Altering muscle activity in the lower extremities by bipedal landing with different drop tasks and shoes

Abstract

BACKGROUND:

The ability of the lower-extremity muscle activation directly affects the performance and in turn interacts with the loading conditions of the muscle itself. However, systematic information concerning the characteristics of lower-extremity muscle during landings is lacking. In particular, the landing height and shoes are also important factors based on the actual situation, which could further contribute to understanding the neuromuscular activity and how biochemical response of the body tissues to double-leg drop landings.

OBJECTIVE:

The study was to investigate the effects of landing tasks on the activation of lower-extremity muscles and explore the relationship among movement control, landing heights, shoe cushioning, and muscle activities.

METHODS:

Twelve male basketball players were recruited to perform drop jump (DJ) and passive landing (PL) from three heights (30, 45, and 60 cm) while wearing highly-cushioned basketball shoes (HC) and less-cushioned control shoes (LC). EMG electrodes were used to record the activities of the target muscles (rectus femoris, vastus lateralis, biceps femoris, tibialis anterior, and lateral gastrocnemius) during the landing tasks.

RESULTS:

Pre- and post-activation activity of the lower-extremity muscles significantly decreased during PL compared with those during DJ ( $p<$ 0.05). No significant shoe effects on the characteristics of muscle activation and coactivation during DJ movements were observed. However, the participants wearing LC showed significantly higher muscle post-activation ( $p<$ 0.05) at the three drop heights during PL compared with those wearing HC. Coactivation of the ankle muscles was higher in LC than in HC during 30-cm PL ( $p<$ 0.05).

CONCLUSIONS:

The activation patterns of lower-extremity muscles can be significantly influenced by landing types. Highly-cushioned basketball shoes would help reduce the risk of injuries by appropriately tuning the muscles during the PL.

Keywords

Landing shoe muscle activation pre-activation post-activation co-contraction

1. Introduction

Participant in any sport carries an inherent risk of injury, especially those that combine jumping and swift changes of direction. These are most often associated with lower-extremity muscle and joint damages [1]. The most fundamental property of muscles is force generation, which is derived from contraction from nervous system activation [2]. The completion of a movement due to force generation is the basic element that constitutes the mechanical role in various sports. In other words, the ability to activate lower-extremity muscles directly affects performance that in turn interacts with the loading conditions of the muscle itself. Therefore, the control mechanism of the human neuro-musculoskeletal system can be understood by studying the characteristics of muscle activation during movements [3].

Sports medicine findings, along with medicine and science findings, inevitably contribute to lower-extremity injuries prevention for players in any sport. Since the 1970s, many researchers have observed muscle activation patterns in running and jumping movements via electromyography [1, 4, 5]. They found the occurrence and development of muscle activation are complex and affect many aspects of an action, such as the positions and loads of lower-extremity joints, the velocity and stiffness of the lower extremities, and the vibration of soft tissue structures. Running and jumping movements show the pre-and post-activation of lower-extremity muscles relative to the condition in the muscle’s working state; thus, the primary goal of muscle activation is to respond rapidly to impact and complete landing tasks before and after ground contact [6, 7]. However, systematic information concerning the characteristics of lower-extremity muscles during landing is lacking. In particular, the landing height is an important factor based on the actual situation, which could further contribute to understanding the neuromuscular activity and how these characteristics are represented, sports-injury mechanisms, and the biochemical response of body tissues to double-leg drop landings.

Additionally, muscle activation is an important biomechanical parameter in double-leg drop landings. Several researchers have put forward a new hypothesis based on original muscle function theories; according to them, one of the main functions of muscle activation is to fine-tune muscle activities in order to reduce the vibration of soft tissues before and after touchdowns [8, 9, 10]. Specifically, the neuromuscular system activates relevant muscles according to the conditions (landing speed, surface hardness, etc.) during running and jumping movements and minimizes the amplitude of soft tissue vibrations close to the resonance frequency to reduce the loads on joints and tendons and prevent the possibility of sports injuries [11, 12, 13]. Muscle activation patterns combining the muscle recruited, the impact force, and the soft tissue resonance frequency have attracted extensive attention in sports medicine and biomechanics, and have gradually been introduced into advancements in sports shoes [10, 14]. In basketball, landing movements often lead to ankle injuries [15], and ankle sprains are identified as one of the most common forms of injuries most players suffer in their careers [16]. Muscle strength is related to the risk of ankle-related injuries according to recent reports [16, 17] Therefore, research on muscle activation is particularly important. Muscle coactivation is related to motor control; it contributes to the accuracy of motion and avoidance of sports injuries [18, 19]. It is worth noting ankle muscle coactivation plays a vital role in affecting the function of the lower extremity and preventing sports injuries during landing movements.

Based on the above consideration, the objectives of this study were to investigate the effects of landing heights and shoe cushioning on lower-extremity muscle activation during two landing tasks and explore the relationship among them. Our findings are expected to provide useful suggestions and insight for athletes and footwear manufacturers. We hypothesized that: (1) the effects of shoe cushioning on muscle activation will differ in passive landing (PL) and drop jump (DJ) tasks from different landing heights, (2) the coactivation characteristics of the target muscles will differ in the target moments, and (3) the muscle pre- and post-activation in PL will be significantly lower than those in DJ.

2. Methods

2.1 Participants

Twelve male basketball players from Shanghai University of Sport (age: 23.8 $\pm$ 1.9 years, height: 177.4 $\pm$ 2.6 cm, body mass: 70.4 $\pm$ 5.3 kg, years of training: 6.7 $\pm$ 2.8 years, shoe size: 27.0 $\pm$ 0.0 cm) were recruited for this study. None of the participants partook in strenuous exercise within 24 hours, had no history of lower-extremity injuries, and had no previous surgery or foot disease within half a year before the tests. The participants were in the good physical condition and had mastered DJ movements in training or practice. All participants were informed about the experimental procedures and requirements, and written consent was obtained before experimental data collection. The study was approved by the Institutional Review Board of the University (date: 15/02/2017; No. 2017007).

2.2 Shoe types

The midsole of highly cushioned shoes (HC) used in this study was made of Phylon (light and soft material that has good cushioning properties and is made by foaming and compression [20]) and equipped with a full-length air cushion unit that has a strong cushioning function. The less-cushioned shoes (LC) had a hard rubber sole, no special midsole material, and no air cushion unit. The size of the two types of testing shoes was 270 mm (EUR 42.5).

2.3 Data collection

Touchdown moments were confirmed with a 90 cm $\times$ 60 cm 3D force platform (9287B, Kistler Corporation, Switzerland) with a frequency of 1200 Hz. An elevated platform was also used in the study, and its height could be adjusted from 20 cm to 65 cm at graduations 5 cm. A perforated bolt was designed at the back of the elevated platform, and it could be manually operated to make the horizontal surface flip suddenly during the DJ tests.

Sixty-four-channel wireless electromyogram (EMG)- electrodes (TrignoTM wireless sEMG, Delsys, USA) collected surface muscle activity at a sampling frequency of 1200 Hz. The sensors were positioned on the dominant leg muscles of the participants by the Surface EMG for the Non-invasive Assessment of Muscles Project [21]. The five test muscles were rectus femoris (RF), vastus lateralis (VL), biceps femoris (BF), tibialis anterior (TA), and lateral gastrocnemius (LG). The electrodes did not influence nor interfere with any of the recorded data.

2.4 Experimental protocol

Each participant completed 12 (2 $\times$ 2 $\times$ 3) sets of tests, including two landing tasks (DJ and PL) at three heights (30, 45, and 60 cm), by using the two differently cushioned shoes in random order. All test items were repeated three times, and the rest interval in each test was 5 min. The two landing tasks [22] were DJ and PL. In DJ, each participant was asked to stand on the elevated platform, with the feet shoulder-width apart. After the synchronization signal started, the participant’s distal foot surfaces slid slowly from the edge of the elevated platform to fall onto the force platform with no vertical initial velocity, and the participant was asked to jump vertically upward after landing (no swinging arms). All participants were asked to complete three tests at the same falling altitude. In PL, each participant was asked to stand on the elevated platform and lean forward slightly with his center of gravity forward to prevent the body from tilting backward after dropping on the ground. The elevated platform was controlled to flip down suddenly by manual operation after the participant was given the “ready” sign so that the participant could land passively without any precognition.

2.5 Data analysis

Means and standard deviations of all tests and statistics were assessed using IBM SPSS Statistics version 19.0 (IBM Co., Armonk, NY, USA). An independent $t$ -test was performed to examine the differences between groups. The statistical significance was set as $\alpha=$ 0.05.

Raw EMG signals were band-pass filtered between 10 and 400 Hz, full-wave rectified and normalized as a percentage of the highest value of each muscle recorded during all tests [23, 24]. The normalized EMG amplitude was calculated by root mean square ( $\textit{EMG}_{\textit{RMS}}$ ) as follows:

$\displaystyle\textit{EMG}_{\textit{RMS}}=\sqrt{\frac{1}{T}\int_{t}^{t+T}% \textit{EMG}^{2}\left(t\right)\cdot dt},$ (1)

where $t$ is the onset time of the signal and $t+T$ is the signal termination time.

The pre-activation phase was defined as 50 ms before ground contact ( $-$ 50 ms, pre-activation), and the post-activation phase was defined as 50 ms after ground contact ( $+$ 50 ms, post-activation) [10] (Fig. 1).

Figure 1.

Normalized EMG–time curve at drop jumps. Note: TA, tibialis anterior; LG, lateral gastrocnemius; RF, rectus femoris; VL, vastus lateralis; BF, biceps femoris.

Figure 2.

Normalized pre-activation EMG ${}_{\textit{RMS}}$ of lower-extremity muscles for the different landing tasks at three landing heights. Note: TA, tibialis anterior; LG, lateral gastrocnemius; RF, rectus femoris; VL, vastus lateralis; BF, biceps femoris. *Denotes significant differences between the two landing tasks at the same height, $p<$ 0.05.

For coactivation, coordination between the agonist and antagonist arises to adjust the timing and strength of muscle contraction to adapt to the demands of the peripheral environment based on the central nervous system [25]. Coactivation regulates the stiffness of the muscles around the lower-extremity joints and thus maintains the stability of the joints when the lower-extremities are affected by the external environment (e.g., violent impact) [26]. Antagonist-agonist coactivation is calculated as the ratio of the activation degree of the antagonist and agonist; it is used to reflect the mode of coordinated movement of the two types of muscles in the same time interval [27].

$\displaystyle\textit{co -- activation}=\frac{\textit{RMS}_{\textit{antagonist % muscle}}}{\textit{RMS}_{\textit{agonist muscle}}}\times 100\%,$ (2)

Figure 3.

Effects of shoe conditions on the pre-activation EMG ${}_{\textit{RMS}}$ (normalized) of lower-extremity muscles at three heights and two landing tasks. Note: HC, highly cushioned shoes; LC, less-cushioned shoes; TA, tibialis anterior; LG, lateral gastrocnemius; RF, rectus femoris; VL, vastus lateralis; BF, biceps femoris. *Denotes significant differences between two shoe conditions at the same height, $p<$ 0.05.

In this study, TA and LG were selected to observe the ankle coactivation characteristics at pre-and post-activation phases [24, 28].

2.6 Statistics

All statistical analyses were performed using SPSS 19.0 (SPSS Inc., Chicago, IL, USA). Results were expressed as mean $\pm$ standard deviation. After testing for normality with the Shapiro-Wilks test, repeated-measure ANOVAs were used to determine the effect of independent variables (dropping heights, landing tasks, and shoe conditions) on muscle activation, and Tukey’s post-hoc test was adopted to determine the significant individual differences. The significance level was set to $p<$ 0.05.

3. Results

3.1 Muscle pre-activation

The muscle pre-activation in PL was much less than that in DJ at the three heights (Fig. 2) ( $p<$ 0.05). The pre-activation $\textit{EMG}_{\textit{RMS}}$ of LG at DJ30; LG and TA at DJ45; TA, VL, BF, and RF at 60DJ were higher than those in PL ( $p<$ 0.05).

The type of footwear did not significantly affect the pre-activation of the lower-extremity muscles (TA, RF, VL, and BF) at the three heights and two landing tasks (Fig. 3). Differences were observed in the pre-activation of LG between the use of LC and HC when the participants performed DJ at the 60 cm height ( $p<$ 0.05).

3.2 Muscle post-activation

Similar to the pre-activation process, the muscle post-activation during PL was still smaller than that during DJ. Significant differences ( $p<$ 0.05) were observed among the muscles under the different heights (Fig. 4). Specifically, the post-activation $\textit{EMG}_{\textit{RMS}}$ of TA, RF, VL, and BF at DJ30; RF at DJ45; and LG, RF, and VL at DJ60 were higher than those in PL ( $p<$ 0.05).

Table 1
Effects of landing tasks and shoe types on ankle coactivation (%) at different dropping heights

Coactivation (%)		DJ			PL
Phase	Groups	30 cm	45 cm	60 cm	30 cm	45 cm	60 cm
Pre-activation	HC	127.2 $\pm$ 37.8	167.6 $\pm$ 50.9	191.1 $\pm$ 48.8 ${}^{\#}$	175.8 $\pm$ 45.9	137.8 $\pm$ 38.1	126.1 $\pm$ 59.2 ${}^{\#}$
	LC	118.8 $\pm$ 41.7	175.7 $\pm$ 57.4	209.8 $\pm$ 39.1 ${}^{\#}$	193.6 $\pm$ 51.3	130.2 $\pm$ 45.1 ${}^{\#}$	112.5 $\pm$ 56.1 ${}^{\#}$
Post-activation	HC	131.1 $\pm$ 48.8	174.6 $\pm$ 53.5	191.1 $\pm$ 39.1 ${}^{\#}$	199.7 $\pm$ 48.6 ${}^{\ast}$	182.0 $\pm$ 47.2	204.9 $\pm$ 63.8
	LC	116.8 $\pm$ 49.1	177.8 $\pm$ 52.9	171.6 $\pm$ 46.8 ${}^{\#}$	238.9 $\pm$ 55.3	212.3 $\pm$ 40.9	215.5 $\pm$ 70.5

Note: HC, highly cushioned shoes; LC, less-cushioned shoes. ${}^{*}p<$ 0.05 indicates a significant difference compared with the LC group under the same height and landing task. ${}^{\#}p<$ 0.05 indicates a significant difference compared with 30 cm under the same shoe condition and landing task.

Figure 4.

Post-activation EMG ${}_{\textit{RMS}}$ of lower-extremity muscles for the different landing tasks at three landing heights. Note: TA, tibialis anterior; LG, lateral gastrocnemius; RF, rectus femoris; VL, vastus lateralis; BF, biceps femoris. *Denotes significant differences between two landing tasks at the same height, $p<$ 0.05.

Figure 5.

Effects of shoe types on the post-activation EMG ${}_{\textit{RMS}}$ (normalized) of lower-extremity muscles at three heights and two landing tasks. Note: HC, highly cushioned shoes; LC, less-cushioned shoes; TA, tibialis anterior; LG, lateral gastrocnemius; RF, rectus femoris; VL, vastus lateralis; BF, biceps femoris. *Denotes significant differences between two shoe types at the same height, $p<$ 0.05.

The different shoe types did not significantly affect the post-activation of the lower-extremity muscles in the DJ task. However, LC could increase post-activation in the PL movement (Fig. 5). The post-activation of TA at the three heights; LG at the 30 and 45 cm heights; RF, VL, and BF at the 60 cm height; and BF at the 45 cm height were all significantly higher in LC than in HC ( $p<$ 0.05).

3.3 Muscle coactivation

Whether for the DJ or PL movement, ankle muscle coactivation of the participants who wore LC showed no significant differences at 50 ms before ground contact compared with those who wore HC. Coactivation of the ankle joint muscle was primarily affected by height (Table 1). Specifically, the coactivation at 60 cm was significantly higher than that at 30 cm for the DJ tasks, but the result was reversed for the PL tasks ( $p<$ 0.05). In the post-activation after ground contact, coactivation for LC was visibly higher than that for HC at the 30 cm height ( $p<$ 0.05), but no significant differences were observed between the two shoe types at the remaining heights.

4. Discussion

4.1 Landing task effects on muscle activation

DJ is one of the technical features of various sports that focus on jumping abilities (basketball, volleyball, etc.); it involves muscle activation and motion characteristics of the typical stretch-shortening circle [29]. DJ has more obvious impact characteristics compared to running and jumping done at normal speeds. In the present study, PL enabled participants to c make lower extremity ground contact without sufficient anticipation; thus, the active regulation of the body is more difficult. The results corroborated that the $\textit{EMG}_{\textit{RMS}}$ values of the main extensor and flexor leg muscles (TA, LG, RF, VL, and BF) were significantly affected by the landing tasks in the pre-and post-activation phases. Specifically, the normalized $\textit{EMG}_{\textit{RMS}}$ values of the muscles in the two phases were smaller during PL than during DJ, and significant differences ( $p<$ 0.05) were found among the muscles for at least one height (Figs 2 and 4).

From a neuromechanical perspective, the human musculoskeletal system is impacted by landing conditions (landing speed, ground hardness, etc.). Soft tissue activity caused by falling’s impact can be minimized by changes in muscle recruitment, which may reduce joint and tendon loads before and after landing during running or jumping [14, 30]. About muscle recruitment, adjustments in joint stiffness and geometry may activate soft tissues at risk of resonance [14, 31, 32]. Research shows the amplitude can be increased when the frequency of the ground impacts approach the natural frequency range of soft tissues, but the amplitude rapidly decays to less than 5% after soft tissues have undergone vibration [33]. The reason is widely believed to be increased muscle activation, that is, the body cuts down the resonance of the soft tissue system by muscle alterations in recruitment, thus reducing the possibility of injuries. The assumptions of previous studies about muscle recruitment and soft tissue resonance coincide with our findings. The human central nervous system can create a string of appropriate adjustments by releasing impulses and activating corresponding muscles during the DJ. This controlled the movement of subjects as they fell or took off and finally relied on forces generated by muscles to complete the movements. However, the activation of muscles (TA, LG, RF, and VL) throughout DJ was much greater than that during PL because of the take-off with the former movement, and the lack of sufficient precognition, and the increased difficulty of active regulation for the latter. Differences between tasks may play an important role, but the difference between self initial and passive initial cannot be ignored. Boyer and Nigg [12] investigated the effect of unknown movement surfaces on the strategies of muscle recruitment during running. In their study, 13 male participants ran 21.9 m at a rate of 4.8 $\pm$ 0.2 m/s on a track composed of soft and hard surfaces, respectively. The control group was informed of the soft and hard track surfaces, whereas the experimental group was not informed. Compared to the controls, the experimental group had no anticipatory lower extremity muscle activity before and after ground contact. Acceleration of the corresponding soft tissues increased significantly after ground contact, thus increasing the possibility of injuries. Our findings are partly consistent with the results of Boyer and Nigg’s as the ability of muscles to self-regulate are affected at pre-and post-activation phases when the participants could not accurately determine ground conditions; this reduced muscle recruitment (as shown by decreased EMG amplitude) could enhance the vibration effect of soft tissues, thereby increasing the risk of resonance injuries [34].

4.2 Shoe effects on muscle activation

Researchers have studied the feedback regulation of muscle activation during vibration shocks since the 1980s [35]. At the beginning of this century, many investigations were conducted to determine if changes in sports shoe characteristics (midsole material, hardness/thickness, etc.) had positive effects on muscle activity. Dupuis et al. [35] found changes in the middle sole hardness of sports shoes (Shore C $=$ 41 vs. Shore C $=$ 61) affected the recruitment of muscle fibers in the impact stage of lower-extremity muscles to change characteristics of muscle activation when the heel encounters impact during running or jumping. In this study, shoe intervention has different effects on pre-activation and post-activation. The pre-activation of LG in LC was significantly higher than that in HC at 60 cm ( $p<$ 0.05) (Fig. 3). And no significant changes in the pre-activation of the lower-extremity muscles with shoe intervention were detected during PL (Fig. 3). This suggests the pre-activation mode for the impending impact may be more closely related to the landing task itself than the shoe intervention.

However, the post-activation at some muscles when falling at different heights (Fig. 5) and ankle muscle coactivation at partial heights (Table 1) was significantly increased when the participants wore LC. The results for PL are similar to those of the human pendulum test conducted in previous studies [9, 36]. The researchers found the activation effect of wearing shoes with a soft midsole on the lower-extremity muscles had no obvious difference compared with that of wearing shoes with a hard midsole in the pre-activation stage before impact. However, the participants who wore the shoes with the hard sole had significantly increased post-activation of TA and quadriceps femoris ( $p<$ 0.05). Boyer and Nigg [10] confirmed this conclusion in their subsequent study. The participants wearing two types of shoes (with elastic or viscous midsole materials) showed no significant difference in the pre-activation of their RF, vastus medialis, and BF in the impact process, but shoe type significantly affected the time-frequency characteristics of these muscles in the post-activation stages.

In summary, the corresponding muscles in the central nervous system were appropriately activated and adjusted to certain stages during DJ, and shoe type as an additional factor did not significantly affect the characteristics of lower-extremity muscle activation and coactivation in such a context. This result is consistent with our previous finding that wearing basketball shoes cannot significantly change the impact forces and vibration performance of soft tissues during DJ [22, 37]. In addition, considering the differences among individuals, we found that the movement control of the participants and their different adaptations to the shoes reduced the variations in the results. The study also found that muscle activation of the lower extremities during PL is much less than that during DJ (Figs 2 and 4). In other words, the lack of self-regulation consciousness and proper control of muscles decreased muscle activation. LC could play an important role in substantially increasing the $\textit{EMG}_{\textit{RMS}}$ of muscle post-activation at different heights and increasing ankle muscle coactivation at specific heights. We speculate that LC causes more post-activation to cope with the impact in PL, but DJ is a max-effort movement, so the human body tries its best to recruit muscles, so the influence of shoe condition is not obvious. Overall whether shoe intervention can alter the effect of muscle recruitment at the landing impact phase to compensate for the lack of muscle activation and whether it can reduce the risk of injuries require further exploration in the future.

Several limitations of this study should be considered. First, the participants were recruited from the same university basketball team and had different training years, which may limit the external validity. Further, the study lacked the comparison of different genders. Finally, generalization and application of the findings may be treated cautiously as only two types of shoes (one mimics the barefoot condition) and three landing heights were adopted in the current study, and kinematics and dynamics should be considered.

5. Conclusion

Different landing tasks from three landing heights can significantly change the characteristics of lower-extremity muscle activation, respectively. Additionally, different shoes have a wider influence on post-activation in PL, but the effect is not significant in a maximum effort movement like DJ. Highly-cushioned basketball shoes would help reduce the risk of injuries by appropriately recruiting muscle during PL. These results suggest the shoe intervention achieved a modest change in the state of muscle activation because the participants lacked the appropriate adjustment at impacts.

Author contributions

CONCEPTION: Weijie Fu.

PERFORMANCE OF WORK: Yang Yang, Changxiao Yu and Chenhao Yang.

INTERPRETATION OR ANALYSIS OF DATA: Yang Yang, Changxiao Yu and Liqin Deng.

PREPARATION OF THE MANUSCRIPT: Yang Yang and Changxiao Yu.

REVISION FOR IMPORTANT INTELLECTUAL CONTENT: Yang Yang, Changxiao Yu and Weijie Fu.

SUPERVISION: Weijie Fu.

Conflict of interest

There is no conflict of interest. Given his role as an Editorial Board Member, Weijie Fu had no involvement nor access to information regarding the peer review of this article.

Ethical considerations

All participants were informed about the experimental procedures and requirements, and written consent was obtained before experimental data collection. The study was approved by the Institutional Review Board of the Shanghai University of Sport (date: 15/02/2017; No. 2017007).

Funding

This research was funded by the National Natural Science Foundation of China (11772201), Shanghai “Aurora Scholar” (19SG47), and Shanghai “Outstanding Young Scholar” project.

Footnotes

Acknowledgments

The authors have no acknowledgments.

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Luo

Wang

Yang

Niu

. Shoe cushioning effects on foot loading and comfort perception during typical basketball maneuvers. Applied Sciences. 2019; 9(18): 3893.

Altering muscle activity in the lower extremities by bipedal landing with different drop tasks and shoes

Abstract

BACKGROUND:

OBJECTIVE:

METHODS:

RESULTS:

CONCLUSIONS:

Keywords

1. Introduction

2. Methods

2.1 Participants

2.2 Shoe types

2.3 Data collection

2.4 Experimental protocol

2.5 Data analysis

3. Results

3.1 Muscle pre-activation

3.2 Muscle post-activation

Table 1 Effects of landing tasks and shoe types on ankle coactivation (%) at different dropping heights

4. Discussion

4.1 Landing task effects on muscle activation

4.2 Shoe effects on muscle activation

5. Conclusion

Author contributions

Conflict of interest

Ethical considerations

Funding

Footnotes

Acknowledgments

References

Table 1
Effects of landing tasks and shoe types on ankle coactivation (%) at different dropping heights