Biomechanics and control of torque production during prehension

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.; 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. 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. (Abstract shortened by UMI.)...