Dynamic performance model of an isometric muscle-joint unit

The dynamic performance model of the medial gastrocnemius muscle of the cat was determined when generating isometric force at its tendon and when transmitting that force across the joint. The frequency response model of the muscle and of the muscle-joint was developed by fitting the experimentally obtained gain and phase Bode plots with a best fit linear second order system determined by recursive least squares. It was shown that the muscle could be represented with double poles at 2.3 Hz and a time delay of 16 ms whereas the muscle-joint was represented with an additional pole at 1.8 Hz, a zero at 3.8 Hz and 16 ms time delay. The harmonic distortion was less than 5% for sinusoidal force output in the frequency range of 0.4-4 Hz, and a force range of 20-80% of the maximal justifying a linear system model. The model is useful in the design of a neuromuscular prosthesis, using electrical stimulation of the muscle nerves, as a rehabilitation procedure for paralysed patients due to spinal cord injury.