Manipulation of Muscle Force with Various Firing Rate and Recruitment Control Strategies

A computer-controlled, dual-channel neuromuscular stimulation system capable of manipulating skeletal muscle force with a wide range of action potential firing rates and motor-unit recruitment control strategies was designed and evaluated on the m. gastrocnemius muscle of the cat. The muscle force could be controlled with control strategies in which motor unit recruitment accounted for from 50 percent and up to 100 percent of the initial muscle force while firing rate induced the remaining force segment. The force response to linearly increasing recruitment was linear, whereas a saturation nonlinearity was evident in response to the firing rate input. Initial and terminal nonlinear force segments during 100 percent recruitment range were shown to be due to the viscoelastic components of the muscle fibers and their tendons. Recruitment of the muscle's motor units at rates that generated from 36 percent/s and up to 360 percent/s of the maximal force range was shown to correlate linearly to the input stimulus (R 0.9889). Reduction of the maximal firing rate from 55 to 40 pps showed that although minimization of fatigue at a cost of 10 percent reduction in the maximal force is possible, the correlation of the force response to the input signal remains high (R 0.9929) and linear. Some preliminary conclusions about rehabilitative applications were drawn based on the data presented in conjunction with previous studies.