Digit force adjustments during finger addition/removal in multi-digit prehension

Abstract

We explored adjustments in multi-digit coordinated action on a hand-held object with finger addition and removal. The subjects (n= 7) kept a vertically oriented handle at rest using a prismatic grasp as if holding a glass of liquid and then either added one finger to the grasp-the index (I) or little (L) finger-or removed one finger. Three external torques were applied on the apparatus: clockwise, counterclockwise, and no torque. The individual digit forces and moments were recorded with 6-component sensors. The change in grasping force depended on the function of the manipulated finger, i.e. on whether the finger resisted external torque (torque agonist) or assisted it (torque antagonist). There was a significant increase of the grasping force when an antagonist was added or when an agonist was removed. These force increases were not necessary for slipping prevention: the normal forces prior to the manipulation were large enough to prevent slipping. All other finger manipulations exhibited no significant change in the grip force, except for the antagonist removal during the supination efforts (after removing the I finger the grasping force decreased). In contrast, the changes in the tangential force depended on the manipulated finger, not on the finger function with respect to external torque. There was a significant thumb force increase when the I finger was added or when the L finger was removed; opposite changes were seen when the L finger was added or the I finger was removed. The changes of the virtual finger (VF) tangential force were equal and opposite to the thumb tangential force alterations; these opposite changes caused changes in the moments these forces generated. The changes in the moments of the tangential forces were counterbalanced by the opposite changes in the moments of normal forces such that the total moment remained constant and the handle orientation was maintained. At the level of individual finger (IF) forces two strategies of error compensation were found: (a) local error compensation -the opposite action of the neighboring finger, i.e. force decrease in response to a force increase (finger addition), and vice versa, and (b) distant error compensation -similar action by a finger that is a torque antagonist to the manipulated finger. During the transient periods, the changes in the thumb and VF forces were simultaneous and equal in magnitude. The normal forces increased or decreased concurrently while the changes in the tangential forces were opposite in direction. The data support the existence of chain effects in the digit force adjustments to finger addition or removal. We conclude that the digit force adjustments during the object manipulation are controlled mainly in a feed-forward manner. The obtained data agree with the principle of superposition reported previously. The findings agree with earlier reports on the limited ability of CNS to organize synergies at two levels of a control hierarchy simultaneously.

Impaired and preserved aspects of independent finger control in patients with cerebellar damage

“…Removing the IF and MF induced force changes in the respective remaining finger and synchronized force changes in the RF and LF. The findings reflect previous results on IF and LF removal (Budgeon et al 2008).…”

“…In this situation, a significant change and redistribution of the individual finger forces is needed to stabilize the object against slippage and avoid residual torque that may cause tilt of the hand. This new force distribution is achieved synchronously and without delay, suggesting feedforward anticipatory adjustments (Budgeon et al 2008;Santello and Soechting 2000).…”

Transfer of Learned Manipulation following Changes in Degrees of Freedom

“…The present findings are evidence of within-hand motor equivalence that uses different fingers as different degrees of freedom. Of particular relevance to the present study are the observations that subjects can change multidigit force coordination patterns after removing or adding one finger to the grip during object hold with no disruption to the task or its mechanical requirements (Santello and Soechting, 2000;Budgeon et al, 2008). These studies showed that motor output can be reorganized within hand while performing the task with online sensing of the desired manipulation goal.…”

Is the Control of Applied Digital Forces During Natural Five-digit Grasping Affected by Carpal Tunnel Syndrome?

“…In addition, our patients with CTS showed few alterations of pair correlations with respect to the phase changes. The task-dependent patterns of five-digit grasping have been examined from the perspectives of kinematics and kinetics, and these are coordinated according to the different requirements of the tasks such as the end position or mass of an object [4,10,11,21,30,33]. During lifting and lowering, more emphasis regarding control of the digits' forces should be placed on acceleration and deceleration of objects.…”