Author(s): YOKOTA, K., TAMAKI, H.2, Institution: NATIONAL INSTITUTE OF FITNESS AND SPORTS, Country: JAPAN, Abstract-ID: 1796

INTRODUCTION:
Introduction: Pre-activation is a feedforward, pre-landing muscle activation that occurs immediately before ground contact and contributes to stiffness regulation. Previous study has proposed that stiffness tuning is task-specific, differing between drop jumps (DJ) and landings [1], which implies distinct feedforward control strategies depending on task demands. The study investigated task- and phase-specific neuromuscular strategies in the triceps surae (lateral gastrocnemius: LG, medial gastrocnemius: MG, and soleus: Sol) during DJ and Landing.
METHODS:
Methods: Eight healthy male sprinters performed DJ and Landing from a 30-cm platform. Surface electromyography (EMG) signals were recorded from the LG, MG, and Sol, and ground reaction forces (GRF) were collected. EMG signals were normalized to the maximal EMG amplitude obtained during maximal voluntary contraction (MVC). Mean EMG was calculated for two-time windows relative to initial contact: 30 ms before contact (pre30) and 30 ms after contact (post30). Mean EMG values were compared between DJ and Landing using paired t-tests (two-tailed), with statistical significance set at p < 0.05.
RESULTS:
Results: During DJ, mean EMG (%MVC) in pre30 was 19.73 ± 7.21 for LG, 30.41 ± 10.06 for MG, and 27.37 ± 11.23 for Sol, and mean EMG in post30 was 32.34 ± 12.37 for LG, 36.69 ± 7.19 for MG, and 41.44 ± 17.52 for Sol. During Landing, mean EMG in pre30 was 15.36 ± 3.14 for LG, 31.54 ± 11.90 for MG, and 14.86 ± 4.62 for Sol, and mean EMG in post30 was 8.19 ± 7.99 for LG, 12.50 ± 8.58 for MG, and 18.57 ± 15.02 for Sol. In pre30, Landing showed lower activation than DJ in Sol (p < 0.05) and a similar tendency in LG (p = 0.08), whereas MG did not differ between tasks (p > 0.05). In post30, EMG was significantly lower during Landing for all muscles (LG and MG: p < 0.001; Sol: p < 0.05).
CONCLUSION:
Conclusion: These findings demonstrate task- and phase-specific modulation of triceps surae activity during DJ and Landing. Compared with DJ, Landing was characterized by reduced pre-activation in Sol (and a trend in LG) and markedly lower post-contact activation across all muscles, suggesting a task-dependent shift toward a more reduced stiffening immediately after ground contact. Importantly, the modulation was not uniform across synergists, indicating muscle-specific contributions within the triceps surae to task-specific stiffness regulation.
Reference: 1. Dyhre-Poulsen, P et al., (1991) J Physiol, 437, 287-304,