Biomechanical effects of squatting movements in Tai Chi on the knee joint

Squatting is essential for daily activities but may risk knee injury due to excessive loads. Tai Chi squatting (TCS), characterized by slow, controlled movements, is hypothesized to reduce joint load compared to standard squatting (SS), though biomechanical comparisons remain scarce. This study aimed to biomechanically compare TCS and SS, with a focus on knee joint kinematics, kinetics, muscle activation, and internal stress distribution.

Twelve experienced Tai Chi practitioners (6 males, 6 females) participated in this study. Participants performed TCS and SS in a controlled laboratory setting. Three-dimensional kinematics were captured with a VICON system, ground reaction forces were measured using AMTI force platforms, and muscle activation was recorded via surface electromyography (sEMG) using the Noraxon Ultium EMG system. The data were processed with an OpenSim musculoskeletal model, and finite element analysis was conducted using Ansys SpaceClaim and Ansys Workbench to evaluate internal knee joint stress distributions.

(1) Kinematics and Kinetics: Compared with SS, TCS produced a markedly smoother knee flexion-extension angle time profile (p < 0.001). TCS elicited significantly greater peak knee extension moments compared with SS (p < 0.001), particularly in male participants, and was associated with significantly elevated activation of the biceps femoris and sartorius, reflecting a shift toward posterior chain co-activation, resulting in a more balanced quadriceps-to-hamstring co-activation pattern. This tendency was particularly pronounced in female participants (p < 0.05). (2) Finite Element Stress Analysis: While the absolute difference in peak Von Mises stress on the femoral cartilage between SS and TCS was modest, TCS produced a qualitatively more homogeneous stress distribution across the articular surface, with the focal high-stress concentration pattern observed under SS notably attenuated. This improvement in load distribution morphology may carry greater functional relevance for long-term cartilage health than peak stress magnitude alone.

TCS promotes more balanced muscle co-activation and a more homogeneous intra-articular stress distribution compared with SS, potentially reducing the risk of localized cartilage fatigue, particularly in female practitioners. These findings support the integration of TCS into rehabilitation and conditioning programs.