FUNCTIONAL-SIGNIFICANCE OF UPPER AND LOWER MOTOR-NEURON IMPAIRMENT INAMYOTROPHIC-LATERAL-SCLEROSIS

The objective of this study was to examine the contribution of lower m otor neuron (LMN) and upper motor neuron (UMN) dysfunction to weakness and impaired motor control in 27 patients with amyotrophic lateral sc lerosis (ALS). Isometric strength was measured by dorsiflexor maximum voluntary contraction force (MVC). LMN function was measured by tetani c force, twitch force, and the amplitude of the compound muscle action potential. UMN function was measured by the speed of rapid foot taps, the maximum rate of rise of voluntary force, and the central activati on ratio [CAR = MVC/(MVC + superimposed tetanic force)l. The results s uggest that (1) LMN loss appears to be the primary cause of progressiv e weakness in ALS; while (2) UMN impairment in ALS leads to slowing of contraction speed and rapid movements, and modest decreases in centra l activation; and (3) during 6 months of progression, LMN changes were greater than UMN changes. (C) 1998 John Wiley & Sons. Inc.