Brain Research 924 (2002) 198–208 www.elsevier.com / locate / bres Research report Central mechanisms of finger interaction during one- and two-hand force production at distal and proximal phalanges * Mark L. Latash , Sheng Li, Frederic Danion, Vladimir M. Zatsiorsky Department of Kinesiology, Rec.Hall-267, Penn State University, University Park, PA 16802, USA Accepted 5 October 2001 Abstract In this study we used changes in the relative involvement of different muscle groups during force production at the distal (DT) and proximal (PR) phalanges to test and modify a hypothesis on the central organization of multi-finger control for tasks involving non-homologous elements in the two hands. Ten subjects produced maximal force with different finger combinations. Two symmetrical (PR / PR and DT / DT) and two asymmetrical (PR / DT and DT / PR) combinations of force application sites in the two hands were used. During one-hand tasks, higher forces were produced at the PR site. In multi-finger tasks, total peak force was smaller than the sum of peak forces in single-finger tasks by the involved fingers (force deficit). Force production by some fingers of a hand was accompanied by involuntary force production by other fingers (enslaving). Force deficit and enslaving were both higher at the PR site. Two-hand tasks were accompanied by an additional drop in the force of individual fingers, i.e., bilateral deficit (BD). When symmetrical sites of force production were used in the two hands, BD was lower for symmetrical finger groups than for asymmetrical groups. During tests at asymmetrical sites, BD was higher and did not depend on symmetry of involved finger groups. We conclude that within-a-hand force deficit and enslaving are likely to be of a central, neural origin. An earlier introduced hypothesis has been expanded assuming that excitatory projections to contralateral finger representations exist only for homologous elements (sub-synergies) of a multi-finger force production synergy, while only inhibitory projections connect non-homologous elements.  2002 Elsevier Science B.V. All rights reserved. Theme: Motor systems and sensorimotor integration Topic: Control of posture and movement Keywords: Finger; Force production; Bilateral deficit; Coordination; Synergy; Human 1. Introduction effectors producing asymmetrical actions, e.g., elbow flexion on one side and elbow extension on the other side When two homologous effectors, i.e., anatomically [51], are not accompanied by BD. symmetrical effectors on the two sides of the body, are Another phenomenon, which resembles BD, is force participating in a maximal voluntary force production deficit (FD) in one-hand multi-finger MVC pressing tasks: (maximal voluntary contraction, MVC) task, the peak force a finger produces less peak force than in its single-finger produced by each effector is typically smaller than when it MVC task [20,32,38]. FD shows an increase with the performs a similar task alone. This phenomenon of bilater- number of fingers involved in a one-hand task [32]. Finger al deficit (BD) has been observed in a variety of tasks interaction during one-hand multi-finger tasks also leads to including bilateral leg extension [21,43,49,50], elbow enslaving, that is involuntary force generation by fingers of flexion and extension [37,39,45], hand grip [39], and wrist the hand that are not explicitly required to produce force extension [23]. However, tasks that involve nonhomo- [13,18,52,53]. The origin of the phenomena of FD and logous effectors, e.g., elbow flexion on one side and knee enslaving is unclear. They have been discussed as resulting extension on the contralateral side [15], or homologous from both peripheral factors such as shared muscles and muscle compartments and inter-digit tendinous connections [18,27,32] and from a central neural organization of multi- *Corresponding author. Fax: 11-814-863-4424. E-mail address: mll11@psu.edu (M.L. Latash). finger control [26,41]. 0006-8993 / 02 / $ – see front matter  2002 Elsevier Science B.V. All rights reserved. PII: S0006-8993(01)03234-6