Neuromuscular factors influencing the maximum stretch limit of the human plantar flexors

Maximum joint range of motion is an important parameter influencing functional performance and musculoskeletal injury risk. Nonetheless, complete description of the muscle architectural and tendon changes that occur during stretch and the factors influencing maximum range of motion is lacking. We measured muscle-tendon elongation and fascicle lengthening and rotation sonographically during maximal plantar flexor stretches in 21 healthy men. Electromyogram (EMG) recordings were obtained synchronously with ultrasound and joint moment data, and H-reflex measurements were made with the ankle at neutral (0°) and dorsiflexed (50% maximal passive joint moment) positions; the maximum H amplitude (normalized to maximum M-wave amplitude; Mmax) and H-amplitude elicited at a stimulation intensity that evoked 10% Mmax were obtained. Maximal stretch was accomplished through significant muscle (14.9%; 30mm) and tendon lengthening (8.4%; 22mm). There were similar relative changes in fascicle length and angle, but planimetric modelling indicated that the contribution of fascicle rotation to muscle lengthening was small (<4mm). Subjects with a greater range of motion showed less resistance to stretch and a greater passive joint moment at stretch termination than less flexible subjects, i.e. greater stretch tolerance. Also, greater fascicle rotation accompanied muscle elongation (9.7 vs. 5.9%) and there was a greater tendon length at stretch termination in more flexible subjects. Finally, a moderate correlation between the angle of EMG onset and maximum ROM was obtained (r=0.60, p<0.05), despite there being no difference in H-reflex magnitudes between the groups. Thus, clear differences in the neuromuscular responses to stretch were observed between 'flexible' and 'inflexible' subjects.