The association of agonist muscle fascicle length and antagonist muscle flexibility on power output during human knee extension tasks.

Recent studies have reported that muscle power can be improved through stretching. However, the mechanisms underlying the power enhancement induced by stretching are not yet well understood. This study aimed to clarify the association of muscle fascicle length and antagonist muscle flexibility on muscle power output and velocity variables. Fascicle length and muscle thickness, pennation angle of vastus lateralis, knee flexor stiffness, and range of motion (ROM) were measured in American football players. Moreover, knee extension torque measurements were taken at five angular velocities (60 deg·s-1 - 300 deg·s-1), and theoretical maximum power (Pmax), maximum force (F0), and maximum angular velocity (V0) were calculated. Pearson's product-moment or Spearman's rank correlation coefficients were calculated for each variable. ROM showed a significant moderate positive correlation with Pmax. This suggests that the flexibility of antagonist muscles may influence the power output of agonist muscles. However, although muscle thickness showed a moderate correlation with F0, V0 did not significantly correlate with any of the variables. This finding suggests that other factors, such as rapid recruitment of motor units and muscle fibre composition, may play a more substantial role in torque at very high angular velocities than muscle morphology and mechanical properties.