Exploring unintentional drifts in finger force production and muscle activity: a study of finger independence.

Unintentional changes in finger force during sustained exertions, known as force drifts, are thought to reflect limits in neural control of independent finger movements. Prior work has examined drifts at the level of finger force, but less is known about how these changes are reflected in muscle activity. Our study sought to quantify the phenomenon of unintentional finger force drifts in digits 2-5, performing both single-finger flexion and extension. Twenty right-handed participants (10 females, 10 males; ages 18-29) performed 30 s isometric flexion and extension exertions with digits 2-5 at 15% and 30% of their maximum voluntary contraction (MVC). Visual feedback was either continuous or removed after 10 s. Finger forces and surface EMG from flexor digitorum superficialis (FDS) and extensor digitorum communis (EDC) were recorded. Removing visual feedback produced consistent downward drifts of instructed finger force in 15% MVC and 30% MVC flexion (0.6-8.4% MVC) and 30% MVC extension (1.4-5.2% MVC). With continuous feedback, the uninstructed fingers showed small upward force drifts (0.5-3.9% MVC), which were largely abolished when feedback was removed. Changes in instructed finger force were accompanied by modest decreases in EMG, whereas upward uninstructed drifts were associated with small increases in EMG; however, these changes were distributed across compartments and not specific to individual fingers. Extension tasks revealed similar patterns to flexion tasks but only at higher force levels, suggesting that both force magnitude and direction shape the conditions under which force drifts emerge. These findings provide evidence that finger force drifts reflect real-time changes to neural constraints on finger independence.