INDEX FINGER POSITION AND FORCE OF THE HUMAN FIRST DORSAL INTEROSSEUS AND 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 systematically compared with regard to the forces generated in different directions (abduction/adduction and flexion) and at different degrees of index finger abduction. With a ''resting'' hand position in which there was no index finger abduction, UNS produced about one-half of the abduction force elicited by an MVC (mean ratio 51%). Qualitatively, such a discrepancy would be expected, because UNS activates two index finger muscles with opposing actions in the abduction/adduction plane of torques: the first dorsal interosseus (FDI) and the first palmar interosseus (FPI). The abduction forces produced by MVC and UNS were very sensitive to index finger abduction angle: at a maximum degree of abduction, the UNS-generated force even reversed its direction of action to adduction (with FPI dominating) and the abduction MVC declined to 37% of that in the resting hand position. Inasmuch as these declines in MVC- and UNS-generated abduction force could not be explained by a change in moment arm, the main alternative seemed to be abduction-associated alterations in FDI fiber length (analysis by previously published biomechanical data). The FDI and PPI were further compared by application of a UNS-generated fatigue test (5-min burst stimulation), with the index finger kept at a ''neutral'' angle, i.e., the abduction angle at which, in the unfatigued state, the forces of the FDI and FPI were in balance (zero net UNS-generated abduction/adduction force). There was no major difference in the fatigability of the two muscles, yet the fatigue reactions of the two muscles were not fully identical.