Unravelling neuromechanical constraints to finger independence

Intentional use of a single finger results in involuntary forces and movements among other fingers. Constraints to finger independence are attributed to both neural and mechanical factors, but the contribution of these factors is debated. We hypothesized that neural factors primarily constrain finger independence during isometric exertions whereas mechanical factors impose larger constraints during movements. We investigated changes in finger independence following a ring finger fatigue protocol. We assumed that with fatigue, the ability to actively transmit forces across fingers through neural pathways will be reduced but force transmission passively through mechanical pathways will remain unaffected. Participants performed isometric finger contractions and flexion-extension movements at baseline and following a ring finger fatigue protocol. At baseline, involuntary ring finger forces ranged from 7.3 to 16.5% MVC. Consistent with our predictions, involuntary ring finger forces decreased by 2.5-8.9% MVC following fatigue. In contrast, involuntary ring finger movement did not change or surprisingly in several cases, increased by greater than 10-20° following fatigue relative to baseline across movement tasks. Our findings demonstrate that the neuromechanical control of finger force versus motion are distinct from each other and can alter the constraints to finger independence in a task-dependent way.