Frequency response model of the cat´s medial gastrocnemius muscle acting through the ankle joint

The effect of the cat ankle joint passive viscoelastic properties on the dynamic response of the Medial Gastrocnemius (MG) was examined under isometric and load-moving conditions. Frequency response models were constructed of the muscle-joint system during sinusoidal isometric force oscillations and constant-mass lifting movements. These responses were compared against those obtained under the same loading condition after separating the muscle from its distal joint insertion. It was found that in isometric conditions, the muscle could be modeled as a critically damped second order system with a pure time delay, with the joint adding a pole-zero combination which reduced the system bandwidth. In load-moving conditions, two sets of double poles, two zeroes and a pure time delay were used to model the muscle, with the joint adding a pole-zero combination affecting the phase with no significant influence in the gain. It was concluded that the joint introduces dynamic effects to the muscle´s response, which vary depending on the type of loading experienced by the joint