The effect of stretch-shortening magnitude and muscle-tendon unit length on performance enhancement in a stretch-shortening cycle.

Stretch-induced residual force enhancement (rFE) is associated with increased performance in a stretch-shortening cycle (SSC). Although the influence of different range of motions and muscle-tendon unit lengths has been investigated in pure stretch-hold experiments in vivo, the contribution to a SSC movement in human muscles remains unclear. In two sessions, 25 healthy participants performed isometric reference (ISO), shortening hold (SHO) and SSC contractions on an isokinetic dynamometer. We measured the net knee-joint torque, rotational mechanical work, knee kinematics and fascicle behavior (m. vastus lateralis) of the upper right leg. In session 1 the SHO- and SSC-magnitude was changed respectively (SHO: 50°-20°, 80°-20° and 110°-20°; SSC: 20°-50°-20°, 20°-80°-20° and 20°-110°-20°) and in session 2 the muscle-tendon unit length (SHO: 50°-20°, 80°-50° and 110°-80°; SSC: 20°-50°-20°, 50°-80°-50° and 80°-110°-80°; straight leg = 0°). In both sessions, rotational work was significantly (p < 0.05) increased in the SSC compared to the SHO contractions (in the range of 8.1-17.9%). No significant difference of joint torque was found in the steady-state for all SSC-magnitudes compared to the corresponding SHO contractions in session 1. In session 2, we found only significantly (p < 0.05) less depressed joint torque in the SSC at the longest muscle-tendon unit length compared to the corresponding SHO condition, without any differences in knee kinematics and fascicle behavior. Therefore, the physiological relevance of rFE might be particularly important for movements at greater muscle-tendon unit lengths.