MOTOR UNIT-ACTIVITY DURING ISOMETRIC AND CONCENTRIC-ECCENTRIC CONTRACTIONS OF THE HUMAN FIRST DORSAL INTEROSSEUS MUSCLE

1. Motor unit activity was recorded with intramuscular fine wire elect rodes during isometric, concentric, and eccentric activity in the huma n first dorsal interosseus muscle. Twenty-one units from 11 subjects w ere sampled. 2. During isotonic cycles of shortening and lengthening, 18 of 21 units were recruited during the concentric phase, increased t heir discharge rates as the concentric movement progressed, then decre ased their discharge rate during the eccentric phase, and were derecru ited. 3. A different pattern of recruitment was observed in recordings from three units. These units were recruited during the eccentric pha se, at a time when other units were decreasing their discharge rate or being derecruited. in two of the units selectively recruited during t he eccentric phase, it was possible to determine their isometric thres holds, which were higher than those of units exhibiting the more commo n pattern of recruitment. 4. For two of the three units exhibiting sel ective recruitment during eccentric contraction, the unit was recorded simultaneously with different pairs of recording wires separated by 5 -10 mm. Each discharge of these units was detected by both electrodes, making it unlikely that movement artifact was responsible for the ini tiation or cessation of discharge. 5. The recruitment patterns observe d suggest that changes in the type or distribution of synaptic inputs to motoneurons during movement can, in some instances, override pre- a nd postsynaptic factors that shape recruitment order in isometric cond itions.