Journal of Neurology, Neurosurgery, and Psychiatry, 1990, Vol 53, 880-885
Motor units in incomplete spinal cord injury: electrical activity, contractile properties and the effects of biofeedback
RB Stein, BS Brucker and DR Ayyar
Department of Neurological Surgery, University of Miami/Jackson Memorial Medical Center, Florida.
The electrical and contractile properties of hand muscles in a selected
population of quadriplegic subjects were studied intensively before and
after EMG biofeedback. Spontaneously active motor units and units that
could only be slowly and weakly activated were observed in these subjects,
in addition to units that were voluntarily activated normally. This
suggests a considerable overlap of surviving motor neurons to a single
muscle that are below, near or above the level of a lesion. Despite the
common occurrence of polyphasic potentials and other signs of neuromuscular
reinnervation, the average twitch tension of single motor units in hand
muscles of quadriplegic subjects was not significantly different from that
in control subjects. Nor did it increase after biofeedback training that
typically increased the peak surface EMG by a factor of 2-5 times. The
percentage of spontaneously active units was also constant. The surface EMG
may be increased during biofeedback by using higher firing rates in motor
units that can already be activated, rather than by recruiting previously
unavailable motor units.
