A characterization of changes in the dynamics of muscle contraction during prolonged electrical stimulation

Reports the use of EMG signals, obtained from surface electrodes, to track changes in the generated torque at the knee, when the quadriceps muscle in paraplegic subjects was electrically stimulated using percutaneous intramuscular electrodes. This was done under isometric conditions, during six minutes of sustained stimulation at constant amplitude, pulse width and pulse frequency. It was observed that the generated torque and the mean absolute value of the evoked EMG decrease, but that the relationship between these to quantities is approximately linear. An autoregressive model of the joint torque was identified and analyzed. It was found that the corresponding z-plane poles move outwards toward the unit circle during sustained stimulation, in a path that indicates decreasing stability, decreasing damping and decreasing natural frequency of a corresponding continuous time model