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. / Thesis / Master of Science (MSc)

<div>Manual function is a key determinant of functional independence. It is well known that manual dexterity declines with aging and negatively impacts quality of life. Therefore, much work has focused on understanding the biomechanics and motor control of manual function in general, and the action of the fingers in particular. Previous research has revealed consistent patterns of interdependence in the action of the fingers that (1) alter with age, and (2) have consequences for manual control, and thereby manual function. Most of this previous work on finger behavior quantifies finger capacities and interactions in terms of maximal forces. However, activities of daily living likely require individuals to rapidly change forces more frequently than produce maximal forces. Therefore, the present work quantifies, for the first time, finger capacities and interactions during rapid increase and decrease in finger forces, and how these quantities change with age. </div><div><br></div><div>Young and older adults performed maximal force production tasks and also tasks that required them to rapidly increase or decrease finger forces from three initial force levels using multiple combinations of the fingers of their dominant hand. The maximal finger forces and force rates, and the interdependence of the fingers (enslaving, individuation, sharing, and deficit) during both behaviors are reported in detail. Overall, similarities in finger behavior patterns obtained from maximal force and maximal force rates were observed. However, some differences are also noted, and novel findings (especially, comparison between force increase and decrease) are reported. Finally, future work that may lead to clinical applications is discussed. </div>

<p>Biomechanical and neural factors have both been suggested to contribute to the limited independence of finger movement and involuntary force production. The purpose of this study was to evaluate the degree of finger independence by examining the activity of the four compartments of extensor digitorum (ED) and flexor digitorum superficialis (FDS) using surface electromyography and involuntary force production in the non-task fingers using methods such as the “enslaving effect” (EE) and the “selectivity index” (SI). Twelve male participants performed a series of 5-second sub-maximal exertions at 5, 25, 50 and 75% of maximum using isometric isotonic and ramp finger flexion and extension exertions. Ramp exertions were performed from 0 to 85% of each finger’s maximum force with ascending and descending phases taking 4.5 seconds each with 0.5 seconds of plateau at 85%. Lower EE and higher SI (more selective force production) was found in flexion exertions compared to extension partially 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 the isotonic or ascending exertions. The selectivity index was also lower during the descending flexion and extension exertions at 25 and 50% MVC exertions. Up to mid-level forces, both finger proximity and contraction mode affects involuntary force production and muscle activation while at higher forces only finger proximity (and not the exertion mode) contributes to finger independence. The fingers were less selective at higher exertion levels (75% MVC) and all 3 exertion modes resulted in similar SI at 75% MVC in all flexion and extension exertions.</p> / Master of Science in Kinesiology