Roles of Femoral Biarticular and Monoarticular Muscles in Cat Walking: A Comparative Review With Dogs, Horses, and Humans

This review applied the two-joint link model to review the functional roles of femoral biarticular, monoarticular and lower leg muscles during walking in domestic cats, with comparative insights from dogs, horses and humans. Electromyographic (EMG) data from the right hindlimb were analyzed to characterize sequential activity switches and coactivation patterns among e-series and f-series muscles. In cats, extensive stance-phase activation of both groups including M. gluteus muscles (f1), M. biceps femoris (f3) and M. vastus muscles (e2) contrasted with humans, in whom f-series activity was minimal and the gastrocnemius (f1) was primarily active during the swing phase. Cross-species comparisons revealed consistent coactivation between two biarticular muscles (e3/f3) or between biarticular and monoarticular muscles (e3/e2), indicating functional parallels in redirecting forces at the ankle. In quadruped cats, dogs and horses, persistent M. gluteus muscle (f1) activity contributed to hip extension and hindlimb propulsion, highlighting its more prominent stance-phase role compared with humans. Mapping muscle activity onto six-sector force outputs demonstrated the model's capacity to interpret directional control of locomotion via combined muscle forces. Collectively, these findings refined the two-joint link model's applicability across tetrapods and provided a foundation for testable hypotheses regarding muscular coordination, limb segment stabilization and evolutionary adaptations in tetrapod locomotion.