M. Lewis, K. Muller, M. Dunn, R. T. Eakin, L. D. Abraham 623.10 Effects of absolute force level and direction of changes in force on accuracy in a cyclic isometric low force pinch task M. Lewis, K. Muller, M. Dunn, R. T. Eakin, L. D. Abraham Motor Coordination Laboratory, Kinesiology and Health Education, The University of Texas at Austin Introduction Objective: To examine differences in task performance in different ranges of force and directions of force change in order to study mechanisms of fine motor control during isometric pinch force regulation. (1 - 5) Aims: Determine differences in tracking accuracy at different force levels and reversals of force change direction in a cyclic pinch force tracking task among three different force ranges, scaled to each participant’s maximum voluntary contraction (MVC), within which the participants produced cyclic repetitions of increasing and decreasing isometric pinch forces. Results Figure 2: Absolute error at 6% MVC was larger when it was the lower end of the force range in a task (valley) than when it was the higher end (peak). Figure 1: Absolute error, measured from the cursor location to the peak or valley (low end) of the target path at the moment the target ball reached that point, was larger as the magnitude of force required at that point increased. Summary of Results Across tasks, absolute error at both reversal points (peak and valley) was greater when the force levels required by the task were greater. (Figure 1) In each task, participants were more accurate (the tracking cursor was closer to the target ball) when the target ball was at the highest force level for that task (the peak) than when the target ball was at the lowest force level for that task (the valley). (Figure 1) However, participants were less accurate at 6%MVC when that force level was at the low point (valley) of a force range than when it was at the high point (peak) of a force range. (Figure 2) There were no significant differences in the absolute error scores between the thumb and index finger. Methods Conclusions Participants: 30 right-handed, healthy volunteers (6 males, 24 females) ages 18-30 years (mean = 22.0, SD = 2.9) Task: Manipulate a computer screen cursor with isometric pinch forces produced by the thumb and index finger of the right hand up and down a 45-degree diagonal line (Figure 4) to continuously match a track ball cycling between two force levels four times each trial. Independent analog force data controlling the cursor location were collected @ 1kHz from both thumb and index finger and compared to the moving target location using our MFQS apparatus (Figure 3) Each participant performed: Pre/post isometric pinch maximal voluntary contractions (MVC) to normalize testing force levels and to rule out fatigue Ten test trials at each of three force ranges, in counterbalanced order: 3% – 6%MVC, 6% – 12%MVC, and 9% – 18%MVC Dependent Measure: Absolute tracking error – absolute distance from tracking cursor to target ball measured instantaneously at each reversal point of the target ball (each peak and each valley in the repeated cycling of the target ball up and down the diagonal path. The thumb error score measured the horizontal difference between the cursor and the target; the index finger error measured the vertical difference between the cursor and the target. These data are consistent with previous reports that force production error magnitude tends to increase as absolute target force magnitude increases. They are also consistent with previous reports that producing very low force levels accurately and consistently may be particularly difficult. The finding that accuracy is greater when the target force level is reversing direction from increasing to decreasing (at a peak) than when the target force is at the same level but is reversing direction from decreasing to increasing (at a valley) is interesting. This finding suggests that control of low-level fine motor force production may involve, in addition to neural factors such as recruitment and firing frequency of motor units, complex muscle mechanical factors, such as stretch in elastic components and excitation-contraction and relaxation dynamics, when tasks require reversals of direction of changing force levels. Index Finger Index Finger Thumb Thumb Apparatus References Harbst K.B., Lazarus J.C., & Whitall J. (2000). Accuracy of dynamic isometric force production: The influence of age and bimanual activation patterns. Motor Control, 4, 232–256. Francis K.L., Macrae P.G., Spirduso W.W., & Eakin T. (2012). The effects of age on precision pinch force control across five days of practice. Current Aging Science, 5, 2-12. Knight C.A. & Kamen G. (2007). Modulation of motor unit firing rates during a complex sinusoidal force task in young and older adults. Journal of Applied Physiology, 102, 122-129. Masumoto J. & Inui N. (2010). Control of increasing or decreasing force during periodic isometric movement of the finger. Human Move Science, 29, 339-348. Park, S. (2012) Effects of Varying the Force Levels and Direction of Force Change on Accuracy and Force Variability in a Cyclic Isometric Pinch Force Tracking Task. Unpublished masters thesis, The University of Texas at Austin. Figure 3: MFQS apparatus for measuring thumb and index finger pinch forces independently to control cursor position Figure 4: The task target path and turn around points (peak and valley).