Independence and control of the fingers depend on direction and contraction mode

Both biomechanical and neural factors are suggested to contribute to the limited independence of finger movement and involuntary force production. The purpose of this study was to evaluate finger independence by examining the activity of the four compartments of extensor digitorum (ED) and flexor digitorum superficialis (FDS) and involuntary force production in the non-task fingers using the "enslaving effect" (EE). Twelve male participants performed a series of 5s sub-maximal exertions at 5%, 25%, 50% and 75% of maximum using isometric isotonic and ramp flexion and extension exertions. Ramp exertions were performed from 0% to 85% of each finger's maximum force with ascending and descending phases taking 4.5s. EE was lower in flexion exertions likely due to the higher activity of the antagonist ED compartments counterbalancing the involuntary activation of the non-task FDS compartments. Minimal FDS activity was seen during extension exertions. At forces up to and including 50%, both EE and muscle activity of the non-task compartments were significantly higher in descending exertions than isotonic or ascending exertions. Up to mid-level forces, both finger proximity and contraction mode affect involuntary force production and muscle activation while only finger proximity contributed to finger independence at higher forces.