Fatigue of intermittently stimulated quadriceps during imposed cyclical lower leg movements

During prolonged experiments the influence of knee angular velocity, and stimulation parameters (interpulse interval (IPI), duty cycle (DC), number of pulses per cycle (NP)) on fatigue-induced torque decline of paralyzed human quadriceps was studied. Identification of torque-angle and -angular velocity was also performed. The overall loss of maximum torque (MT) and torque-time integral (TTI) per cycle during sustained intermittent stimulation during isokinetic movement had a typical exponential decay reaching asymptotic values. Larger knee velocities resulted in a significantly faster and relative larger decay of MT and TTI. The rate and relative magnitude of fatigue during concentric contractions are in direct relation to NP. The results may be valuable in the design of optimal control systems for FES which pursue minimization of muscle fatigue.