Adaptive control of functional neuromuscular stimulation-induced knee extension exercise

An automated system for exercising the paralyzed quadriceps muscles of spinal cord injured patients using functional neuromuscular stimulation (FNS) has been developed. It induces smooth concentric and eccentric contractions in both limbs to enable bilateral 70 degree knee extensions in an asynchronous pattern. External load resistance is applied at the ankle level to "overload" the muscles and bring about training effects. The system uses adaptive control methods to adjust FNS current output (threshold level and the ramp slope) to the quadriceps muscles to maintain performance as the muscles fatigue. Feedback control signals for limb movement and knee extension angle are used to continuously adjust the FNS current parameters so that the external load is moved through the preset zero to 70 degree angle range. Typically, the threshold current level and the FNS current increase as the muscles fatigue to maintain performance with repetitive contractions. Fatigue is defined as the inability to extend the knee to 50 percent of the 70 degree target angle. When this occurs, FNS is automatically terminated for the fatigued leg, while the functioning leg continues to exercise. The automated nature of this system appears to be advantageous as compared to a manually operated system for subject safety, convenience, and uniformity of exercise bouts. Simulated safety problems, such as hyperextension of the knee joint, open circuitry, muscle spasms, and low battery power, were successfully detected by the logic circuitry, and the system followed appropriate safety procedures to minimize risk.