Mechanisms underlying reversal of motor unit activation order in electrically evoked contractions after spinal cord injury.

Extracellular stimulation normally activates larger-diameter axons, innervating motor units producing higher force, at lower stimulation intensities than required to activate small-diameter axons innervating motor units producing low force. However, activation of weaker thenar motor units at lower stimulation intensities than required to activate strong motor units has been reported during extracellular stimulation of the median nerve in persons with chronic cervical spinal cord injury. We used a computational model that reproduced this experiment to identify the potential mechanisms for the observed reversal of the inverse recruitment order, including preferential death of large motoneurons, demyelination and remyelination, and denervation and reinnervation of muscle fibers.