Influence of preactivation on fascicle behavior during eccentric contraction

Because muscle fascicle behavior affects to the force-generating capability, understanding of muscle fascicle length changes during dynamic movements is important. Preactivation may affect the muscle fascicle length changes, especially in the case of eccentric contraction. However, its influence has not been clarified. To this end, muscle fascicle behavior during eccentric contraction was compared between preactivation and no-preactivation conditions.

Seven healthy men (24.6 ± 2.2 years, 169 ± 2 cm, 68.0 ± 5.1 kg) participated in this study. An eccentric knee extension controlled by a Biodex system was adopted as the testing motion. Muscle fascicle behavior of vastus lateralis during eccentric knee extensions was compared following two conditions. In preactivation condition, isometric preactivation was conducted before initiating eccentric contraction. On the other hand, in no-preactivation condition, muscle contraction was initiated immediately after initiating the knee angle change induced by a dynamometer. The muscle fascicle length at the onset of eccentric contraction was significantly shorter in preactivation condition than in no-preactivation condition (Cohen's d = 0.98, p < 0.001) although that at the end of eccentric contraction was not different (Cohen's d = 0.08, p = 0.844). The muscle fascicle was elongated throughout the eccentric contraction phase in preactivation condition. On the other hand, muscle fascicle was shortened in the first part, and then elongated in the latter part of the eccentric contraction phase in no-preactivation condition.

The muscle fascicle is shortened even during eccentric contraction phase. However, this shortening is disappeared when preactivation is conducted.