Maximum joint range of motion is an important parameter influencing functional performance and musculoskeletal injury risk. Nonetheless, a 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; M(max)) and H-amplitude elicited at a stimulation intensity that evoked 10% M(max) were obtained. Maximal stretch was accomplished through significant muscle (14.9%; 30 mm) and tendon lengthening (8.4%; 22 mm). There were similar relative changes in fascicle length and angle, but planimetric modeling indicated that the contribution of fascicle rotation to muscle lengthening was small (<4 mm). 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 range of motion 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.