In vivo moment generation and architecture of the human plantar flexors after different shortening-stretch cycles velocities

The purpose of this study was to examine the moment generation of the human plantar flexors and the architecture of the gastrocnemius medialis muscle during and after shortening-stretch cycles in vivo. Fourteen male subjects (30 ± 7 years, 177 ± 7 cm, 80 ± 9 kg) performed a series of electro-stimulated shortening-stretch plantar flexion contractions. The shortening-stretch cycles were performed at three constant angular velocities (25°/s, 50°/s, 100°/s), two amplitudes (15° and 25° ankle angle changes) and at two different stimulation frequencies (30 Hz and 85 Hz). The resultant ankle joint moments were calculated through inverse dynamics. Pennation angle and fascicle length of the m. gastrocnemius medialis at rest and during contractions were measured using ultrasonography. The corresponding ankle moments, kinematics and changes in muscle architecture were analysed at seven time intervals. A three-way analysis of variance (amplitude × velocity × stimulation frequency) and post-hoc test with Bonferroni correction were used to check the amplitude, velocity and stimulation level related effects on moment enhancement (α = 0.05). The results show an ankle joint moment enhancement after shortening-stretch cycles influenced by muscle architectural changes. We found 2-3% isometric ankle joint moment enhancement at steady state, 1.5-2.0 s after the shortening-stretch cycle. However, the observed alteration in muscle architecture after the imposed perturbation, could lead to an underestimation (1-3%) of joint moment enhancement due to the force-length relationship of the triceps surae. Furthermore, the enhancement observed was independent of the shortening-stretch amplitude, velocity and stimulation frequency.