Motor unit forces and recruitment patterns after cervical spinal cord injury.

Force was measured from triceps brachii motor units in individuals with chronic cervical spinal cord injury (SCI) and in able-bodied (A-B) control subjects using spike-triggered averaging (175 and 48 units, respectively). Eleven percent of units from the SCI population generated normal electromyograms (EMGs) but exerted no measurable force, 65% generated force comparable to the control data, while 24% were stronger than usual. Weak units probably reflect disuse. Muscle shortening, densely innervated territories, and polyphasic EMG potentials suggested strong units resulted from intact axons sprouting to reinnervate denervated muscle. Many units from SCI subjects had faster than normal contraction times (CTs). The force and CT distributions from the SCI and A-B populations differed significantly. Motor units of SCI subjects were recruited in order of increasing force output and increasing contraction time. Chronic cervical SCI therefore seems to alter the expected triceps brachii motor unit force-speed relations.