THE DYNAMIC-RESPONSE OF THE CAT ANKLE JOINT DURING LOAD-MOVING CONTRACTIONS

The dynamic response of the cat's ankle joint during load-moving activ ation of the medial gastrocnemius was determined, Sinusoidal-input osc illations of ankle plantar flexion were performed by the muscle at fre quencies ranging from 0.4 to 5 Hz against a 10-N load acting via a cab le through a pulley with a 2 cm radius, This was followed by sinusoida l muscle length changes against the same load while excluding the join t, The frequency responses of the two conditions were compared and dec omposed in terms of their relative phase and gain, and best-fit pole-z ero models to yield the dynamic model of the joint isolated from the m uscle properties, The muscle displacement transfer function M(jw) was characterized as two sets of double poles at 2.1 and 3.2 Hz, with a pa ir of zeros at 0.92, and 20 Hz, and pure time delay of 8 mS. The joint model J(jw) was obtained by adding a pole at 5 Hz and a zero at 13 Hz . It was concluded that the ankle joint acts as a lag system, introduc ing significant increase in the phase lag between stimulus input and m echanical output without affecting the frequency-dependent attenuation of gain, Average harmonic distortion was less than 5% in all cases, T his particular finding reveals that, despite its inherently nonlinear mechanical characteristics, the joint introduces no degradation in the simplified linear behavior of the muscle-joint system, This model is useful in the design of systems employing electrical stimulation to re store movement to limbs paralyzed by spinal cord injury or stroke, In this application, it is suggested that a linear systems approach may b e reasonable, but that the joint's dynamic response decreases the cont rol system design stability margins of the muscle-joint unit, The join t model J(jw), albeit obtained from a condition in which only one musc le was activated, could be used in complex conditions where several mu scles are cocontracting independently as agonists or antagonists to mo dify the torque, angle, or stiffness of the joint.