Although electromyographic (EMG) activities of the gastrocnemius muscle (GAS) increase while a body sways voluntarily and rises on tiptoe from a standing position, the effects of movement speed on the upright postural task-dependent GAS fibers are unclear. The present study was to demonstrate GAS fiber movement during anterior-posterior bodily swaying and tiptoe-heel standing tasks using a time series of ultrasound images. Subjects achieved two postural tasks, each under three movement frequencies, 0.1Hz, 0.2Hz, and 0.3Hz, while sinuous-like modulations of ankle angular displacement patterns were recorded. The GAS fiber movements revealed by the ultrasound images were analyzed with respect to ankle angular displacement and movement velocity. The results showed that GAS fibers shortened when the body was in an incline position while swaying back and forth and standing on tiptoes. The amplitude of the GAS fiber movement increased with the increase of peak-to-peak ankle angular displacement and was less dependent on joint angular velocities. These results suggest that the roles of shortened GAS fiber movements are to produce ankle plantarflexion torque to prevent a fall while in an upright posture. Further, increased electromyography levels of the GAS reflect the effect of voluntary motor commands since GAS fiber movement would follow geometric constraints, such as ankle angular displacement.