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Variable Heights Influence Lower Extremity Biomechanics and Reactive Strength Index during Drop Jump: An Experimental Study of Male High Jumpers

Variable Heights Influence Lower Extremity Biomechanics and Reactive Strength Index during Drop... Hindawi Journal of Healthcare Engineering Volume 2021, Article ID 5185758, 7 pages https://doi.org/10.1155/2021/5185758 Research Article Variable Heights Influence Lower Extremity Biomechanics and Reactive Strength Index during Drop Jump: An Experimental Study of Male High Jumpers 1 1 2 1 3 Zehao Tong , Feng Zhai , Hang Xu , Wenjia Chen , and Jiesheng Cui College of Physical Education, China University of Mining and Technology, Xuzhou, Jiangsu, China Department of Medical Imaging, Xuzhou Medical University, Xuzhou, Jiangsu, China National University of Singapore 119077, Singapore Correspondence should be addressed to Feng Zhai; zhaifeng123@126.com Received 29 October 2021; Revised 11 November 2021; Accepted 20 November 2021; Published 1 December 2021 Academic Editor: Kalidoss Rajakani Copyright © 2021 Zehao Tong et al. -is 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. Introduction. -is study finds the lower limbs’ reactive strength index and biomechanical parameters on variable heights. Objective. -is research aims to reveal the effects of drop height on lower limbs’ reactive strength index and biomechanical parameters. Methods. Two AMTI force platforms and Vicon motion capture system were used to collect kinematic and dynamic signals of the lower limbs. Results. -e drop height had significant effects on peak vertical ground reaction force and peak vertical ground reaction force in the extension phase, lower limbs’ support moment, eccentric power of the hip joint, eccentric power of the knee joint, eccentric power of the ankle joint, and concentric power of the hip joint. -e drop height had no significant effects on the reactive strength index. Reactive strength index (RSI) had no significant correlations with the personal best of high jumpers. -e optimal loading height for the maximum reactive strength index was 0.45 m. Conclusion. -e optimal loading height for the reactive strength index can be used for explosive power training and lower extremity injury prevention. Current research studies reported that DJ height for the 1. Introduction maximum RSI could enhance sport performance. Ramirez- Campillo et al. reported that optimal loading height for the Drop jump (DJ) is always a core part of plyometric RSI could significantly increase the 20 m sprint of soccer training for high jumpers. It has also been used to find the players by 7 weeks of intervention, but did not point out the effect of variable heights on lower limbs’ biomechanical DJ height for the RSI [4]. Boullosa et al. selected the optimal parameters and reactive strength index (RSI). Kipp et al. loading height for the RSI during warm-up, which improved found the effect of variable heights on the biomechanical the CMJ performance of long-distance runners, but failed to indicator and RSI in male basketball players, but RSI, increase 1000 m performance [5]. Smirniotou et al. found vertical stiffness, and other parameters were unaffected by that the RSI was highly correlated with sprint speed in young drop height [1]. Wu et al. indicated that DJ height marked sprinters, but did not apply the result to intervention [6]. impacted the peak of vertical ground reaction force Hennessy and Kilty revealed that BDJ index has a significant (vGRF) in male paratroopers [2]. Peng found that negative correlation with 30 m and 100 m performance in female power and vGRF at higher loading heights were higher athletes, but did not use for further training [7]. It is quite than low heights in college students [3]. It is known that clear that optimal loading height for the RSI is hard to the same DJ height has different effects on the RSI and improve the personal best (PB) directly for most of sports, biomechanical parameters of various subjects, so it is but it should be stressed that the ideal PB is related to sport necessary to determine an appropriate loading height of performance. DJ for each sport event. 2 Journal of Healthcare Engineering High jump includes fast run-up takeoff and high flight reserved for motion capture. -e location and name of the height. Studies showed that RSI can effectively evaluate the markers are shown in Figure 1. Vicon Nexus (v.2.10.1, Vicon, UK) was used to syn- ground contact time and flight height. Flanagan et al. [8] and Ebben and Petushek reported that RSI has high reliability on chronously collect kinematic and dynamic data. Vicon the evaluation of lower limbs’ explosive power [9]. Markwick Nexus was also used to identify the reflective markers during et al. found that RSI can effectively assess the plyometric static calibration and export them in a C3D format. Visual training of male basketball players [10]. Meanwhile, Louder 3D (v.6.01.36, C-Motion, USA) was used to build bony et al. confirmed the high internal validity of the RSI [11]. models of human lower limbs and process original dynamic Some research studies believed that PB was affected by and kinematic signals to obtain dynamic and kinematic data vertical height of high jump. Mateos-Padorno et al. reported of lower limb joints. -e marker trajectory and analog data that vertical displacement in the sagittal plane has a high were filtered with a fourth-order low-pass Butterworth filter correlation with PB [12]. Viitasalo and Aura found that the at 6 Hz and 15 Hz, respectively, to reduce the noise in the vertical height of the jumpers was increased at the game signals [20]. Reactive strength index (RSI) was worked out as the ratio of flight height and ground contact time. -e season [13]. -erefore, the improvement of RSI may be helpful for PB. In addition, high jumpers were always tor- processed data were calculated using the following formula tured by lower extremity injury. Schmitt et al. believed that [21, 22]: most of elite high jumpers have injured ankle and knee joints 1 Tf in the takeoff leg [14]. Schmitt et al. reported that a certain Hf � g􏼠 􏼡 (m), 2 2 proportion of high jumpers suffered by hip arthrosis [15]. (1) Schmitt et al. found that the jumpers with ankle injury have a Hf low radiological score [16]. It should be emphasized that RSI � . Tc inappropriate training results in injury [17]. To our knowledge, no studies found the effects of var- -e average value for three effective DJs at the same iable heights for high jumpers. Many studies showed that height was calculated. By quadratic polynomial regression RSI could significantly improve the sprint and CMJ per- analysis, the effects of drop height on the biomechanics formance, but did not clarify the correlations between the parameters and RSI of the lower limbs were obtained. One- RSI and the PB of high jumpers. -erefore, the aim of this way repeated measure analysis of variance (ANOVA) was study was to investigate the effects of variable drop heights used for comparison of differences and selected the sig- on vGRF, RSI, joint power, and moment of lower limbs, find nificant variables of them for Bonferroni correction. -e the correlations between RSI and PB, and determine the maximum average value was taken from each group as the optimal loading height for the maximum RSI. Hypotheses optimal loading height for the RSI. By Pearson’s test, the include the following: (1) drop height had significant effects correlations of RSI and PB were investigated. -e above data on the reactive strength index and biomechanical parame- were processed with SPSS (v. 25.0, IBM, USA). ters. (2) -e correlation between the RSI and PB would be significant. 3. Results 2. Methods Table 1 shows the result of quadratic polynomial regression analysis. -e drop height significantly affected vGRF Ten high jumpers were recruited (males: age: 20.7± 2.32 y, (P< 0.001) and vGRFep (P< 0.001), lower limbs’ support training background: 5.1± 1.51 y, weight: 72.33± 5.36 kg, moment (P< 0.05), eccentric power of the hip joint height: 1.89± 0.04 m, and personal best: 2.065± 0.04 m). Vicon motion capture system (Vicon, UK) and AMTI (P< 0.05), eccentric power of the knee joint (P< 0.001), eccentric power of the ankle joint (P< 0.001), and concentric HPS400600 force platforms (AMTI, USA) were used to collect kinematic and dynamic signals of the lower limbs. power of the hip joint (P< 0.001). It has no significant effects on the hip extension moment (P> 0.05), knee extension Plyo Soft Box was used for the DJ test (Escape Fitness, UK). -e warm-up protocol included 10 min of jogging at a moment (P> 0.05), ankle plantarflexion moment (P> 0.05), and RSI (P> 0.05). Table 2 shows the result of one-way moderate self-selected pace [5] and 5 min of static stretching repeated measure analysis. vGRF (P< 0.001), vGRFep for each leg. After hearing the “start” order, raise the (P< 0.001), hip extension moment (P< 0.05), knee exten- dominant leg, naturally lean forward, and drop freely. When sion moment (P< 0.05), eccentric power of the hip the feet touch the ground, the subject must jump vertically as fast as possible. Requirements of effective DJ were as follows: (P< 0.05), knee (P< 0.05), and ankle joint (P< 0.05), and concentric power of the hip joint (P< 0.05) have statistical do not move center of gravity upwards after leaving the box. Both feet should touch the ground at the same time. Jump significance at 0.3 m, 0.45 m, 0.6 m, and 0.75 m. Table 3 reflects mean values (±SD) and the result of upwards with full strength, and keep both hands on hips [18]. Vicon motion capture system was used to capture Bonferroni correction during DJs. Relative to the drop height of 0.3 m, vGRF (P< 0.008) increased significantly at kinematic signals of the lower limbs at a frequency of 100 Hz. 0.45 m, 0.6 m, and 0.75 m. vGRF at the DJ height of 0.75 m According to the pasting scheme of Plug-In Gait Lower- was significantly larger than at 0.45 m and 0.6 m (P< 0.008). Limb Ai 2.3 Marker [19], 28 reflective markers were pasted Compared with the drop height of 0.3 m, vGRFep on the bony landmarks of the lower limbs. 24 markers were Journal of Healthcare Engineering 3 RPSI LPSI RASI LASI LTHAP LTHI LTAD LKNM LKNE LTIAP LTIB LTIAD LMED LHEE LANK LTOE Calibration only Optional (a) (b) Figure 1: -e location of reflective markers. (a) Location on the lower limbs. (b) Specific location of each marker. Table 1: Quadratic polynomial regression analysis for variables during DJs. 2 2 S. no. Variables R R Adjusted R F β t P Constant 1 vGRF 0.686 0.471 0.457 33.852 8.149 7.233 ≤0.001 27.742 2 vGRFep 0.54 0.292 0.273 15.667 −7.275 −3.958 ≤0.001 63.006 3 Hip extension moment 0.114 0.013 −0.013 0.5 0.09 0.707 0.484 1.43 4 Knee extension moment 0.281 0.079 0.055 3.26 0.158 1.806 0.079 3.106 5 Ankle plantarflexion moment 0.171 0.029 0.004 1.141 0.111 1.068 0.292 3.035 6 Lower limbs’ support moment 0.35 0.123 0.1 5.314 0.359 2.305 0.027 7.571 7 Eccentric power of the hip joint 0.49 0.24 0.22 12 −9.883 −3.464 0.001 −4.342 8 Eccentric power of the knee joint 0.676 0.457 0.443 31.985 −6.142 −5.656 ≤0.001 −8.729 9 Eccentric power of the ankle joint 0.531 0.282 0.263 14.946 −3.563 −3.866 ≤0.001 −4.465 10 Concentric power of the hip joint 0.589 0.347 0.33 20.171 3.556 4.491 ≤0.001 4.519 11 Concentric power of the knee joint 0.119 0.014 −0.012 0.547 0.457 0.739 0.464 19.267 12 Concentric power of the ankle joint 0.046 0.002 −0.024 0.081 0.202 0.284 0.778 22.514 13 RSI 0.064 0.004 −0.022 0.155 0.002 0.393 0.696 0.156 vGRF: the peak of vertical ground reaction force; vGRFep: the peak of vertical ground reaction force in the extension phase; RSI: reactive strength index. P< 0.05 indicates significant effects, and bold is used to indicate it, which means drop height significantly affects the indicator. Table 2: One-way repeated measure analysis of variance for variables during DJs. S. no. Variables Mauchly’s sphericity test F P Partial eta [2] 1 vGRF 0.004 47.709 ≤0.001 0.841 2 vGRFep 0.403 27.577 ≤0.001 0.754 3 Hip extension moment 0.043 8.059 0.019 0.472 4 Knee extension moment 0.005 3.752 0.023 0.294 5 Ankle plantarflexion moment 0.028 0.84 0.484 0.085 6 Lower limbs’ support moment 0.036 2.533 0.119 0.22 7 Eccentric power of the hip joint 0.006 32.686 ≤0.001 0.784 8 Eccentric power of the knee joint 0.01 42.591 ≤0.001 0.826 9 Eccentric power of the ankle joint 0.482 22.819 0.001 0.907 10 Concentric power of the hip joint 0.174 12.905 0.001 0.589 11 Concentric power of the knee joint 0.057 1.179 0.336 0.116 12 Concentric power of the ankle joint 0.091 0.48 0.699 0.051 13 RSI 0.196 0.585 0.63 0.061 vGRF: the peak of vertical ground reaction force; vGRFep: the peak of vertical ground reaction force in the extension phase; RSI: reactive strength index. P< 0.05 indicates a significant difference, and bold is used to indicate it. -e value of this index at the drop height of 0.3m, 0.45m, 0.6m, 0.75m is significantly different from each other. 4 Journal of Healthcare Engineering Table 3: Mean values (±SD) with the Bonferroni correction of variables for DJs. Drop height S. no. Variables 0.3 m 0.45 m 0.6 m 0.75 m 1 vGRF (N/kg) 35.32± 8.97 44.84± 9.53 52.31± 10.1∗# 59.99± 11.82∗ +^ 2 vGRFep (N/kg) 57.01± 13.15 ^# 47.7± 14.94 38.87± 10.32 35.7± 14.09∗ 3 Hip extension moment (Nm/kg) 1.52± 0.68 ^ 1.66± 0.98 1.6± 0.86∗ 1.84± 1.1 4 Knee extension moment (Nm/kg) 3.17± 0.52 3.49± 0.53 3.74± 0.64 3.61± 0.77 5 Ankle plantarflexion moment (Nm/kg) 3.09± 0.61 3.34± 1.08 3.35± 0.46 3.46± 0.73 6 Lower limbs’ support moment (Nm/kg) 7.78± 0.69 8.49± 1.32 8.69± 1 8.91± 1.36 7 Eccentric power of the hip joint (Nm/s) −16.55± 11.41# −21.92± 12.65# −31.39± 20.8# −46.34± 31.15∗ + ^ 8 Eccentric power of the knee joint (Nm/s) −15.47± 5.28+^# −20.09± 6.43∗ # −27.2± 8.19∗ # −33.57± 10.56∗ +^ 9 Eccentric power of the ankle joint (Nm/s) −7.93± 4.72^# −11.28± 7.62# −16.07± 6.24∗ −18.21± 7.66∗ + 10 Concentric power of the hip joint (Nm/s) 9.2± 4.33 10.38± 4.43# 14.33± 5.51∗# 19.73± 7.65∗ + 11 Concentric power of the knee joint (Nm/s) 18.9± 2.3 21.33± 3.31# 20.8± 5.72 20.6± 5.4+ 12 Concentric power of the ankle joint (Nm/s) 21.99± 4.54 24.02± 7.07 23.1± 3.1 22.97± 4.93 13 RSI (m/s) 0.15± 0.03 0.17± 0.04 0.16± 0.03 0.16± 0.04 vGRF: the peak of vertical ground reaction force; vGRFep: the peak of vertical ground reaction force in the extension phase; RSI: reactive strength index. ∗ indicates a significant difference relative to 0.3 m (P< 0.008), + indicates a significant difference relative to 0.45 m (P< 0.008), ^indicates a significant difference relative to 0.6 m (P< 0.008), and # indicates a significant difference relative to 0.75 m (P< 0.008). (P< 0.008) increased significantly at 0.6 m and 0.75 m. Drop height had a significant impact on vGRFep, low Relative to the DJ height at 0.3 m, the hip extension moment limb support moment, and concentric power of the hip joint. (P< 0.008) increased significantly at 0.6 m. -e eccentric vGRFep represented the level of active muscle contraction power of the hip joint (P< 0.008) at 0.75 m was significantly [26]. -e jumpers could achieve maximum vGRFep at a larger than at 0.3 m, 0.45 m, and 0.6 m. Compared with DJ lower DJ height, caused by short-term stretch-shortening height at 0.3 m, eccentric power of the knee joint (P< 0.008) cycle (SSC). It means more active muscle contraction can be was increased significantly at 0.45 m, 0.6 m, and 0.75 m, generated in shorter coupling time during this process. In while it increased significantly at 0.6 m relative to 0.3 m and addition to the short-term SSC, short-latency reflex (SLR) 0.45 m (P< 0.008). -e eccentric power of the knee joint also cannot be ignored in the active contraction of the lower (P< 0.008) at the DJ height of 0.45 m was significantly larger limbs. Taube et al. found that drop jump controlled SLR and than at 0.3 m. Relative to the drop height of 0.3 m, eccentric stretch reflex activation of the lower limb [27]. Komi and power of the ankle joint (P< 0.008) increased significantly at Gollhofer reported that when the drop height was controlled 0.6 m and 0.75 m, while it increased significantly at 0.75 m within 0.2 m–0.6 m, the SLR of the soleus was positively relative to 0.45 m (P< 0.008). -e eccentric power of the correlated with the drop height and significantly decreased ankle joint at 0.6 m was significantly larger than at 0.3 m at 0.8 m [28]. It indicated that the muscle-tendon complex (P< 0.008). Compared with the drop height of 0.3 m and was increasing stiffness and storing more elastic energy by 0.45 m, the concentric power of the hip joint (P< 0.008) the increasing SLR and stretch reflex level at a suitable range increased significantly at 0.75 m, while it increased signifi- of DJ heights. When the loading height exceeds the ap- cantly at 0.6 m relative to the drop height of 0.3 m propriate value, stretch reflex and SLR activation were de- (P< 0.008). -e RSI (0.17 ± 0.04) was the largest average in creased, resulting in the low stiffness of the muscle-tendon the group, so the optimal loading height for the RSI was complex for reducing the excessive impact [29]. Active 0.45 m. -e result of Pearson’s test reflected that RSI has no contraction was limited by the low SLR and stretch reflex significant correlations with PB (r � 0.149, P> 0.05). activation, presenting a decrease in vGRFep at excessive drop heights. Meanwhile, the concentric power of the hip joint was increased with DJ height, which may be related to 4. Discussion the technical requirements of high jump because the jumpers performed takeoff several times when extending the vGRF and eccentric joint power were significantly affected hip during their training. Avela et al. believed that high by drop height in the lower limbs. Studies have found that jumpers could adapt the high impact during DJ [25]. Kim vGRF was affected by drop height, which was further suggested an elite jumper to improve the extension strength demonstrated in this research [2, 3, 23, 24]. According to the [30]. -erefore, high jumpers could quickly extend the hip law of free-fall, the higher drop height converted gravita- and generate the greater concentric power of the hip joint tional potential energy into kinetic energy, and a greater when performing DJs under excessive heights. momentum was generated at the landing phase, so a greater -e result indicated that lower limbs’ support moment vGRF can be generated at a higher drop height. Eccentric was significantly affected by drop height and had no sig- joint power was increased with the increase of DJ heights; it nificant effects on knee extension moment and ankle is caused by the neuromuscular adaptation [25]. It means plantarflexion moment. It was related to the neuromuscular that high jumpers can keep the shorter contact time at the adaptation of high jumpers to high impact loads. It indicated landing phase. -erefore, the eccentric joint power of high that high jumpers can still keep the extension of lower limb jumpers was increased with DJ height. Journal of Healthcare Engineering 5 joints, even at high DJ height. Avela et al. showed that high result of this study indicated that the drop height had sig- jumpers have no significant changes in soleus H-reflex and nificant effects on vGRF, vGRFep, lower limbs’ support SLR sensitivity during DJ [25]. Evidence shows that the moment, eccentric joint power, and concentric power of the modulation of spinal nerves could maintain the H-reflex and hip joint. -e drop height had no significant effects on the SLR sensitivity of the soleus at a low level under rapid RSI. RSI had no significant correlations with the PB of high stretching and high impact load and prevent the muscle- jumpers. -e optimal loading height for the maximum RSI tendon complex from excessive stress in the landing phase was 0.45 m. -e optimal loading height can be used for [25, 29]. Nevertheless, joint extension of the lower limbs will explosive power training and injury prevention. result in the increase of the Achilles tendon in the landing phase. Laurent et al. found that the stiffness of the Achilles Data Availability tendon in the knee extension group increased significantly at Data sharing is not applicable to this article as no datasets 0.2 m, 0.4 m, and 0.6 m [31]. Previous research studies were generated or analyzed during the current study. showed that most of lower extremity injuries were induced by excessive stiffness. Brazier et al. reported that much lower Conflicts of Interest or higher stiffness in the lower limbs leads to an increased risk of injury [32]. Choi and Cho showed that excessive joint -e authors declare no conflicts of interest with respect to stiffness is one of the reasons of stress fractures [33]. Faria the research, authorship, and/or publication of this article. et al. believed that excessive stiffness of the muscle-tendon is a risk factor for lower extremity injuries [34]. -erefore, high Authors’ Contributions jumpers should control stiffness, when it is at an excessive level. It is important to use maximum RSI to reduce the Zehao Tong wrote the article. Hang Xu designed the pro- worse effect of this special landing style (lower limbs’ ex- tocol and completed the experiment. Wenjia Chen and tension and fast takeoff) of high jumpers. Jiesheng Cui processed the data. 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Variable Heights Influence Lower Extremity Biomechanics and Reactive Strength Index during Drop Jump: An Experimental Study of Male High Jumpers

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Copyright © 2021 Zehao Tong 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.
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Hindawi Journal of Healthcare Engineering Volume 2021, Article ID 5185758, 7 pages https://doi.org/10.1155/2021/5185758 Research Article Variable Heights Influence Lower Extremity Biomechanics and Reactive Strength Index during Drop Jump: An Experimental Study of Male High Jumpers 1 1 2 1 3 Zehao Tong , Feng Zhai , Hang Xu , Wenjia Chen , and Jiesheng Cui College of Physical Education, China University of Mining and Technology, Xuzhou, Jiangsu, China Department of Medical Imaging, Xuzhou Medical University, Xuzhou, Jiangsu, China National University of Singapore 119077, Singapore Correspondence should be addressed to Feng Zhai; zhaifeng123@126.com Received 29 October 2021; Revised 11 November 2021; Accepted 20 November 2021; Published 1 December 2021 Academic Editor: Kalidoss Rajakani Copyright © 2021 Zehao Tong et al. -is 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. Introduction. -is study finds the lower limbs’ reactive strength index and biomechanical parameters on variable heights. Objective. -is research aims to reveal the effects of drop height on lower limbs’ reactive strength index and biomechanical parameters. Methods. Two AMTI force platforms and Vicon motion capture system were used to collect kinematic and dynamic signals of the lower limbs. Results. -e drop height had significant effects on peak vertical ground reaction force and peak vertical ground reaction force in the extension phase, lower limbs’ support moment, eccentric power of the hip joint, eccentric power of the knee joint, eccentric power of the ankle joint, and concentric power of the hip joint. -e drop height had no significant effects on the reactive strength index. Reactive strength index (RSI) had no significant correlations with the personal best of high jumpers. -e optimal loading height for the maximum reactive strength index was 0.45 m. Conclusion. -e optimal loading height for the reactive strength index can be used for explosive power training and lower extremity injury prevention. Current research studies reported that DJ height for the 1. Introduction maximum RSI could enhance sport performance. Ramirez- Campillo et al. reported that optimal loading height for the Drop jump (DJ) is always a core part of plyometric RSI could significantly increase the 20 m sprint of soccer training for high jumpers. It has also been used to find the players by 7 weeks of intervention, but did not point out the effect of variable heights on lower limbs’ biomechanical DJ height for the RSI [4]. Boullosa et al. selected the optimal parameters and reactive strength index (RSI). Kipp et al. loading height for the RSI during warm-up, which improved found the effect of variable heights on the biomechanical the CMJ performance of long-distance runners, but failed to indicator and RSI in male basketball players, but RSI, increase 1000 m performance [5]. Smirniotou et al. found vertical stiffness, and other parameters were unaffected by that the RSI was highly correlated with sprint speed in young drop height [1]. Wu et al. indicated that DJ height marked sprinters, but did not apply the result to intervention [6]. impacted the peak of vertical ground reaction force Hennessy and Kilty revealed that BDJ index has a significant (vGRF) in male paratroopers [2]. Peng found that negative correlation with 30 m and 100 m performance in female power and vGRF at higher loading heights were higher athletes, but did not use for further training [7]. It is quite than low heights in college students [3]. It is known that clear that optimal loading height for the RSI is hard to the same DJ height has different effects on the RSI and improve the personal best (PB) directly for most of sports, biomechanical parameters of various subjects, so it is but it should be stressed that the ideal PB is related to sport necessary to determine an appropriate loading height of performance. DJ for each sport event. 2 Journal of Healthcare Engineering High jump includes fast run-up takeoff and high flight reserved for motion capture. -e location and name of the height. Studies showed that RSI can effectively evaluate the markers are shown in Figure 1. Vicon Nexus (v.2.10.1, Vicon, UK) was used to syn- ground contact time and flight height. Flanagan et al. [8] and Ebben and Petushek reported that RSI has high reliability on chronously collect kinematic and dynamic data. Vicon the evaluation of lower limbs’ explosive power [9]. Markwick Nexus was also used to identify the reflective markers during et al. found that RSI can effectively assess the plyometric static calibration and export them in a C3D format. Visual training of male basketball players [10]. Meanwhile, Louder 3D (v.6.01.36, C-Motion, USA) was used to build bony et al. confirmed the high internal validity of the RSI [11]. models of human lower limbs and process original dynamic Some research studies believed that PB was affected by and kinematic signals to obtain dynamic and kinematic data vertical height of high jump. Mateos-Padorno et al. reported of lower limb joints. -e marker trajectory and analog data that vertical displacement in the sagittal plane has a high were filtered with a fourth-order low-pass Butterworth filter correlation with PB [12]. Viitasalo and Aura found that the at 6 Hz and 15 Hz, respectively, to reduce the noise in the vertical height of the jumpers was increased at the game signals [20]. Reactive strength index (RSI) was worked out as the ratio of flight height and ground contact time. -e season [13]. -erefore, the improvement of RSI may be helpful for PB. In addition, high jumpers were always tor- processed data were calculated using the following formula tured by lower extremity injury. Schmitt et al. believed that [21, 22]: most of elite high jumpers have injured ankle and knee joints 1 Tf in the takeoff leg [14]. Schmitt et al. reported that a certain Hf � g􏼠 􏼡 (m), 2 2 proportion of high jumpers suffered by hip arthrosis [15]. (1) Schmitt et al. found that the jumpers with ankle injury have a Hf low radiological score [16]. It should be emphasized that RSI � . Tc inappropriate training results in injury [17]. To our knowledge, no studies found the effects of var- -e average value for three effective DJs at the same iable heights for high jumpers. Many studies showed that height was calculated. By quadratic polynomial regression RSI could significantly improve the sprint and CMJ per- analysis, the effects of drop height on the biomechanics formance, but did not clarify the correlations between the parameters and RSI of the lower limbs were obtained. One- RSI and the PB of high jumpers. -erefore, the aim of this way repeated measure analysis of variance (ANOVA) was study was to investigate the effects of variable drop heights used for comparison of differences and selected the sig- on vGRF, RSI, joint power, and moment of lower limbs, find nificant variables of them for Bonferroni correction. -e the correlations between RSI and PB, and determine the maximum average value was taken from each group as the optimal loading height for the maximum RSI. Hypotheses optimal loading height for the RSI. By Pearson’s test, the include the following: (1) drop height had significant effects correlations of RSI and PB were investigated. -e above data on the reactive strength index and biomechanical parame- were processed with SPSS (v. 25.0, IBM, USA). ters. (2) -e correlation between the RSI and PB would be significant. 3. Results 2. Methods Table 1 shows the result of quadratic polynomial regression analysis. -e drop height significantly affected vGRF Ten high jumpers were recruited (males: age: 20.7± 2.32 y, (P< 0.001) and vGRFep (P< 0.001), lower limbs’ support training background: 5.1± 1.51 y, weight: 72.33± 5.36 kg, moment (P< 0.05), eccentric power of the hip joint height: 1.89± 0.04 m, and personal best: 2.065± 0.04 m). Vicon motion capture system (Vicon, UK) and AMTI (P< 0.05), eccentric power of the knee joint (P< 0.001), eccentric power of the ankle joint (P< 0.001), and concentric HPS400600 force platforms (AMTI, USA) were used to collect kinematic and dynamic signals of the lower limbs. power of the hip joint (P< 0.001). It has no significant effects on the hip extension moment (P> 0.05), knee extension Plyo Soft Box was used for the DJ test (Escape Fitness, UK). -e warm-up protocol included 10 min of jogging at a moment (P> 0.05), ankle plantarflexion moment (P> 0.05), and RSI (P> 0.05). Table 2 shows the result of one-way moderate self-selected pace [5] and 5 min of static stretching repeated measure analysis. vGRF (P< 0.001), vGRFep for each leg. After hearing the “start” order, raise the (P< 0.001), hip extension moment (P< 0.05), knee exten- dominant leg, naturally lean forward, and drop freely. When sion moment (P< 0.05), eccentric power of the hip the feet touch the ground, the subject must jump vertically as fast as possible. Requirements of effective DJ were as follows: (P< 0.05), knee (P< 0.05), and ankle joint (P< 0.05), and concentric power of the hip joint (P< 0.05) have statistical do not move center of gravity upwards after leaving the box. Both feet should touch the ground at the same time. Jump significance at 0.3 m, 0.45 m, 0.6 m, and 0.75 m. Table 3 reflects mean values (±SD) and the result of upwards with full strength, and keep both hands on hips [18]. Vicon motion capture system was used to capture Bonferroni correction during DJs. Relative to the drop height of 0.3 m, vGRF (P< 0.008) increased significantly at kinematic signals of the lower limbs at a frequency of 100 Hz. 0.45 m, 0.6 m, and 0.75 m. vGRF at the DJ height of 0.75 m According to the pasting scheme of Plug-In Gait Lower- was significantly larger than at 0.45 m and 0.6 m (P< 0.008). Limb Ai 2.3 Marker [19], 28 reflective markers were pasted Compared with the drop height of 0.3 m, vGRFep on the bony landmarks of the lower limbs. 24 markers were Journal of Healthcare Engineering 3 RPSI LPSI RASI LASI LTHAP LTHI LTAD LKNM LKNE LTIAP LTIB LTIAD LMED LHEE LANK LTOE Calibration only Optional (a) (b) Figure 1: -e location of reflective markers. (a) Location on the lower limbs. (b) Specific location of each marker. Table 1: Quadratic polynomial regression analysis for variables during DJs. 2 2 S. no. Variables R R Adjusted R F β t P Constant 1 vGRF 0.686 0.471 0.457 33.852 8.149 7.233 ≤0.001 27.742 2 vGRFep 0.54 0.292 0.273 15.667 −7.275 −3.958 ≤0.001 63.006 3 Hip extension moment 0.114 0.013 −0.013 0.5 0.09 0.707 0.484 1.43 4 Knee extension moment 0.281 0.079 0.055 3.26 0.158 1.806 0.079 3.106 5 Ankle plantarflexion moment 0.171 0.029 0.004 1.141 0.111 1.068 0.292 3.035 6 Lower limbs’ support moment 0.35 0.123 0.1 5.314 0.359 2.305 0.027 7.571 7 Eccentric power of the hip joint 0.49 0.24 0.22 12 −9.883 −3.464 0.001 −4.342 8 Eccentric power of the knee joint 0.676 0.457 0.443 31.985 −6.142 −5.656 ≤0.001 −8.729 9 Eccentric power of the ankle joint 0.531 0.282 0.263 14.946 −3.563 −3.866 ≤0.001 −4.465 10 Concentric power of the hip joint 0.589 0.347 0.33 20.171 3.556 4.491 ≤0.001 4.519 11 Concentric power of the knee joint 0.119 0.014 −0.012 0.547 0.457 0.739 0.464 19.267 12 Concentric power of the ankle joint 0.046 0.002 −0.024 0.081 0.202 0.284 0.778 22.514 13 RSI 0.064 0.004 −0.022 0.155 0.002 0.393 0.696 0.156 vGRF: the peak of vertical ground reaction force; vGRFep: the peak of vertical ground reaction force in the extension phase; RSI: reactive strength index. P< 0.05 indicates significant effects, and bold is used to indicate it, which means drop height significantly affects the indicator. Table 2: One-way repeated measure analysis of variance for variables during DJs. S. no. Variables Mauchly’s sphericity test F P Partial eta [2] 1 vGRF 0.004 47.709 ≤0.001 0.841 2 vGRFep 0.403 27.577 ≤0.001 0.754 3 Hip extension moment 0.043 8.059 0.019 0.472 4 Knee extension moment 0.005 3.752 0.023 0.294 5 Ankle plantarflexion moment 0.028 0.84 0.484 0.085 6 Lower limbs’ support moment 0.036 2.533 0.119 0.22 7 Eccentric power of the hip joint 0.006 32.686 ≤0.001 0.784 8 Eccentric power of the knee joint 0.01 42.591 ≤0.001 0.826 9 Eccentric power of the ankle joint 0.482 22.819 0.001 0.907 10 Concentric power of the hip joint 0.174 12.905 0.001 0.589 11 Concentric power of the knee joint 0.057 1.179 0.336 0.116 12 Concentric power of the ankle joint 0.091 0.48 0.699 0.051 13 RSI 0.196 0.585 0.63 0.061 vGRF: the peak of vertical ground reaction force; vGRFep: the peak of vertical ground reaction force in the extension phase; RSI: reactive strength index. P< 0.05 indicates a significant difference, and bold is used to indicate it. -e value of this index at the drop height of 0.3m, 0.45m, 0.6m, 0.75m is significantly different from each other. 4 Journal of Healthcare Engineering Table 3: Mean values (±SD) with the Bonferroni correction of variables for DJs. Drop height S. no. Variables 0.3 m 0.45 m 0.6 m 0.75 m 1 vGRF (N/kg) 35.32± 8.97 44.84± 9.53 52.31± 10.1∗# 59.99± 11.82∗ +^ 2 vGRFep (N/kg) 57.01± 13.15 ^# 47.7± 14.94 38.87± 10.32 35.7± 14.09∗ 3 Hip extension moment (Nm/kg) 1.52± 0.68 ^ 1.66± 0.98 1.6± 0.86∗ 1.84± 1.1 4 Knee extension moment (Nm/kg) 3.17± 0.52 3.49± 0.53 3.74± 0.64 3.61± 0.77 5 Ankle plantarflexion moment (Nm/kg) 3.09± 0.61 3.34± 1.08 3.35± 0.46 3.46± 0.73 6 Lower limbs’ support moment (Nm/kg) 7.78± 0.69 8.49± 1.32 8.69± 1 8.91± 1.36 7 Eccentric power of the hip joint (Nm/s) −16.55± 11.41# −21.92± 12.65# −31.39± 20.8# −46.34± 31.15∗ + ^ 8 Eccentric power of the knee joint (Nm/s) −15.47± 5.28+^# −20.09± 6.43∗ # −27.2± 8.19∗ # −33.57± 10.56∗ +^ 9 Eccentric power of the ankle joint (Nm/s) −7.93± 4.72^# −11.28± 7.62# −16.07± 6.24∗ −18.21± 7.66∗ + 10 Concentric power of the hip joint (Nm/s) 9.2± 4.33 10.38± 4.43# 14.33± 5.51∗# 19.73± 7.65∗ + 11 Concentric power of the knee joint (Nm/s) 18.9± 2.3 21.33± 3.31# 20.8± 5.72 20.6± 5.4+ 12 Concentric power of the ankle joint (Nm/s) 21.99± 4.54 24.02± 7.07 23.1± 3.1 22.97± 4.93 13 RSI (m/s) 0.15± 0.03 0.17± 0.04 0.16± 0.03 0.16± 0.04 vGRF: the peak of vertical ground reaction force; vGRFep: the peak of vertical ground reaction force in the extension phase; RSI: reactive strength index. ∗ indicates a significant difference relative to 0.3 m (P< 0.008), + indicates a significant difference relative to 0.45 m (P< 0.008), ^indicates a significant difference relative to 0.6 m (P< 0.008), and # indicates a significant difference relative to 0.75 m (P< 0.008). (P< 0.008) increased significantly at 0.6 m and 0.75 m. Drop height had a significant impact on vGRFep, low Relative to the DJ height at 0.3 m, the hip extension moment limb support moment, and concentric power of the hip joint. (P< 0.008) increased significantly at 0.6 m. -e eccentric vGRFep represented the level of active muscle contraction power of the hip joint (P< 0.008) at 0.75 m was significantly [26]. -e jumpers could achieve maximum vGRFep at a larger than at 0.3 m, 0.45 m, and 0.6 m. Compared with DJ lower DJ height, caused by short-term stretch-shortening height at 0.3 m, eccentric power of the knee joint (P< 0.008) cycle (SSC). It means more active muscle contraction can be was increased significantly at 0.45 m, 0.6 m, and 0.75 m, generated in shorter coupling time during this process. In while it increased significantly at 0.6 m relative to 0.3 m and addition to the short-term SSC, short-latency reflex (SLR) 0.45 m (P< 0.008). -e eccentric power of the knee joint also cannot be ignored in the active contraction of the lower (P< 0.008) at the DJ height of 0.45 m was significantly larger limbs. Taube et al. found that drop jump controlled SLR and than at 0.3 m. Relative to the drop height of 0.3 m, eccentric stretch reflex activation of the lower limb [27]. Komi and power of the ankle joint (P< 0.008) increased significantly at Gollhofer reported that when the drop height was controlled 0.6 m and 0.75 m, while it increased significantly at 0.75 m within 0.2 m–0.6 m, the SLR of the soleus was positively relative to 0.45 m (P< 0.008). -e eccentric power of the correlated with the drop height and significantly decreased ankle joint at 0.6 m was significantly larger than at 0.3 m at 0.8 m [28]. It indicated that the muscle-tendon complex (P< 0.008). Compared with the drop height of 0.3 m and was increasing stiffness and storing more elastic energy by 0.45 m, the concentric power of the hip joint (P< 0.008) the increasing SLR and stretch reflex level at a suitable range increased significantly at 0.75 m, while it increased signifi- of DJ heights. When the loading height exceeds the ap- cantly at 0.6 m relative to the drop height of 0.3 m propriate value, stretch reflex and SLR activation were de- (P< 0.008). -e RSI (0.17 ± 0.04) was the largest average in creased, resulting in the low stiffness of the muscle-tendon the group, so the optimal loading height for the RSI was complex for reducing the excessive impact [29]. Active 0.45 m. -e result of Pearson’s test reflected that RSI has no contraction was limited by the low SLR and stretch reflex significant correlations with PB (r � 0.149, P> 0.05). activation, presenting a decrease in vGRFep at excessive drop heights. Meanwhile, the concentric power of the hip joint was increased with DJ height, which may be related to 4. Discussion the technical requirements of high jump because the jumpers performed takeoff several times when extending the vGRF and eccentric joint power were significantly affected hip during their training. Avela et al. believed that high by drop height in the lower limbs. Studies have found that jumpers could adapt the high impact during DJ [25]. Kim vGRF was affected by drop height, which was further suggested an elite jumper to improve the extension strength demonstrated in this research [2, 3, 23, 24]. According to the [30]. -erefore, high jumpers could quickly extend the hip law of free-fall, the higher drop height converted gravita- and generate the greater concentric power of the hip joint tional potential energy into kinetic energy, and a greater when performing DJs under excessive heights. momentum was generated at the landing phase, so a greater -e result indicated that lower limbs’ support moment vGRF can be generated at a higher drop height. Eccentric was significantly affected by drop height and had no sig- joint power was increased with the increase of DJ heights; it nificant effects on knee extension moment and ankle is caused by the neuromuscular adaptation [25]. It means plantarflexion moment. It was related to the neuromuscular that high jumpers can keep the shorter contact time at the adaptation of high jumpers to high impact loads. It indicated landing phase. -erefore, the eccentric joint power of high that high jumpers can still keep the extension of lower limb jumpers was increased with DJ height. Journal of Healthcare Engineering 5 joints, even at high DJ height. Avela et al. showed that high result of this study indicated that the drop height had sig- jumpers have no significant changes in soleus H-reflex and nificant effects on vGRF, vGRFep, lower limbs’ support SLR sensitivity during DJ [25]. Evidence shows that the moment, eccentric joint power, and concentric power of the modulation of spinal nerves could maintain the H-reflex and hip joint. -e drop height had no significant effects on the SLR sensitivity of the soleus at a low level under rapid RSI. RSI had no significant correlations with the PB of high stretching and high impact load and prevent the muscle- jumpers. -e optimal loading height for the maximum RSI tendon complex from excessive stress in the landing phase was 0.45 m. -e optimal loading height can be used for [25, 29]. Nevertheless, joint extension of the lower limbs will explosive power training and injury prevention. result in the increase of the Achilles tendon in the landing phase. Laurent et al. found that the stiffness of the Achilles Data Availability tendon in the knee extension group increased significantly at Data sharing is not applicable to this article as no datasets 0.2 m, 0.4 m, and 0.6 m [31]. Previous research studies were generated or analyzed during the current study. showed that most of lower extremity injuries were induced by excessive stiffness. Brazier et al. reported that much lower Conflicts of Interest or higher stiffness in the lower limbs leads to an increased risk of injury [32]. Choi and Cho showed that excessive joint -e authors declare no conflicts of interest with respect to stiffness is one of the reasons of stress fractures [33]. Faria the research, authorship, and/or publication of this article. et al. believed that excessive stiffness of the muscle-tendon is a risk factor for lower extremity injuries [34]. -erefore, high Authors’ Contributions jumpers should control stiffness, when it is at an excessive level. It is important to use maximum RSI to reduce the Zehao Tong wrote the article. Hang Xu designed the pro- worse effect of this special landing style (lower limbs’ ex- tocol and completed the experiment. Wenjia Chen and tension and fast takeoff) of high jumpers. Jiesheng Cui processed the data. All the works were sup- -e drop height had no significant effects on the RSI, ported by Feng Zhai in theory. mainly due to the neuromuscular adaptation. -e modu- lation of the muscle H-reflex and SLR sensitivity of high References jumpers by upper spinal nerves showed no significant changes before and after DJ [25], while shorter landing time [1] K. Kipp, M. T. Kiely, M. D. Giordanelli, P. J. Malloy, and C. F. Geiser, “Biomechanical determinants of the reactive could be maintained at a higher drop height. -e special strength index during drop jumps,” International Journal of landing strategy of high jumpers was induced by neuro- Sports Physiology and Performance, vol. 13, no. 1, pp. 1–20, muscular adaptation in the landing phase. Studies have shown that, with this special landing strategy, greater RSI [2] D. Wu, C. Zheng, J. 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