Role of intertendinous connections in distribution of force in the human extensor digitorum muscle

The human extensor digitorum (ED) muscle gives rise distally to multiple tendons that insert onto and extend digits 2-5. It has been shown previously that the spike-triggered average forces of motor units in ED are broadly distributed across many tendons. Such force dispersion may result from linkages between the distal tendons of ED and may limit the ability to move the fingers independently. The purpose of this study, therefore, was to determine the extent to which the connections between tendons of ED distribute force across the fingers. Stimulation of ED muscle fibers was performed at 107 different sites in four subjects. The isometric force exerted on digits 2-5 resulting from the stimulation was measured separately. Stimulus-triggered averaging of each of the four force channels yielded the force contribution to each of the digits due to the stimulation at each site. A selectivity index from 0 (a site that distributes force equally across the fingers) to 1.0 (a site that produces force on a single finger) was computed to describe the distribution of force across the four fingers. The selectivity index resulting from electrical stimulation of ED averaged 0.70 +/- 0.21. These selectivity index values were significantly greater (P < 0.001) than those obtained for single motor units using spike-triggered averaging. These findings suggest that linkages between the distal tendons of ED probably play only a minor role in distributing force across the fingers and, therefore, other factors must be primarily responsible for the inability to move the fingers independently.