Electrical stimulation of skeletal muscle—A study of muscle as an actuator

In many forms of disability in humans due to paralysis, it appears possible to regain some measure of function through direct electrical stimulation of paralysed muscles. The work described in this paper represents some of the first steps taken in a broad research program toward controlled electrical stimulation of muscles in human beings. From an engineering point of view, it is important to characterize the muscle as an operational element. This means that the force and movement responses to input stimuli must be analysed and modeled so that effective control systems can be designed. Such analytic models must take into account physiologic factors such as metabolic capability, maintenance and regeneration of muscle tissue, and comfort and fatigue factors. Most of the data reported herein have been obtained from normal human beings. In general, comfort and pain have been used to define limits which are assumed to be equally applicable to involved persons. Initial studies have been directed to determine the nature of electrical signals which produce the maximum stimulation effects with minimum discomfort and pain. Constant current stimulation was chosen as an optimum mode. Studies were made to determine the nature and location of motor points. Plots showing isometric torque developed about the elbow as a function of stimulation frequency and intensity have been made. Single muscle twitch responses were recorded and a mathematical model derived for a single muscle twitch. Tetanic torque was predicted from single muscle twitch response. A mathematical model for a total muscle which is consistent with the concept of a summation of muscle twitches to produce tetanic contraction has been proposed.