Effect of wrist posture, rate of force development/relaxation, and isotonic contractions on finger force independence
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The multi-articular anatomy and inter-connections of the extrinsic finger muscles suggests that wrist posture may affect enslaved finger forces. The purpose of this study was to determine the effects of (i) wrist posture on enslaved finger forces during ramp and isotonic exertions, and (ii) the rate of force development on enslaved forces and error. Twelve men performed 3 repetitions of isometric finger flexion and extension force with index and ring fingers with the wrist in 30° flexion, neutral, and 30° extension. Trials consisted of an isotonic contraction at 25% of maximum, and two ramp contractions performed at 25% MVC/s and 10% MVC/s up to 50% MVC, returning to zero at the same rate. Electromyography was recorded from compartments of extensor digitorum (ED) and flexor digitorum superficialis and analyzed at 25% MVC. Wrist posture significantly affected enslaving effect (EE) during extension exertions (F4, 44 > 2.6, p < .05) with higher EE, error, and muscle activity of ED in wrist extension. Contraction condition significantly affected EE for index and ring finger exertions (p = .001). In fingers adjacent to the task finger, descending phase EE was higher than ascending phase, independent of muscle activity. Mechanical factors such as posture, and neural factors both contribute to enslaved finger actions.