MOTOR UNIT RECRUITMENT ORDER DURING VOLUNTARY AND ELECTRICALLY-INDUCED CONTRACTIONS IN THE TIBIALIS ANTERIOR

The recruitment order of motor units (MU) was compared during voluntar y and electrically induced contractions. With the use of spike-trigger ed averaging, a total of 302 MUs with recruitment thresholds ranging f rom 1% to 88% of maximal voluntary contraction were recorded in the hu man tibialis anterior muscle in five subjects. The mean (+/-SD) MU for ce was 98.3+/-93.3 mN (mean torque 16.8+/-15.9 mNm) and the mean contr action time (CT) 46.2+/-12.7 ms. The correlation coefficients (r) betw een MU twitch force and CT versus the recruitment threshold in volunta ry contractions were +0.68 and -0.38 (P<0.001), respectively. In volun tary contractions, MUs were recruited in order of increasing size exce pt for only 6% of the cases; whereas, during transcutaneous electrical stimulation (ES) at the muscle motor point, MU pairs showed a reversa l of recruitment order in 28% and 35% of the observations, respectivel y, when the pulse durations were 1.0 ms or 0.1 ms. This recruitment re versal during ES was not related to the magnitude of the difference in voluntary recruitment thresholds between MUs. It is concluded that if the reversal of MU recruitment observed during ES is biophysically co ntrolled by differences in their nerve axon input impedance, in percut aneous stimulation at the motor point, other factors such as the size and the morphological organisation of the axonal branches can also inf luence the order of activation.