A phenomenological model for electrical stimulation of the upper limb

Neuromuscular electrical stimulation (NMES) reanimates the paralyzed limb by using electrical currents to stimulate the motor nerves to produce a muscle contraction. The resulting limb movement is difficult to control, as the relationship between stimulation intensity and muscle force is highly non-linear and dependent on initial conditions such as joint angle and velocity. NMES systems that enable a user to perform arbitrary movements conventionally require the user to control the amount of stimulation manually, using residual functions to manipulate input devices such as a joystick. Alternatively, constant stimulation parameters are used to induce full range movements, for example, a full hand grasp/release. This paper proposes a model of the biceps and triceps muscles to determine the intensity of NMES required to flex or extend the arm by a specific angle. The muscle model is based on a phenomenological approach, with parameters that are easily measured and can thus be conveniently implemented. Preliminary trials with two healthy subjects showed that the muscle model was successful in moving the limb to within ±5° from the desired angle.