Fig. 1.

Left: illustration of the terminology and the changes after spinal cord injury (SCI). The drive arising from supraspinal areas is defined as voluntary drive; the voluntary drive that passes the SCI and reaches the first dorsal interosseous (FDI) motoneuron pool is referred to as SCI net effective drive, and the drive reaching the muscle fibers is referred to as muscle activation. The difference between the voluntary drive and the SCI net effective drive equals the SCI-impaired drive. Right: above the SCI, neurons (gray matter) receive normal descending and spinal input (but less ascending input, not shown), and muscle function remains intact. At the epicenter, there is neuronal death, axon disruption, and demyelination (white matter) resulting in paralysis of some (but not all) muscle fibers. The net muscle force will depend on the number of motor units that are still under voluntary control. Motoneuron pools below the SCI receive reduced supraspinal input (SCI-effective drive) resulting in reduced muscle activation (reduced motor unit recruitment and less rate modulation).