EFFECT OF WRIST POSTURE AND RATE OF FORCE DEVELOPMENT ON FINGER CONTROL AND INDEPENDENCE

Abstract

The anatomical structure of the extrinsic finger muscles suggests that posture may play a role in the production of enslaved forces in the fingers. This phenomenon also appears dependent on contraction conditions. The purpose of this thesis was to determine the effect of: (i) wrist posture on the enslaving effect (EE) during ramp and isotonic exertions, and (ii) the rate of force development on EE and accuracy during ramp exertions. Twelve male participants performed 3 submaximal finger flexion and extension trials with the index and ring fingers at 30° wrist flexion, neutral, and 30° wrist extension. Trials consisted of a 5 second isotonic contraction at 25% MVC (maximum voluntary contraction), and two ramp contractions. Ramp contractions were performed at 25% MVC/s and 10% MVC/s up to 50% MVC, a 0.5 second hold, and decreased to zero at the same rate. Surface electromyography was recorded from the compartments of extensor digitorum and flexor digitorum superficialis and analyzed at 25% of maximum. Wrist posture had a significant effect on EE during extension exertions (F4, 44 > 2.6, p < 0.05); specifically, higher EE, error, and muscle activity were found at shorter muscle lengths. Contraction condition significantly affected EE for both index (p = 0.001) and ring finger exertions (p = 0.001). In the fingers adjacent to the task finger, descending phase EE was higher than the ascending phase, which appeared independent of muscle activity. This thesis found that, in extension exertions, neural factors affecting EE were dependent on muscle length, while mechanical factors appeared dependent on the type of exertion. These findings further our knowledge of the complex relationship between neural and mechanical control of the hand and fingers.