Comparison of contractile properties of single motor units in human intrinsic and extrinsic finger muscles

1. The purpose of the present study was to compare the contractile properties of single motor units in the intrinsic (short) and extrinsic (long) finger muscles in awake human subjects using intraneural motor axon stimulation. 2. Twitch properties were measured for 17 intrinsic and 11 extrinsic motor units by selective stimulation of a single motor axon in the ulnar or the median nerve. Force was measured from the appropriate digit, just distal to the muscle's point of insertion and single unit EMG was recorded with surface electrodes. Force-frequency relationships (2-100 Hz) were established for 16 of these units (7 intrinsic, 9 extrinsic). Across the 16 motor units for which force-frequency data were obtained, twitch contraction time (63·7±6·ms, mean ± S.E.M.) was inversely correlated with the frequency required to generate half-maximum tetanic force (12·0±1·1 Hz). 3. We found no systematic differences between the contractile properties of intrinsic and extrinsic motor units. There was no evidence of a bimodal distribution into large/small or fast/slow units based on maximum force or contraction times, although both fast and slow motor units were encountered. 4. The peak slope of the sigmoidal force-frequency relationship for intrinsic motor units (9·1 and 4·4-12·9 mN Hz^-1, median and interquartile range) was significantly higher than that for extrinsic motor units (2·9 and 2·3-3·1 mN Hz^-1; P = 0·028), i.e. greater force steps were produced by the intrinsic motor units for a given change in stimulation frequency. This difference suggests that motor units in the modulating intrinsic muscles of the hand are more sensitive to modulating force output by changes in motoneurone firing rates than are those in the extrinsic muscles. This reflects the important role of the intrincic hand muscle in the fine manipulation of objects.