Excitation-contraction fatigue during sustained electrical stimulation of paralyzed muscle

The authors consider the use of the evoked EMG (EEMG) signal that is obtained using surface electrodes, to track changes in the torque generated at the knee when the vastus-lateralis muscle in paraplegic subjects is stimulated via percutaneous intramuscular electrodes. This was done under isometric conditions, during six minutes of sustained stimulation at constant amplitude, pulse width and pulse frequency. The authors show that there are three states in muscle behavior during prolonged stimulation potentiation, fatigue and maximal fatigue. These three states can be characterized from time-domain and frequency-domain parameters of the EEMG signal. It was observed that the amplitude of the EEMG increases during potentiation, decreases during fatigue, and begins to increase during maximal fatigue. During maximal fatigue, the firing rate of motor unit action potentials decreases. In all three states, there is a high correlation between the generated torque and the EEMG.