Is the thumb a fifth finger? A study of digit interaction during force production tasks

Abstract

We studied indices of digit interaction in single-and multi-digit maximal voluntary contraction (MVC) tests when the thumb acted either in parallel or in opposition to the fingers. The peak force produced by the thumb was much higher when the thumb acted in opposition to the fingers and its share of the total force in the five-digit MVC test increased dramatically. The fingers showed relatively similar peak forces and unchanged sharing patterns in the four-finger MVC task when the thumb acted in parallel and in opposition to the fingers. Enslaving during one-digit tasks showed relatively mild differences between the two conditions, while the differences became large when enslaving was quantified for multi-digit tasks. Force deficit was pronounced when the thumb acted in parallel to the fingers; it showed a monotonic increase with the number of explicitly involved digits up to four digits and then a drop when all five digits were involved. Force deficit all but disappeared when the thumb acted in opposition to the fingers. However, for both thumb positions, indices of digit interaction were similar for groups of digits that did or did not include the thumb. These results suggest that, given a certain hand configuration, the central nervous system treats the thumb as a fifth finger. They provide strong support for the hypothesis that indices of digit interaction reflect neural factors, not the peripheral design of the hand. An earlier formal model was able to account for the data when the thumb acted in parallel to the fingers. However, it failed for the data with the thumb acting in opposition to the fingers.

Limits to the Control of the Human Thumb and Fingers in Flexion and Extension

“…However, during the multidigit tasks, total enslavement was similar to that when the thumb acted in parallel (ϳ15% MVC; see Fig. 5 in Olafsdottir et al 2005). Furthermore, the summed force deficits in the present study were also similar to the data for the thumb acting in parallel (ϳ30% MVC here; see Fig.…”

“…Neural interactions involving drive to the long flexors of the fingers and thumb have been reported (Hockensmith et al 2005;Kilbreath and Gandevia 1994;Yu et al 2007). During a pressing task with the fingers and thumb, Olafsdottir et al (2005) reported that the degree of thumb enslavement depended on its position. There was more enslavement when the thumb was positioned opposite the fingers than when it acted in parallel with the fingers.…”

“…Despite many studies on the interaction between forces produced by the thumb and fingers (e.g., Li 2002;Santello and Soechting 2000;Sosnoff et al 2005;Yu et al 2007), only one assessed the force enslavement at the thumb in flexion (Olafsdottir et al 2005; see also Kilbreath and Gandevia 1994). There was more enslavement when the thumb opposed the fingers than when the thumb was parallel to the fingers, but the precise interaction was not given (Olafsdottir et al 2005).…”

“…There was more enslavement when the thumb opposed the fingers than when the thumb was parallel to the fingers, but the precise interaction was not given (Olafsdottir et al 2005). …”

Accurate production of time-varying patterns of the moment of force in multi-finger tasks

“…Shim et al 2004b), and involvement of the thumb (Olatsdottir et al 2005). We would like to note, however, that at least one study reported high indices of moment-stabilizing synergies during more natural grasping tasks (Shim et al 2004a).…”