Effect of muscle length on voluntary activation of the plantar flexors in boys and men.

PURPOSE: The aim of the present study was to compare the effect of muscle length on the maximal voluntary activation level (VA) of the plantar-flexors between children and adults.

METHODS: Fourteen boys (10.0 ± 1.0 years) and fifteen men (24.6 ± 4.2 years) performed 5-s maximal isometric voluntary contractions (MVC) of the plantar-flexor muscles at seven ankle angles [from 10° in dorsi-flexion (DF) to 20° in plantar-flexion (PF); 0° = reference position; the angle between the plantar surface and leg is a right angle]. Single magnetic stimulations were delivered to the posterior tibial nerve during MVCs to determine VA.

RESULTS: Results showed a higher absolute torque of the plantar-flexor muscles at long (10° DF) than at short muscle length (20° PF) in men (89.4 ± 19.4 vs. 46.8 ± 17.0 N m, P < 0.001) and boys (44.9 ± 18.5 vs. 26.6 ± 12.8 N m, P < 0.001). On average, VA was significantly higher in men than in boys (92.4 ± 1.7 vs. 87.6 ± 1.6 %, P < 0.05). However, no significant main effect of the ankle angle was observed on VA.

CONCLUSIONS: The VA partly accounts for the plantar-flexors MVC torque difference between children and adults but is not affected by the muscle length changes in both groups. Therefore, VA cannot account for the shape of the torque-angle relationship on the plantar-flexor muscles.