Effects of varying force levels and combinations of force application and release during an isometric pinch force task
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Fine motor control is important for the completion of many activities of daily living, such as writing, eating, buttoning a shirt, and texting. All these tasks require a high amount of continuous coordination and regulation of increasing and decreasing forces between multiple digits using sub-maximal force levels to successfully accomplish the task. Thus understanding the coordination of force regulation by the thumb and index finger at these sub-maximal force levels is a relevant topic especially for rehabilitation and instrumentation. This study was designed to investigate how accuracy and smoothness of performance of an isometric pinch force tracking task is affected by changing the level of forces required to perform the task and by different combinations of application and release of force by the thumb and index finger. Twenty two healthy, right handed adult participants between the ages of 18-30 were asked to manipulate a cursor to track a moving target ball counterclockwise around a prescribed path using the thumb and index finger of the right hand only. The goal of the task was to keep the cursor as close as possible to the moving target throughout the entire trial. Each participant was given 50 practice trials: 25 at 24% MVC and 25 at 12% MVC. For the 40 experimental trials, participants returned 24 hours later to complete 10 trials at each of the following force levels: 4%, 8%, 16%, and 32%. RMSE and CVE were calculated for each digit (thumb and index finger) as well as the combined digits and were used as indicators of accuracy and smoothness, respectively. Results showed significant differences in all dependent variables with p-values less than 0.05. Task performance accuracy was found to decrease as force level increased, whereas smoothness was found to decrease as force level decreased for all three. These findings suggest that varying force levels and combinations of force application and force release can change performance of this fine motor task and should be further investigated in order to better understand mechanisms involved and for implementing new designs of equipment and diagnostic tools.