In a recent study, it was shown that medial gastrocnemius (GAS) muscle fiber movements while Standing upright were paradoxical compared to anatomical demands, which suggested that this movement is involved in ankle support torque. In this respect, we hypothesized that the GAS movements while standing upright depend on the positions of the center of mass (COM) relative to the center of rotation of the ankle joint The objective of this study was to investigate changes in the GAS fiber length while subjects stood upright performing one of two postural tasks: swaying their bodies or standing on their tiptoes or heels. Ultrasonography of the GAS was recorded to detect the changes in the GAS fiber length in viva In addition, 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 an anterior-posterior direction, the GAS fiber lengths were shortened, whereas they were lengthened while standing on their tiptoes or heels. Namely, the length of the GAS fiber changes when the body is swaying, which demonstrates an anti-phasic pattern within the dorsiflexed ankle, whereas, when subjects were on their tiptoes or heels, the movement was in-phase. The results suggest that the changes in the length of the GAS fiber while Standing upright depend on the COM position relative to the center of rotation of the ankle joint.