Biomechanics and Control of Torque Production During Prehension

Open Access
Author:
Gregory, Robert William
Graduate Program:
Kinesiology
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
July 11, 2002
Committee Members:
  • Vladimir M Zatsiorsky, Committee Chair
  • John Henry Challis, Committee Member
  • Mark Latash, Committee Member
  • David A. Rosenbaum, Committee Member
Keywords:
  • human
  • voluntary movement
  • motor redundancy
  • fingers
  • hand
Abstract:
The coordinated action of fingers during static tasks involving exertion of force and torque on a hand-held object was studied. Subjects were asked to keep a handle with an attachment that allowed for independent change of the suspended load (0.5-2.0 kg) and external torque (0.375-1.5 Nm) in a vertical position while applying minimal effort. Normal and shear forces were measured from the thumb; normal forces only were measured from the four fingers. Experimental results: (1) The thumb shear force increased during supination efforts and decreased during pronation efforts. (2) The total moment of the normal finger forces counterbalanced only approximately 50% of the external torque. Hence, shear forces accounted for approximately one-half of the total torque exerted on the object. (3) The total normal force increased with external torque. The total force magnitude did not depend on the torque direction. (4) The forces of the ‘peripheral’ (index and little) fingers depended mainly on the torque while the forces of the ‘central’ (middle and ring) fingers depended both on the load and torque. (5) There was a monotonic relationship between the mechanical advantage of a finger (i.e., its moment arm during torque production) and the force produced by that finger. (6) Antagonist finger moments acting opposite to the intended direction of the total moment were always observed. At low external torques, the antagonist moments were as high as 40-60% of the agonist moments. Modeling: A three-zone model of coordinated finger action is suggested. In the first zone of load/torque combinations, activation of antagonist fingers, i.e., fingers that generate antagonist moments, is necessary to prevent slipping. In the second zone, the activity of agonist fingers is sufficient for preventing slips. In the third zone, the performer has freedom to choose between either activating the antagonist fingers or redistributing the forces amongst the agonist fingers. Optimization: Optimization modeling was performed using as criteria the norms of (a) finger forces, (b) relative finger forces normalized with respect to the maximal forces measured in single-finger tasks, (c) relative finger forces normalized with respect to the maximal forces measured in a four-finger task, (d) relative finger forces normalized with respect to the maximal moments measured in single-finger tasks, and (e) relative finger forces normalized with respect to the maximal moments measured in a four-finger task. All five criteria failed to predict antagonist finger moments when these moments were not imposed by the task mechanics. Reconstruction of neural commands: The vector of neural commands [c] was reconstructed from the equation [c] = [W]-1[F], where [W] is the finger interconnection weight matrix adjusted from Zatsiorsky et al. (1998) and [F] is the vector of finger forces. The neural commands ranged from 0 (no voluntary force production) to 1 (maximal voluntary contraction). For fingers producing moments counteracting the external torque (‘agonist’ fingers), the intensity of the neural commands was well correlated with the relative finger forces normalized to the maximal forces in a four-finger task. When fingers worked to produce moments in the direction of the external torque (‘antagonist’ fingers), the relative finger forces were always larger than those expected from the intensity of the corresponding neural commands. The individual finger forces were decomposed into forces due to ‘direct’ commands and forces induced by enslaving effects. Optimization of neural commands resulted in the best correspondence between the actual and predicted finger forces. The antagonist moments were, at least in part, due to enslaving effects: strong commands to agonist fingers also activated antagonist fingers.