Nonlinear summation in muscles of normal subjects and paraplegic patients: measurements and simulation

The nonlinear summation behaviours of joint torques produced by the neuromuscular electrical stimulation of different leg muscles were investigated in neurologically intact subjects and patients with complete thoracic spinal cord injury. Pulse trains with different numbers of pulses, interpulse intervals, and pulse widths were applied via surface electrodes. Nonlinear summation was found in all the muscles of normal subjects investigated, but not in those of paraplegic patients. Whereas various muscles of normal subjects showed considerable differences in dynamics, the same muscles in paraplegic patients exhibited a uniform behaviour. The observations could also be predicted by model simulations. We conclude that some of the beneficial effects of variable frequency pulse trains discussed in the literature may not be relevant to functional electrical stimulation (FES) in patients with upper motor neurone lesions