The effect of a fatiguing exercise by the index finger on single- and multi-finger force production tasks

Abstract

We studied the effects of fatigue, induced by a 60-s maximal isometric force production with the index finger, on multi-finger coordination and force production by the other fingers of the hand. Finger forces were measured during single- and multi-finger maximal voluntary force production (MVC) at two sites, the middle of the distal or the middle of the proximal phalanges. Two fatiguing exercises involving force production by the index finger were used, one at the distal phalanx and the other at the proximal phalanx. The MVC of the index finger dropped by about 33% when it was produced at the site involved in the fatiguing exercise. In addition, large transfer effects of fatigue were observed across sites of force application and across fingers. Force deficit increased under fatigue, especially due to a drop in the recruitment of the index finger. Under fatigue, the index finger was less enslaved during force production by other fingers. During multi-finger tasks, the percentage of total force produced by the index finger was significantly reduced after the fatiguing exercise. The principle of minimization of secondary moments was violated under fatigue. We suggest that the most impaired (fatigued) finger shows less interaction with other fingers or, in other words, is being progressively removed from the multi-finger synergy. Some of the observed changes in finger coordination suggest effects of fatigue at a central (neural) level.

Fig. 1

A The experimental setup (adapted with permission from Li et al. 2000). B Side view of the hand fixation and finger configuration. The hand is clamped and the forearm is strapped to a solid board (adapted with permission from Li et al. 2000). C A typical set of data recorded during a mini-session (adapted with permission from Danion et al. 2000). For each test, the force of each finger is reported at the time of maximal voluntary contraction (MVC; in newtons). Numbers in bold represent the forces produced by the master fingers, and the others are the forces produced by the slave fingers. See text for more details of enslaving, force deficit, and sharing (I index finger, M middle finger, R ring finger, L little finger, IMRL all four fingers acting together)

Fig. 2

Drop in MVC induced by the fatiguing exercise. A Averaged across subjects, values are presented for each task, exercise site, and test site of force production. Error bars show the standard errors. B Averaged across subjects, values are presented for each finger depending on the nature of the task (single- or multi-finger exercise). Data for tests performed at the site of the fatiguing exercise. Stars indicate significant differences in force loss between multi- and single-finger tests. (Prox. Proximal, Dist. distal, I index finger, M middle finger, R ring finger, L little finger, IMRL all four fingers acting together)

Fig. 3

Force deficit in the four-finger tests. Averaged across subjects, values are presented for each exercise site and test site of force production. Error bars show the standard errors of the means. For abbreviations, see Fig. 2

Fig. 4

Force-sharing patterns in the four-finger tests. Averaged values across subjects are presented for each exercise site and test site of force production. Note the reduced share of the index finger across all experimental conditions. For abbreviations, see Fig. 2