FIRING PATTERN OF TYPE-IDENTIFIED WRIST EXTENSOR MOTOR UNITS DURING WRIST EXTENSION AND HAND CLENCHING IN HUMANS

1. Single motor unit activity was investigated in the extensor carpi r adialis muscles during voluntary isometric contraction involving eithe r the coactivation of the wrist agonist extensor muscles (wrist extens ion) or the coactivation of the wrist and finger antagonist extensor a nd flexor muscles (hand clenching). 2. The motor units were found to b e activated at a similar level of motoneurone pool drive during both w rist extension and hand clenching, as indicated by the fact that the E MG activity at which they were recruited was practically the same in b oth cases (mean +/- S.D.: 20 +/- 26 and 21 +/- 25 mV, respectively). I n addition, the net excitatory drive exerted on the motoneurones, as a ssessed from the mean interspike intervals, did not differ significant ly between the two tasks (mean +/- S.D.: 104.57 +/- 17.24 and 103.01 /- 16.26 ms, for wrist extension and hand clenching, respectively). 3. However, the discharge variability, in terms of the coefficient of va riation of the interspike intervals, was slightly but significantly gr eater during hand clenching than during wrist extension (0.213 +/- 0.0 49 and 0.198 +/- 0.045, respectively). This increase involved all type s of motor units, regardless of their contractile force. 4. We suggest that the greater motoneurone discharge variability observed during ha nd clenching may be attributable to an increase in the synaptic noise. This increase might be due to the activation of numerous afferent pat hways mediating reciprocal interactions between antagonist motoneurone pools, as well as to the activation of hand cutaneous receptors that play a major role in the regulation of handling and gripping motor act ivities.