Progress In Motor Control Abstract | 2005

MOTOR UNIT SYNCHRONIZATION IS GREATER WITHIN THAN BETWEEN FUNCTIONAL REGIONS OF BICEPS BRACHII Barry BK1,2, Pascoe MA2, Riek S1, Carson RG1, Enoka RM2 1.

School of Human Movement Studies, The University of Queensland 2. Department of Integrative Physiology, University of Colorado

Introduction Short-term synchronization is greater between pairs of motor units located within functionally distinct regions of individual extrinsic hand muscles than for pairs of motor units recorded across these regions (Keen & Fuglevand 2004; Reilly & Schieber 2004). The purpose of the experiment was to compare the level of synchronization exhibited by pairs of motor units located within and between functionally distinct regions of the long head of the biceps brachii muscle. Methods Fifteen experiments were conducted with 7 men (25 – 39 yrs). Pairs of single motor units were recorded using separate electrodes located in the lateral and medial aspects of the long head of the biceps brachii muscle. Participants were required to exert a combination of flexion and supination torques so that both motor units discharged at approximately 10 pps for ≥200 s. When motor unit recordings were sufficiently stable at the completion of this synchrony task, a series of ramp contractions with multiple combinations of flexion and supination torques were performed to characterize the recruitment thresholds of the motor units. Results Cross-correlograms were generated from the discharge trains (mean ± SD; 223 ± 37 s) of 44 motor unit pairs. The mean discharge rate for all motor units was 11.8 ± 1.9 pps and an average of 2447 ± 538 events were used to generate the cross-correlograms. Twenty-six motor unit pairs were recorded from within either the medial (12 pairs) or the lateral (14 pairs) compartments of the muscle, and 18 motor unit pairs were recorded between the two compartments. The mean common input strength (CIS) for all pairs was comparable with previous reports (0.22 ± 0.18 extra pps). Synchrony was greater (P < 0.01) for motor unit pairs located within the same region of the long head of biceps brachii (CIS = 0.28 ± 0.20) than for those between regions (CIS = 0.13 ± 0.13). Synchrony did not differ (P = 0.83) between motor unit pairs recorded from within the medial (CIS = 0.29 ± 0.27) or lateral regions (CIS = 0.27 ± 0.10). Recruitment thresholds were obtained for a subset of 13 motor unit pairs in each of the different force combinations. Although there was no clear relation between CIS and recruitment threshold, the lowest CIS values were found for motor unit pairs with the least similar recruitment thresholds. Discussion The amount of short-term synchronization is typically used as an index of the relative extent of branched common input received by motor neuron pools. The current results are consistent with previous reports in two respects: (1) motor unit synchronization is less in proximal arm muscles than has been found for hand muscles; (2) motor neurons that innervate the same functional region of a muscle receive greater amounts of common input than those that innervate different regions of the same muscle. The implied differential distributions of synaptic inputs across the two regions of the long head of biceps brachii likely facilitate independent activation of functionally distinct parts of the muscle (ter Haar Romeny 1984, 1982). Funding: Australian Research Council to RGC and SR, National Health and Medical Research Council to RGC and NIH (NS042734) to RME.