Motor unit firing rates in postpolio and control subjects during submaximal contraction

Postpolio patients have a deficit in strength recovery after isometric activity. The cause for this is unknown, but may be the result of higher motor unit firing rates during the activity, which leads to excessive fatigue of the motor units. The purpose of this study was to determine whether postpolio subjects recruited motor units at higher firing rates than control subjects. Twelve control and seven postpolio subjects were tested for maximal voluntary contraction of the quadriceps isometrically. Randomly, subjects performed isometric contractions for five 10-s periods freely against gravity (threshold) and at 20% and 40% of maximal voluntary contraction. Decompositional motor unit electromyographic analysis was used to measure motor unit amplitude, motor unit firing rate and counted number of motor units identified. Analysis was by univariate analysis of variance. Motor unit firing rate was not significantly greater (P > 0.05) in postpolio subjects than control subjects at all three levels of contraction. Thus, it is unlikely that an increased motor unit firing rate leads to the deficit in recovery of strength in postpolio subjects.