A peculiar aspect of the muscular organization of the human hand is that the main flexors and extensors of the fingers are muscles that each give rise to four parallel tendons that insert on all the fingers. It has been hypothesized that these multi-tendoned muscles are comprised of functional compartments, with each finger controlled by a discrete population of motor units. The purpose of this study was to determine the force distribution across the four fingers for motor units in human extensor digitorum (ED), a multi-tendoned muscle that extends the fingers. The force distribution was assessed by spike-triggered averaging and intraneural microstimulation for 233 and 18 ED units, respectively. A selectivity index from 0 (force equally distributed across the fingers) to 1.0 (force concentrated on a single finger) was used to quantify the distribution of motor unit force across the four digits. The mean selectivity index was high for ED motor units assessed with intraneural microstimulation (0.90 ± 0.28) and was significantly greater than that obtained with spike-triggered averaging (0.38 ± 0.14). Therefore it is likely that each finger is acted on by ED through a discrete population of motor units and that weak synchrony between motor units in different compartments of ED may have contributed to the appearance of spike-triggered average force on multiple fingers. Moreover, the high selectivity of motor units for individual fingers may provide the mechanical substrate needed for highly fractionated movements of the human hand.
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