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

Abstract

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.

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