INDEX FINGER POSITION AND FORCE OF THE HUMAN FIRST DORSAL INTEROSSEUSAND ITS ULNAR NERVE ANTAGONIST

In normal subjects, maximum voluntary contraction (MVC) and electrical ulnar nerve stimulation (UNS; 30-Hz bursts of 0.33 s) were systematic ally compared with regard to the forces generated in different directi ons (abduction/adduction and flexion) and at different degrees of inde x finger abduction. With a ''resting'' hand position in which there wa s no index finger abduction, UNS produced about one-half of the abduct ion force elicited by an MVC (mean ratio 51%). Qualitatively, such a d iscrepancy would be expected, because UNS activates two index finger m uscles with opposing actions in the abduction/adduction plane of torqu es: the first dorsal interosseus (FDI) and the first palmar interosseu s (FPI). The abduction forces produced by MVC and UNS were very sensit ive to index finger abduction angle: at a maximum degree of abduction, the UNS-generated force even reversed its direction of action to addu ction (with FPI dominating) and the abduction MVC declined to 37% of t hat in the resting hand position. Inasmuch as these declines in MVC- a nd UNS-generated abduction force could not be explained by a change in moment arm, the main alternative seemed to be abduction-associated al terations in FDI fiber length (analysis by previously published biomec hanical data). The FDI and PPI were further compared by application of a UNS-generated fatigue test (5-min burst stimulation), with the inde x finger kept at a ''neutral'' angle, i.e., the abduction angle at whi ch, in the unfatigued state, the forces of the FDI and FPI were in bal ance (zero net UNS-generated abduction/adduction force). There was no major difference in the fatigability of the two muscles, yet the fatig ue reactions of the two muscles were not fully identical.