This paper proposes a new method of muscle force estimation based on the relationship between the muscle space and the task space using a general lower extremity model in the sagittal plane. In addition, we report a verification experiment using several active motion patterns with a training robot for the lower extremities. The results show that the muscle forces estimated by the proposed method exhibit good correlation with surface electromyograms. Moreover, it is confirmed that the proposed method can capture some muscle activities undetectable by conventional methods.