Common inputs to motor units are modulated by force profile and muscle length

Groups of motor units receive common synaptic inputs from specialized networks of premotor neurons to produce movement. Here, we test whether common inputs received by motor units in the first dorsal interosseous are fixed or task-dependent during submaximal isometric contractions. Discharge times of motor units were identified and tracked across a set of conditions. Factor analysis identified a single motor unit mode that captured most of the covariation in the discharge rates of motor units, presumably reflecting common synaptic inputs. This was supported by the high correlation values between motor unit pairs from the same participant. Next, displacement analysis on the discharge rates of motor units was computed to capture the magnitude of departures from a monotonic control structure expected from fixed common inputs. For the same force profile, activity of motor units was limited to a set of discharge rate-time trajectories across trials, suggesting a constrained pattern of premotor inputs to motor units. Importantly, changes in force profile and muscle length modulated synaptic inputs to motor units, as observed in the divergent rate-time trajectories of motor units and high displacement values. These findings highlight that common input is a key feature in the neural control of motor units and is subject to substantial modulation. The composition of inputs to motor units depends on the force profile and muscle length, and the system relies on reusing a common input structure when performing the same task.