates (40 ∗ 60 2.5. Statistical Analysis. The MATLAB software (R2016a; cm) were used to collect GRF data during each trial at a sam- MathWorks, Inc., Natick, MA) was used for various statisti- pling frequency of 400 Hz (BTS P6000, BTS Bioengineering, cal analysis. The variables were analyzed using one-way Milano, Italy). The infrared camera data were synchronized ANOVA repeated measure for DJH30, DJH40 and DJH50 with the force plate data via the Qualisys 64 channel A/D at DJ1, DJs50, DJs100, DJs150, and DJs200. When signiﬁcant plate. results were found, post hoc analysis was performed with LSD (p <0:05) pairwise comparisons to compare the mea- 2.4. Data Analysis. A kinematic model was generated by sured values between diﬀerent drop heights. The eﬀect size deﬁning the skeletal segments (foot, talus, shank, thigh of (ES) is used to determine whether a diﬀerence is a practical both extremities, and pelvis) in the standing trial. The central correlation diﬀerence. The modiﬁed Cohen scale was used position of the hip joint was calculated by the method pro- to determine the size of variation diﬀerences in three drop posed by Bell et al. . The center of the ankle joint was height, <0.2 means trivial diﬀerence, 0.2-0.6 means small dif- deﬁned as the midpoint between the medial and lateral ference, 0.6-1.2 means moderate diﬀerence, and 1.2-2.0 malleolus. The midpoint between the medial and lateral epi- means large diﬀerence . condyles was deﬁned as the knee joint center. The anatomic coordinate systems of the thigh and shank were determined 3. Results by the static calibration test. The vertical axis was deﬁned as the line from the distal to proximal centers of the joint, Figure 2 presents the mean deviations of each dependent while the anteroposterior axis was perpendicular to the verti- kinematic variable of the lower extremities. Jumping height cal axis; the third axis was deﬁned as the cross product of the and contact time increased signiﬁcantly overall (all p < anteroposterior and vertical axes and used to obtain the 0:005) across the three increasing drop heights, and post dynamic coordinate systems of the pelvis, thigh, and shank. hoc analysis revealed signiﬁcant diﬀerences between We performed all calculations using a custom MATLAB DJH30, DJH40, and DJH50. Our results show that jumping program (Mathworks, Natick, RI, USA). height was 1.05-, 1.08-, 1.04-, 1.07-, and 1.08-fold higher The landing phase was deﬁned as the time interval from (all p <0:048; ES varying from 0.48 to 0.73) during DJH40 when the foot contacted the ground to the lowest center-of- than DJH30; 1.13-, 1.15-, 1.07-, 1.13-, and 1.16-fold higher mass position. A fourth-order low-pass Butterworth digital (all p <0:006; ES varying from 0.71 to 1.35) during DJH50 ﬁlter with a cutoﬀ frequency of 50 Hz was used to smooth than DJH30; and 1.07-, 1.07-, 1.04-, 1.06-, and 1.07-fold the GRF data. Jumping height (JH) was calculated by the fol- higher (all p <0:046; ES varying from 0.48 to 0.67) during lowing formula: JH = gT /8. Contact time (CT) was deﬁned DJH50 than DJH40 at DJ1, DJs50, DJs100, DJs150, and as the time from initial ground contact to toe-oﬀ during the DJs200, respectively (Figure 2(a)). The post hoc comparisons foot ground contact phase. The reaction strength index showed that contact time was 1.08-, 1.08-, 1.09-, 1.08-, and (RSI) was calculated as follows: RSI = JH/CT. The peak 1.07-fold higher (all p <0:049; ES varying from 0.47 to vertical ground reaction force (PVGRF) was deﬁned as the 0.68) during DJH40 than DJH30; was 1.19-, 1.23-, 1.24-, maximum PVGRF at the initial point of contact with the 1.23-, and 1.18-fold higher (all p <0:002; ES varying from ground to the maximum angle of knee ﬂexion. The PGRF 0.85 to 1.57) during DJH50 than DJH30; and 1.10-, 1.13-, was normalized by the subjects’ body weight (BW). 1.13-, 1.14-, and 1.11-fold higher (all p <0:041; ES varying K was calculated using the following formula: leg from 0.49 to 0.82) during DJH50 than DJH40 at DJ1, DJs50, DJs100, DJs150, and DJs200, respectively (Figure 2(b)). The reaction strength index value during vertical GRF ht hip−lowest DJH50 was signiﬁcantly higher than DJH30 and DJH40 K BW ∗ = , ð1Þ leg rad ΔL leg (p <0:050), and the post hoc comparisons showed that dur- ing DJH50 was 1.14-fold (p =0:048; ES = 0:47) higher than DJH30 and 1.14-fold (p =0:021; ES = 0:56) higher than where the vertical GRF at the lowest position of the hip joint DJH40 at DJ1. The reaction strength index value during is the vertical GRF and ΔL represents the vertical DJH40 was signiﬁcantly higher than DJH30 and DJH50 (all hip−lowest leg displacement of the hip from the contact position to the p <0:050), and the post hoc comparisons showed that the lowest position . values were 1.08-, 1.06-, 1.12-, and 1.17-fold higher (all 4 Applied Bionics and Biomechanics Running on a treadmill at a speed Warm up of 8km/h for 10 minutes Markers attach Jumping with the maximum effort Five DJs for practice in the shortest ground contact time DJs200 from DJH30 cm R andomization DJ1, DJs50, DJs100, DJs150 and DJs200 from DJH40 cm DJs200 for data analysis DJs200 from DJH50 cm Figure 1: Framework for the proposed methodology. p <0:028; ES varying from 0.54 to 0.74) during DJH40 than decreased signiﬁcantly overall (all p <0:050) across the three DJH30 and were 10.26%, 13.94%, 14.87%, and 13.14% lower increasing drop heights, with the post hoc results showing during DJH50 than DJH40 (all p <0:029; ES varying from diﬀerences between DJH30, DJH40, and DJH50. The post 0.53 to 0.70) at DJs50, DJs100, DJs150, and DJs200, respec- hoc comparisons showed that the leg stiﬀness was lower dur- tively (Figure 2(c)). ing DJH40 than DJH30, with ∇ values of 10.46%, 15.43%, Figure 2 presents the mean deviations of each dependent 17.62%, 12.38%, and 7.53% (all p <0:047; ES varying from ground reaction force variable for the lower extremities. The 0.48 to 0.64) and lower during DJH50 than DJH30, with ∇ peak ground reaction force and leg ground reaction force sig- values of 20.97%, 30.05%, 32.03%, 25.06%, and 23.82% (all niﬁcantly increased overall (all p <0:050) across the three p <0:030; ES varying from 0.53 to 0.87) at DJ1, DJs50, increasing drop heights, with the post hoc results showing DJs100, DJs150, and DJs200, respectively; the values were diﬀerences between DJH30, DJH40, and DJH50. The post lower during DJH50 than DJH40, with ∇ values of 17.28%, hoc comparisons showed that peak ground reaction force 18.31%, 14.47%, and 17.62% (all p <0:043; ES varying from was 1.05-, 1.05-, and 1.07-fold higher (all p <0:018; ES vary- 0.49 to 0.58) at DJs50, DJs100, DJs150, and DJs200, respec- ing from 0.59 to 0.84) during DJH40 than DJH30 at DJs100, tively (Figure 3(a)). The post hoc comparisons showed that DJs150, and DJs200, respectively; 1.20-, 1.18-, 1.20-, 1.17-, ankle stiﬀness during DJH40 was lower than DJH30, with ∇ and 1.21-fold higher (all p <0:001; ES varying from 1.03 to values of 19.35%, 32.00%, 25.00%, 19.30%, and 19.09% (all 2.23) during DJH50 than DJH30; and 1.19-, 1.15-, 1.14-, p <0:040; ES varying from 0.18 to 0.91) and lower during 1.11-, and 1.13-fold higher (all p <0:001; ES varying from DJH50 than DJH30, with ∇ values of 25.81%, 30.40%, 0.91 to 1.82) during DJH50 than DJH40 at DJ1, DJs50, 37.88%, 23.68%, and 30.00% (all p <0:019; ES varying from DJs100, DJs150, and DJs200, respectively (Figure 2(e)). The 0.22 to 0.78) at DJ1, DJs50, DJs100, DJs150, and DJs200, post hoc comparisons showed that the right leg ground reac- respectively (Figure 3(d)). There were no signiﬁcant diﬀer- tion force was 1.11-, 1.05-, and 1.06-fold higher (all p <0:022; ences in knee or hip stiﬀness between DJH30, DJH40, and ES varying from 0.56 to 0.86) during DJH40 than DJH30 at DJH50 (Figures 3(b) and 3(c)). DJs100, DJs150, and DJs200, respectively; 1.17-, 1.21-, 1.23-, 1.16-, and 1.18-fold higher (all p <0:002; ES varying from 0.87 to 1.25) during DJH50 than DJH30; and 1.24-, 1.15 4. Discussion 1.11-, 1.11-, and 1.11-fold higher (all p <0:008; ES varying from 0.67 to 1.77) during DJH50 than DJH40 at DJ1, The purposes of this study were to investigate the eﬀects of DJs50, DJs100, DJs150, and DJs200, respectively highly repetitive DJs from DJH30, DJH40, and DJH50 on (Figure 2(d)). The post hoc comparisons showed that the left lower extremity kinematics and stiﬀness and to determine leg ground reaction force was 1.15-, 1.13-, 1.14-, 1.14-, and the appropriate drop height and training volume of drop 1.15-fold higher (all p <0:050; ES varying from 0.56 to jump training. The results show that jumping height and 1.04) during DJH50 than DJH30 at DJ1, DJs50, DJs100, contact time reached the maximum values within DJs200 at DJs150, and DJs200, respectively, and 1.08-, 1.10-, and DJH50, and training at this height and volume can improve 1.11-fold (all p <0:005; ES varying from 0.72 to 0.81) higher jumping performance; however, the large ground reaction than during DJH40 at DJs100, DJs150, and DJs200, respec- force generates a high impact force, which can easily lead to tively (Figure 2(f)). lower extremity injury. Leg and ankle stiﬀness are maximal Figure 3 presents the mean deviations of each dependent at DJH30, which can reduce the risk of lower extremity mus- lower extremity stiﬀness variable. The leg and ankle stiﬀness culoskeletal injury and eﬀectively utilize stretch-shortening Applied Bionics and Biomechanics 5 DJs100 DJs100 42 †‡ †‡ † DJs150 DJs150 0.4 † †‡ †‡ DJs50 †‡ DJs50 †‡ † 0.2 † 0 0.0 DJs200 DJs200 –14 †‡ –0.2 †‡ †‡ †‡ –28 DJ1 DJ1 –0.4 –42 (a) (b) DJs100 DJs100 †‡ 1.8 DJs150 DJs150 1.2 24 DJs50 † DJs50 †‡ †‡ 0.6 0.0 –12 DJs200 –0.6 DJs200 †‡ †‡ †‡ –24 –1.2 DJ1 DJ1 –36 –1.8 –48 (c) (d) DJs100 DJs100 †‡ †‡ DJs150 DJs150 1.2 †‡ †‡ † DJs50 †‡ DJs50 0.6 0 0.0 –1 DJs200 DJs200 –0.6 †‡ † †‡ †‡ –2 DJ1 DJ1 –1.2 –3 DJH30 DJH40 DJH50 (e) (f) Figure 2: The jumping height, contact time, reaction strength index, the peak vertical GRF, and right and left leg GRF during drop jumps from three heights at DJ1, DJs50, DJs100, DJs150, and DJs200. Asterisk † indicates that a signiﬁcant diﬀerence with DJH30; ‡ indicates that a signiﬁcant diﬀerence with DJH40; § indicates that a signiﬁcant diﬀerence with DJH50. p values <0.05 were considered to signiﬁcantly diﬀer. cycle beneﬁts to store and release elastic energy to improve can follow the natural trend of the substrate recoil, thus wast- jumping performance. ing a minimum amount of energy . In this study, jump- In this study, within DJs200, jumping height and contact ing height increased gradually with drop height, which may time gradually increased with increasing drop height. Previ- have been caused by the jumping ability increasing with ous studies have shown that jumping height and contact time increasing contact time. Therefore, the DJH50 height had a increase with increasing drop height [6, 30] and that the longer contact time than did diﬀerent jumps and exhibited training intensity can be enhanced and the values of jump a jumping height increase, which may have produced a better parameters such as jumping height and contact time can be training eﬀect. In this study, reaction strength index was inﬂuenced by the drop height . However, some studies higher during DJH50 than during DJH30 and DJH40 at have shown that jumping height decreases with drop height DJ1, while when landing, the reaction strength index was . Increased contact time results in increased knee ﬂexion higher during DJH40, than during DJH30 and DJH50 at to absorb the increased landing force and thus greater jump- DJs50, DJs100, DJs150, and DJs200. The diﬀerences in reac- ing ability [7, 31]. And the long contact time with the ground tion strength index according to the drop height and jump Jumping height (cm) PGRF (BW) RSI (cm/s) LGRF (BW) RGRF (BW) Contact time (s) 6 Applied Bionics and Biomechanics DJs100 DJs100 0.08 DJs150 ‡ 0.06 DJs150 3.2 DJs50 0.04 DJs50 1.6 0.02 0.0 0.0 –0.02 DJs200 DJs200 –1.6 –0.04 DJ1 DJ1 –3.2 –0.06 –0.08 (a) (b) DJs100 DJs100 0.15 0.2 DJs150 DJs150 0.10 DJs50 DJs50 0.1 0.05 0.0 0.00 DJs200 DJs200 –0.05 –0.1 DJ1 DJ1 –0.10 –0.2 –0.15 DJH30 DJH40 DJH50 (c) (d) Figure 3: The right leg, hip, knee, and ankle stiﬀness during drop jumps from three heights at DJ1, DJs50, DJs100, DJs150, and DJs200. Asterisk † indicates that a signiﬁcant diﬀerence with DJH30; ‡ indicates that a signiﬁcant diﬀerence with DJH40; § indicates that a signiﬁcant diﬀerence with DJH50. p values <0.05 were considered to signiﬁcantly diﬀer. time are due to the changes in jumping height and contact ﬁndings, our results show that the resistance training inten- time. The higher the jumping height and the shorter the con- sity can be controlled by the drop height, resulting in the ground reaction force gradually increasing with increasing tact time are, the higher the reaction strength index [33, 34]. Too high of a drop height altitude will produce a large land- drop height . High drop heights cause individuals to land ing impact and is not conducive to muscle ﬁne-tuning which during DJs with high impact intensity , which can lead to can even decrease rather than increase power output; after ankle sprains, anterior cruciate ligament tears, and patellofe- repeated DJs, muscle fatigue will lead to a smaller stretch- moral pain syndrome . Therefore, landing with high shortening cycle beneﬁt and reaction strength index diﬀer- impact during DJH50 can cause lower extremity injury and ence at diﬀerent drop heights [4, 8, 19]. Thus, the gradual is not suitable for repeated contact time training. There were increase in drop height may lead to a change in the reaction signiﬁcant diﬀerences in ground reaction force between strength index diﬀerence when the DJH40 and DJH50 jump DJH30 and DJH40 at DJs100, DJs150, and DJs200. These dif- times are diﬀerent. This study showed that after DJ1, the ferences may have been caused by the diﬀerences in the initial DJH40 height produces a larger reaction strength index, peak vertical ground reaction force at ground contact due to which can have a larger training eﬀect, while after repeat fatigue and individual changes in joint stiﬀness [17, 37]. drop jump training at the DJH50 height, muscle fatigue Therefore, the impact force can be changed by controlling may decrease the stretch-shortening cycle beneﬁt, resulting the training intensity at diﬀerent drop heights. In this study, in a decrease in the reaction strength index. Therefore, drop smaller ground reaction forces reduced the risk of lower heights of DJH40 and DJH50 can produce greater reaction extremity musculoskeletal injury when landing at DJ1, strength index values, and the reaction strength index value DJs50, DJs100, DJs150, and DJs200 from the height of may be more suitable for drop jump training; however, train- DJH30. In this study, during landing from DJH40 and DJH50 ing from DJH50 for 200 consecutive times produces a lower reaction strength index, which may easily cause muscle within DJs200, the stiﬀnesses of the legs and ankle joints were fatigue and poor jumping performance. lower than those during landing from DJH30, while there The results of this study show that the ground reaction were no diﬀerences in the stiﬀnesses of the knees and hips force produced at DJH50 within DJs200 is greater than those between other drop heights. Consistent with previous ﬁnd- ings, the stiﬀnesses of the legs and ankle gradually decreased produced at DJH30 and DJH40. Consistent with previous Hip stiffness(BW ht/rad) Leg stiffness(BW ht/rad) ⁎ ⁎ Ankle stiffness(BW ht/rad) Knee stiffness(BW ht/rad) Applied Bionics and Biomechanics 7 and stiﬀness by the drop height will aﬀect the stability of with increasing drop height, while the stiﬀnesses of the knees and hips did not signiﬁcantly diﬀer across drop heights [6, the knee joint. Within DJs200, training at the height of 26, 38]. The ankle stiﬀness decreased gradually with increas- DJH50 can yield better jumping performance; however, ing drop height, resulting in a decrease in the stretch- because the high impact easily leads to lower extremity shortening cycle beneﬁt when drop jump training ; there- injury, training at the drop height of DJH30 can increase fore, the smaller ankle stiﬀness may have aﬀected the training the stiﬀnesses of the legs and ankle joints, thus eﬀectively uti- eﬀect. Increasing stiﬀness enables better storage and release lizing the SSC beneﬁts to store and release elastic energy, of stretch-shortening cycle-based elastic energy . In addi- improving jumping performance and reducing the risk of tion, jumping training can increase the joint stiﬀness of the lower extremity musculoskeletal injury. lower extremities during landing, reduce the risk of injury, and improve athletic performance by strengthening the Abbreviations lower extremity muscles . The higher leg and ankle stiﬀ- DJ: Drop jump nesses generated by highly repetitive drop jump training at DJH: Drop jump height DJH30 in this study can lead to greater stretch-shortening JH: Jumping height cycle beneﬁts regarding the storage and release of elastic CT: Contact time energy and reduce the incidence of lower extremity injuries, SSC: Stretch-shortening cycle which may be suitable for repetitive drop jump training. Past RSI: Reaction strength index studies have shown that the longer the contact time at drop BWs: Body weights jump landing is, the lower the stiﬀness of the lower extremi- ES: Eﬀective size ties . In this study, contact time increased gradually with GRF: Ground reaction force increasing drop height, so the stiﬀnesses of the legs and ankle PGRF: Peak ground reaction force may be related to the increase in contact time. Therefore, if RGRF: Right leg ground reaction force athletes can consciously control the contact time during LGRF: Left leg ground reaction force training, they may adjust the lower extremity stiﬀnesses at PVGRF: Peak vertical ground reaction force. landing according to the drop height. The stiﬀnesses of lower extremity joints are aﬀected by joint torques. In this study, the knee and hip stiﬀnesses showed no diﬀerences after Data Availability repeated drop jump training, which may have occurred The data results are included in the manuscript. because the knee and hip torques did not change signiﬁcantly between the drop heights. In summary, drop jump training Conflicts of Interest with the appropriate lower extremity stiﬀness or stiﬀness adjustments during repeated jumping training can reduce All authors declare that they have no conﬂicts of interest with the risk of lower extremity injury and enhance the training regard to the contents of this article. eﬀect. This study showed that landing from the DJH30 height within DJs200 produces larger leg and ankle stiﬀnesses, Authors’ Contributions which can yield greater stretch-shortening cycle beneﬁts, thereby improving jumping performance and reducing the YMC and ILW designed the experiments. YMC, ILW, and risk of lower extremity injury, so these parameters are YSC carried out the supplement preparation and laboratory suitable for repeated drop jump training. experiments. ILW, WCC, and YMC analyzed the data, inter- preted the results, prepared the ﬁgures, and wrote the 4.1. Limitations. Limitations need to be considered when manuscript. WCC, SZ, and YMC contributed the reagents, interpreting the results. Firstly, amounts of drop height are materials, and the analysis platforms. not suﬃcient, so we need lower and higher DJH such as 20 cm and 60 cm. Secondly, the subjects were obviously not Acknowledgments blinded to the DJH, so the central nervous system may apply a protective strategy, and this could introduce performance The authors would like to thank all colleagues and students bias. Thirdly, electromyographic was not used in this study, who contributed to this study. This work was supported by so the activity of the lower limb muscles is unknown during the Research on Hubei province education department landing and jumping. science and technology research program (grant number D20202502). This manuscript was edited by American Jour- 5. Conclusion nal Experts Editing. In summary, with increasing drop height, the kinematics and References stiﬀnesses of the lower extremities varied during landing. DJ training from a high drop height produces a high impact  M. Walsh, A. Arampatzis, F. Schade, and G. P. Brüggemann, intensity, resulting in a greater impact. 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Applied Bionics and Biomechanics – Hindawi Publishing Corporation
Published: Mar 4, 2021