Objective To investigate the influence of Tai Chi posture on knee joint load and the strategy of coordinated muscle contraction. Methods Twenty Tai Chi practitioners with more than 3 years of Tai Chi practice was recruited. By using high-speed motion capture system andthree-dimensional force plate, the adduction angle, tibial angle, joint force, joint torque of the supporting leg during the Tai Chi step lunge movement were collected, and the quadriceps and gastrocnemius muscle strength were obtained through simulation method. The muscle strength performance, recruitment mode and activation method of baseline step, toe-out step and toe-in step were compared. ResultsThe tibial angle and adduction force increased in the toe-out step but those parameters in the toe-in step remained unchanged; the strength of the medial femoris, semimembranosus, semitendinosus and lateral gastrocnemius muscle increased in the toe-out step; the external femoral muscle and medial gastrocnemius muscle increased in the toe-in step. The gastrocnemius muscle was preferentially activated than the quadriceps muscle, and the way of muscle recruitment changed. Conclusions Tai Chi posture will change the way of muscle force recruitment and affect the knee joint function. Standardized movements can be used as the reference for osteoarthritis exercise therapy and have some significance in clinical application.