MOTOR UNIT-ACTIVITY DURING LONG-LASTING INTERMITTENT MUSCLE CONTRACTIONS IN HUMANS

Changes accompanying long-lasting intermittent muscle contractions (30 %-50% of the maximal) were investigated by tracing the activity of 38 motor units (MU) of the human biceps brachii muscle recorded from fine -wire branched electrodes. The motor task was a continuous repetition of ramp-and-hold cycles of isometric flexion contractions. During ramp -up phases a significant decline in recruitment thresholds was found w ith no changes in the discharge pattern. During ramp-down phases the u nchanged mean value of derecruitment thresholds during the task was ac companied by increased duration of the last two interspike intervals ( ISI). These findings would suggest that during fatigue development the main compensatory mechanism during ramp-up contractions is space codi ng while for ramp-down contractions it is rate coding. During the stea dy-state phases the mean value of ISI, as well as the firing variabili ty, had increased by the end of the task in most of the MU investigate d. In addition 17 recruited MU were also investigated. These units rev ealed a lower initial discharge rate and a faster decrease in the mean discharge rate with the development of fatigue. The gradual reduction of the recruitment threshold of already active MU and the recruitment of new units demonstrated an increased excitability of the motorneuro n pool during fatigue. A typical recruitment pattern (a first short IS I followed by a long one) was observed during ramp-up contractions in units active from the very beginning of the task, as well as during su stained contractions at the onset of the stable discharge of the addit ionally recruited MU.