Task-specific motor units in the extrinsic hand muscles control single- and multidigit tasks of the human hand.

Movements of the hand require a precise distribution of synaptic inputs to spinal motor neurons innervating intrinsic and extrinsic hand muscles. Humans can generate complex multidigit tasks as well as separate motions of individual digits. The specific mechanisms by which the central nervous system controls multidigit and single-digit tasks on a motor neuron level remain poorly understood. We recorded synchronized three-dimensional hand kinematics and high-density surface electromyographic data from extrinsic hand muscles, including all extrinsic thumb and digit flexors and extensors. Twelve participants each performed 13 dynamic periodic single-digit flexion and extension- and multidigit grasping tasks for 45 s per task. Multidigit tasks were composed of combinations of the performed single-digit tasks. We decoded single motor unit (MU) activity from 7.8 ± 1.8 MUs (means ± SD) per task and participant in all muscles and identified MUs across tasks. For single-digit tasks, as expected, the activity of some MUs was associated with digit kinematics (task-modulated MUs), whereas other MUs discharged in a tonic way with little modulation of their discharge rate. MUs showed task-modulated activity only for one specific single digit. Moreover, a relatively small proportion of task-modulated MUs active during single-digit tasks could be identified during the multidigit grasping tasks [median 7.5%, interquartile range (IQR) 2.2%-15.0%]. Similarly, only few task-modulated MUs were identified in more than one multidigit task (median 3.6%, IQR 0%-18.4%). These results indicate a high task specificity in the control of MUs determining hand motions.NEW & NOTEWORTHY We investigated the neural control of motor units in the extrinsic hand muscles during dynamic single- and multidigit movements. We consistently found motor units that modulated the generation of flexion and extension movements of individual digits. Only a small number of motor units were active in both single- and multidigit tasks. The findings suggest high neural specificity in the recruitment and modulation of discharge rates for motor units controlling single- and multidigit hand tasks.