In recent studies, it is shown that gastrocnemius (GAS) muscle fiber movements in upright standing were paradoxical as compared to the movement of whole muscle length depended on the ankle joint movement. We hypothesized these muscle fiber movement depend on the position of the center of mass (COM) relative to the center of rotation of the ankle joint. The purpose of this study was to examine the change of muscle fiber length in GAS and tibialis anterior (TA) muscles while upright standing postures: swaying anterior-posterior and standing tiptoe-heel. Ultrasonography of the GAS and TA was recorded to detect the change of muscle fiber length in vivo. The center of pressure and the ankle joint angular displacements were recorded to use as visual feedback signals in two postural tasks. When subjects swayed their body in anterior-posterior direction repeatedly, the relationship between the whole muscle length and the muscle fiber length was an anti-phasic pattern in both muscles. However, when subjects stood on their tiptoe/heel repeatedly, the relationship between the whole muscle length and the muscle fiber length was an in-phasic pattern. These results suggest that the changes in the muscle fiber length of the lower leg while standing upright depend on the COM position relative to the center of rotation of the ankle joint.