Abstract: To study muscular force distribution during the impact phase and push-off phase in running, a simulation model with 3 DOF and covering 9 muscles in the lower extremity is presented. On the basis of this model, an optimum model is built up to solve the problem of redundant muscle forces. In this model, the minimum degree of muscular fatigue is chosen as the objective function subject to dynamics constraint and muscle parameters. Simulated annealing algorithm based on the multiplier method is used to calculate redundant muscular forces. Numerical results quantitatively indicate variation rules of the 9 muscular forces in these phases and their important role in forming normal running motion. This provides a theoretical basis for muscle strength training and improvement in physical training. In addition, it can be applied to solve problems on muscular force distribution in other similar motions.