We assessed the effects of muscle forces on ankle joint kinetics during postural balance control of human boy. Nine male subjects (mean age of 25.8 yrs) participated in the experiment. An ankle joint model assumed ball and socket joint was used, which was capable of three dimensional rotations. A six-camera VICON system was used for motion analysis. Waist pulling system and force platform were adopted for forward sway and GRF (ground reaction force) measurement. We used linear optimization programs to calculate the variation of muscle forces and angular displacements of shank and foot segments. With the experimental data and linear programs, we could calculate joint reaction forces, and bone-on-bone forces. The results presented in this study give us the insights to understand the roles of lower limb muscles during postural balance control and ankle injury mechanism.