The factors influencing ankle range of motion were investigated for 185 middle-aged and elderly subjects (116 women and 69 men, aged 48-86 years) . Each subject was seated with the right knee extended, and the ankle joint was passively dorsiflexed by a dynamometer with torque just tolerable for each subject, to measure the maximal dorsiflexion angle. During passive loading, elongation of muscle fibers in the gastrocnemius and Achilles tendon was determined in vivo by ultrasonography. There was a difference between women and men for the passive dorsiflexion angle (men smaller than women), which negatively correlated with muscle thickness of the posterior portion of the leg determined by ultrasonography. Both in women and men, the passive dorsiflexion angle negatively correlated with age, even after normalizing for maximal voluntary plantar flexion torque. Both elongation of muscle fibers and tendon was related to the passive dorsiflexion angle, and the ratio of tendon elongation to muscle fiber elongation positively correlated with the passive dorsiflexion angle. The active dorsiflexion angle, measured separately with the subject maximally dorsiflexing the ankle with no load, correlated with the passive dorsiflexion angle but not with age, and there was no gender difference. From the results it was suggested 1) that the mobility of the ankle joint is affected by elongation of both muscle fibers and tendon, but with the effect of the tendon being greater than that of muscle fibers, and 2) that muscle mass negatively affects passively-induced joint range of motion. Actively performed joint range of motion would be affected by elongation of the muscle-tendon corn plex and force-generating capability of the ankle. Gender difference in joint range of motion and the aging effect are related to these factors.