Control of finger force direction in the flexion-extension plane

Abstract

We have examined the interaction among individual finger forces in tasks that required the production of the total force by a subset of fingers in a particular direction in the flexion-extension plane. Nine subjects produced fingertip forces in a prescribed direction with a maximum voluntary contraction (MVC) effort and held the peak force for two seconds. Six finger combinations were tested, four single-finger tasks-Index (I), Middle (M), Ring (R) and Little (L)-one two-digit task (IM), and one four-digit task (IMRL). The subjects were asked to generate the finger forces in two directions, 0° (perpendicular to the surface of the transducer) and 15° toward the palm. In all task conditions, there were two experimental sessions, with and without visual feedback on the task force vector. The main findings were:1. The target direction significantly affected the constant error (CE) but not the variable error (VE) while removal of the feedback resulted in an increase in VE. 2.The direction of the forces produced by fingers that were not explicitly required to produce force (enslaved fingers) depended on the target direction. 3.In multi-finger tasks, the individual fingers produced force in directions that could differ significantly from the target direction, while the resultant force pointed in the target direction. There was a negative co-variation among the deviations of the directions of the individual finger forces from the target direction. If a finger force vector deviated from the target, another finger force vector was likely to deviate in the opposite direction. We conclude that a multi-finger synergy is involved in the control of the finger force direction.

Hierarchies of synergies: an example of two-hand, multi-finger tasks

“…Examples include the coordination of articulators during speech (Abbs and Gracco 1984), of arms during loading and unloading (Dufosse et al 1985;Paulingnan et al 1989), of joints during pointing, reaching, standing-up, and swaying (Domkin et al 2002;Freitas et al 2006;Scholz and Schöner 1999;Wang and Stelmach 1998), of muscles during standing and stepping (Krishnamoorthy et al 2003;Wang et al 2005), and of digits during pressing, grasping, and holding an object (Gao et al 2005;Santello and Soechting 2000;Scholz and Latash 1998;Shim et al 2005). Very few studies, however, have addressed synergies at different levels of an involved control hierarchy.…”

“…Very few studies, however, have addressed synergies at different levels of an involved control hierarchy. Those include the studies of digit coordination during prehension (Baud-Bovy and Soechting 2001;Gao et al 2005;Shim et al 2005;Zatsiorsky et al 2003) and of two-arm interaction during pointing (Domkin et al 2002).…”

Multi-digit maximum voluntary torque production on a circular object

“…Humans usually exert a larger normal force on an object than absolutely necessary to prevent the object from slipping (Gordon et al 1993, Johansson and Cole 1994, Hager-Ross et al 1996, Gao et al 2005a). The present study also found significantly larger individual digit normal forces than the normal forces that are required mechanically.…”

Forces and moments generated by the human arm: variability and control

“…Studies on the effects of the force direction on the force variability in multilink tasks (which is a topic of the present study) have been mainly limited to fingertip force production in the flexion–extension plane. It was observed that during the fingertip force production the target direction significantly affected the variable error of the force direction, but not the constant error (Gao et al 2005). Fingertip force direction variability was shown to be larger for the force exerted downward and towards the body as compared to other directions (Kapur et al 2010).…”