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Flexibility is often evaluated from the joint range of motion (ROM), but the mechanisms underlying gender differences in joint ROM have not been elucidated. The purpose of this study was to investigate the factors influencing the difference in ankle joint ROM between men and women with respect to the extensibility of muscle-tendon complex. Eighteen men (21-26 yr, 66.4±6.0kg, 173.7±7.4cm mean±SD) and 12 women (19-27yr, 52.9±4.8kg, 163.4±4.3cm) participated in this study. Each subject was seated with the knee extended, and the ankle joint was attached to a foot plate, by which the ankle joint was passively dorsiflexed with torque gradually increasing from zero to a value at which the passive loading to the ankle joint was just tolerable for each subject. During the passive loading, real-time ultrasonogram was taken to track the movement of MTJ (muscle-tendon junction of the gastrocnemius medialis and Achilles tendon) as the elongation of muscle belly (dMus). The change of MTC (muscle-tendon complex) length (dMTC) during the passive dorsiflexion was estimated from changes in ankle joint angle. Tendon elongation (dTen) was calculated by subtracting dMus from dMTC. There was no significant difference in normalized passive torque during passive dorsiflexion between men and women. Women were more flexible, i.e., they demonstrated greater dMTC, which was accompanied by greater dTen at lower torque levels. However, dMus was not different between men and women. It was concluded that gender difference in the joint ROM at the ankle reflects more compliant Achilles tendon in women than in men.
Kato et al. (Sat,) studied this question.