Voluntary Dissociation of Motor Unit Activity in the Vastii Muscles.

The central nervous system controls movement with consistent activation patterns across muscles and motor units (MUs), suggesting the presence of a relatively fixed and high-dimensional number of neural constraints on voluntary actions. In the human quadriceps, the vastus medialis (VM) and vastus lateralis (VL) contribute to the knee extensor torque and are considered a synergistic pair largely activated by shared neural inputs. However, some evidence suggests that these muscles or even subregions within them can be controlled independently. We investigated whether humans can dissociate neural input to VM and VL during isometric contractions. Ten participants (six males, four females) received real-time feedback from multiple intramuscular electromyography (EMG) electrodes inserted into different regions of the VM and VL while attempting to activate each muscle or region selectively. Nine out of ten participants were able to separate VM and VL activity based on the intramuscular EMG feedback. However, MU decomposition from the intramuscular EMGs revealed that selective recruitment of a unique set of MUs was possible only within the proximal region of VM. In contrast, we found highly correlated activity between MUs in VL and distal VM. Correlation analyses confirmed that the proximal VM exhibited distinct activation profiles compared with both distal VM and VL, supporting the existence of compartmentalized control within VM. These findings demonstrate that it is possible to dissociate the activation of MUs within this synergistic muscle group during low-force isometric contractions.