<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>

<div>Stability of motor performance is important for voluntary movement control, but it should not be maximized to the exclusion of all else. To transition to a new task, the current task must be destabilized. When expecting to switch tasks, people are known to reduce their stability prior to initiating the change. Here, we determine if the observed stability modulation is influenced by the expectation of future movement, is a relic of the movements performed in the recent past, or is a consequence of both those processes. Furthermore, this work explores the relation between stability modulation observed in isometric finger force production tasks to cognitive flexibility and clinical measures of manual dexterity. Stability modulation can be viewed as a motor response to the recognition of altered environmental demands or internally generated desires to change body movements or postures. Therefore, it is hypothesized that cognitive flexibility – the efficacy of cognitive processing – will relate to stability modulation. Finally, it is hypothesized that the motor adjustments in response to changing task/environment demands will correlate with clinical tests of manual dexterity that involve placing pegs into holes.</div><div>Twenty-two young-adult participants (age 21.05 +/- 0.44 years) completed tasks in the three domains. The Grooved Pegboard and NIH 9-Hole tests of manual dexterity measured their manual function by time to complete the tests. Cognitive flexibility was measured by a task-switching task which required adjusting to a changing set of rules, and the reaction time and accuracy costs of task-switching were recorded. Lastly, participants’ stability of performance in an isometric finger-pressing task was assessed using the uncontrolled manifold analysis and root-mean-square error (RMSE) in the performance. Participants produced pressing forces with four fingers to match a single total force targets presented as feedback on a computer screen. In the ‘Steady’ task, target remained motionless. In the ‘Future Effects’ task, the target remained motionless for several seconds and then began moving. The ‘Past Effects’ task comprised of a dynamic initial portion followed by a stationary target. Lastly, the ‘Combined’ task had a constant force section flanked on either side by epochs of target movement. </div><div>The RMSE results confirmed the existence of stability modulation and established that this is driven by the expectation of future movement, and not by the history of previous movements. The Steady and Past Effects tasks exhibited higher stability than the Future Effects and Combined tasks. The stability estimates obtained from the uncontrolled manifold analysis showed similar trends. Cognitive flexibility (quantified as global accuracy cost) correlated with stability modulation indicating that individuals who show greater cognitive flexibility tend to demonstrate greater stability modulation. However, an association between stability modulation and clinical pegboard tests of manual function were not observed. This may possibly be due to the homogeneity of the test sample, or because the finger-force-production task and pegboard task measure disparate aspects of manual function. </div><div><br></div>