A three-dimensional biomechanical analysis of the cat ankle joint complex:II. Effects of ankle joint orientation on evoked isometric joint torque

Muscle actions are often defined within a single anatomical reference plane . Yet animals must control posture and movement within a three-dimensional (3-D) environment responding to a 3-D array of perturbing forces. Based on information gained regarding the 3-D muscle mechanics at the cat ankle join t complex (companion paper), we decided to study how alterations in the 3-D AJC orientation might affect ankle joint postural control. We used a 6 deg ree-of-freedom force-moment sensor to assess the affect of ankle joint orie ntation on the 3-D isometric joint torques evoked by electrical stimulation of muscles crossing the ankle joint complex (AJC) in the deeply anesthetiz ed cat. An orthogonal axis system was established at the designated ankle r otation center, such that pitch (flexion-extension), yaw (abduction-odducti on), and roll (inversion-eversion) axis torques were calculated. Experiment al results suggest that both the magnitude and sign of extra-sagittal torqu es from the gastrocnemius muscles are joint angle dependent. Also, the hind limb levering system stabilizes the AJC against large yaw and roll rotatio ns away from the control position.