STABILITY MODULATION IN FINGER-FORCE PRODUCTION TASKS
Stability is the ability of a system to reject noise and maintain or return to the desired movement pattern and is an important feature of a motor system. In contrast, maneuverability is the ability of a system to transition between different motor states. A system that prioritizes stability inhibits its ability to transition between different motor states in a dexterous fashion. Since stability and maneuverability are opposing characteristics of a system, stability could be traded off to increase maneuverability. This study focuses on isometric finger force production, and its goals were to identify whether (1) the amount of information available about an upcoming motor transition influences the reduction in stability of total isometric force produced by the fingers, (2) stability reduction was correlated with greater maneuverability, i.e., less time for initiating a change in the total force, (3) the amount of stability reduction is correlated across tasks with different amount of information regarding the upcoming force changes, and (4) the times required to change force correlated across tasks with different informational content.
Twenty-nine young adults (17 women; age, 23.3 ± 4.3 years) participated in this study and completed three different finger force tasks. For each task, the participants modulated the total pressing force produced by the four fingers of their right hand to track a target presented on a computer screen. In each task, participants began by producing a consistent (10% of their maximum voluntary contraction, MVC) background force with their fingers. In the Steady task, the target remained stationary and participants knew the target would not move. In the Reaction Time (RT) task, the target moved randomly in the vertical direction and participants knew that this could happen at any point in time. In the Self-paced task, participants started producing a background force and then produced a quick increase in total force using a predefined target that was displayed at the beginning of the trial, and visible throughout the trial.
The uncontrolled manifold analysis was used to assess the stability of the total force during each task. This assessment was performed when the participants produced the same force (10% MVC), but expected different upcoming force changes, and had different amount of information about these upcoming force changes. This analysis yielded a stability index, and measures of the variance structure in the finger forces, computed across multiple repetitions. The reaction time and the movement time in the RT and the Self-paced tasks, respectively, was computed to quantify maneuverability.
In contrast to previous findings and our expectations, the stability index was not statistically different for the Steady, RT, and Self-paced tasks, meaning that stability of the total force was not reduced in response to the mere expectation of an upcoming change in total force. However, the stability index reduced immediately before individuals changed their total force in the Self-paced tasks, which supports findings from previous studies. The stability modulation between the Steady and RT tasks did not correlate with the RT, and the stability modulation between the Steady and Self-paced tasks did not correlate with the movement time. Therefore, this study did not reveal a stability-maneuverability trade-off in isometric finger force production tasks. The movement time for the RT and Self-paced tasks were also not correlated. However, the novel finding of this study was that participants changed stability similarly for the RT and Self-paced tasks.
Finally, the variance components obtained from the uncontrolled manifold analysis were higher in the RT task compared to the Steady task, consistent with previous reports. In fact, the increase in the performance error (greater variability in total force) while expecting to change total force in uncertain conditions (RT tasks) is the most striking and consistent result across multiple similar studies. This result indicates that despite the inconsistent results regarding the stability index, the performance of the current task (producing a constant total force) is hampered by the uncertainty and the expectation of upcoming changes in total force.
It is likely that the stability-maneuverability trade-off is not essential for young, healthy adults in manual force production tasks. Investigations that include participants across the lifespan will shed light on this relation and help identify whether it plays a salient role in understanding loss of manual dexterity with healthy aging.