Author(s): YAMAMOTO, K., YAMAZAKI, Y., KATO, R., MIYAMOTO, K., TAKAHASHI, H., YANAGIYA, T., Institution: JUNTENDO UNIVERSITY, Country: JAPAN, Abstract-ID: 2141

INTRODUCTION:
During calf-raise exercise, gastrocnemius medialis fascicles elongate and shorten in phase with the muscle–tendon unit (MTU) at low movement frequency, but with increasing movement frequency, they shift to quasi-isometric and eventually to antiphase behavior relative to the MTU [1]. Thus, the MTU exhibits movement frequency-dependent behavior. Quasi-isometric fascicle behavior is beneficial for producing high forces rapidly, due to the force–velocity relationship and the utilization of elastic energy stored in tendinous tissue. Since sprinting requires high force production within short ground contact times [2], differences in MTU behavior in response to movement frequency may be associated with sprint performance. Therefore, this study aimed to examine whether gastrocnemius medialis MTU behavior with increasing calf-raise frequency differs according to sprint performance level.
METHODS:
Twenty-four male sprinters participated in this study. The participants were divided into two groups based on their best seasonal times using the World Athletics score, with the top 50% assigned to the high-performance group and the bottom 50% assigned to the low-performance group. They performed single-leg calf-raise exercises at four frequencies (2.67, 3.00, 3.33, and 3.67 Hz) for 10 s. During each exercise, the behavior of the gastrocnemius medialis was recorded using an ultrasound apparatus. The MTU, fascicle, and tendinous tissue amplitudes were determined for each trial. The mean shortening velocity of the fascicle was calculated during plantarflexion and dorsiflexion phases, and the proportion of positive mechanical work of the fascicle with respect to that of the MTU was computed. These variables were compared between groups at each frequency.
RESULTS:
No significant differences were observed in the amplitudes of the MTU, fascicle, or tendinous tissue between the high- and low-performance groups across all frequency conditions. As movement frequency increased, mean fascicle shortening velocity increased during dorsiflexion and decreased during plantarflexion (becoming more negative, indicating greater eccentric velocity). Between-group comparisons revealed that the high-performance group had significantly higher shortening velocity at 3.00 Hz during dorsiflexion, while showing significantly more negative velocities at 3.00 Hz and 3.33 Hz during plantarflexion. The proportion of positive mechanical work performed by the fascicle relative to that of the MTU did not differ between movement frequencies or performance levels.
CONCLUSION:
During high-frequency calf-raise exercises, differences in sprint performance were not associated with the amplitudes or mechanical work of the MTU and its constituent tissues, but rather with how the fascicle shortening velocity responded to increasing movement frequency.

References
1. Takeshita et al., J Appl Physiol, 2006.
2. Weyand et al., J Appl Physiol, 2010.