DIRECTIONAL FORCE CONTROL OF THE THUMB

Laurel Kuxhaus, Daniel A. Harkness, and Zong-Ming Li

Hand Research Laboratory, Departments of Orthopaedic Surgery and Bioengineering University of Pittsburgh, Pittsburgh, PA, USA

E-mail: zmli@pitt.edu

INTRODUCTION A well-controlled thumb requires successful modulation of force magnitude and application direction via muscle synergy, (Kaufman et al., 1999, Valero-Cuevas, 2000) joint-torque balance, and cortical regulation (Georgopoulos et al., 1992). We investigated the control of submaximal thumb forces in eight directions and predicted that this control is influenced by and aligned with the target direction. METHODS Eleven self-reported right-handed male volunteers indicated their informed consent via a form approved by the University of Pittsburgh’s Institutional Review Board. The subjects were 26±3.5 (mean±SD) years and free of upper extremity trauma or disorders. We splinted the right thumb’s interphalangeal joint and standardized the arm position. An aluminum ring around the thumb’s proximal phalanx coupled it to a six-degree-of-freedom force/torque sensor (Mini40, ATI, Apex, NC). Subjects produced maximal forces in eight different randomly-presented directions spanning the dorsal-ulnar plane of the thumb. We provided visual feedback about force production in the dorsal-ulnar plane via custom LABVIEW (National Instruments, Austin, TX) software and collected force data at 100 samples-per-second. We then asked subjects to produce and hold 25% of their maximal force in each direction for sixty seconds. The target force was

displayed with the subject’s current force production. Two minutes’ rest was required between trials. To eliminate ramp-up and down effects, only the center fifty seconds of data were analyzed. Custom MATLAB (The Math Works, Natick, MA) routines expressed the force data relative to the target force in the radial and tangential directions (Figure 1), fit an ellipse to each force cluster, computed its major and minor axes relative to the target radial direction, and computed the force variations in the radial (force magnitude) and tangential (directional control) directions and their ratio. Repeated-measures ANOVAs using SPSS (SPSS Inc., Chicago, IL) assessed the significance of target direction on the radial and tangential force variations and on the major axis orientation (α=0.05). RESULTS AND DISCUSSION Thumb forces closely matched the target forces (Figure 1). Mean radial and tangential variations ranged from 0.49N to 1.1N (Table 1). Direction influenced the relative radial and tangential variations (p<0.02). For example, the variation in the tangential direction was half the radial in extension and flexion (#2, #6, Table 1, tan/rad), and less extreme for other directions. The orientation of each cluster’s major axis relative to its target is influenced by target direction (p<0.004) (e.g. #6 only deviates -0.34±4.3° from its target direction). In addition, the major and minor

axis lengths were significantly different (p<0.001). A longer major axis (#2, #6, Figure 2) indicates different directional control than magnitude control, while comparable lengths of major and minor axes (#0, #4) indicate similar variations in all directions and more homogeneous control around the target. Radial variation

dominance agrees with the idea that muscle activation patterns for submaximal forces are scaled versions of those for maximal forces (Valero-Cuevas 2000). However, the similar radial and tangential variations (#0, #4) suggest modulation of both the activation level of and the synergy among the muscles. The general alignment of the major axes with the target direction suggests that there may be distinct control signals at the cortical level to regulate forces (Georgopoulos et al., 1992). In conclusion, we have shown that the controllability of submaximal thumb forces changes with the target direction. Our future work will examine the temporal force variation, correlate muscle coordination patterns with our results, explore the role of feedback on force control, and study the effects of peripheral neuropathies on the ability to control thumb forces. REFERENCES Georgopoulos, A.P., et al. (1992). Science,

19(256),1692-1695. Kaufman K.R., et al. (1999). Clin. Biomech.,

14(2),141-50. Valero-Cuevas, F.J. (2000). J

Neurophysiol., 83, 1469-1479. ACKNOWLEDGEMENTS This material is based upon work supported under a National Science Foundation Graduate Research Fellowship (LK) and the Whitaker Foundation (ZML).

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Figure 1: Force clusters from one representative subject. Numerals indicate the eight directions. Inset shows the radial and tangential directions, the major axis and its angle relative to the target direction.

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Figure 2: Major and minor axes for each force cluster.

Table 1: Force variations. Mean (SD).

01234567

radial variation (N)

tangential variation (N)direction

0.65 (0.16)0.83 (0.27)1.0 (0.25)

0.75 (0.26)0.65 (0.19)0.91 (0.27)

1.1 (0.36)0.99 (0.26)

0.92 (0.27)0.61 (0.18)0.49 (0.16)0.74 (0.22)0.80 (0.21)0.77 (0.32)0.50 (0.19)0.73 (0.16)

tan/rad1.4 (0.19)

0.75 (0.16)0.49 (0.10)1.0 (0.20)1.2 (0.15)

0.83 (0.15)0.45 (0.069)0.76 (0.12)