The effect of fatigue on multifinger co-ordination in force production tasks in humans

Abstract

1. This study investigated the effects of fatigue, induced by production of maximal isometric force for 60 s with four fingers, upon indices of multifinger co-ordination. 2. Measurements of individual finger forces were performed during single- and multifinger maximal force production (maximal voluntary contraction, MVC) for two sites of force application, the middle of the distal or the middle of the proximal phalanxes. Two fatiguing exercises were used, involving force production at the distal phalanxes and at the proximal phalanxes. Fourteen subjects were tested. 3. The total force in four-finger tasks dropped by about 43 % when it was produced at the site involved in the fatiguing exercise. During force production at the other site, MVC dropped by 23 %. During single-finger MVC tests, force drop with fatigue was similar across all four fingers (about -25 % of their corresponding MVCs). 4. Force production by one finger was accompanied by involuntary force production by other fingers (enslaving). Enslaving remained unchanged by fatigue when measured during force generation at the site involved in the fatiguing exercise, but increased during force production at the other site. 5. The total MVC of four fingers acting in parallel was smaller than the sum of the MVCs of these fingers in single-finger tasks (force deficit). The force deficit increased with fatigue. Force-sharing patterns during four-finger tasks showed only minor changes under fatigue. 6. These results indicate that the effects of fatigue were not limited to changes in the force-generating capabilities of the muscles. In particular, fatigue could lead to a reorganisation at a neural level that defines commands to individual fingers.

Figure 1. An illustration of a possible mechanism leading to changes in force deficit under fatigue

Total force (shown by the horizontal bars) is produced by small motor units (MUs; F₁), medium-size MUs (F₂) and large MUs (F₃). Force deficit during four-finger tests prior to fatigue (D_pre) is due to the lack of recruitment of the largest MUs (cf. the upper and lower left drawings). Fatigue leads to derecruitment of medium-size MUs (upper right drawing). Absolute force deficit is expected to be constant, whereas relative force deficit is expected to increase under fatigue (D_post; lower right drawing).

Figure 2. Method

A, schematic drawing of the experimental set-up (adapted with permission from Li, 1998). ADC, analog-to-digital converter. B, schematic illustration of the protocol. Abbreviations: PROX, proximal; DIST, distal; I, index finger; M, middle finger; R, ring finger; L, little finger; IMRL, all four fingers acting together. C, a typical trial by a representative subject at a MVC test using the middle finger. The vertical line indicates the moment of maximal force production by the middle finger when all the measurements were taken. Note the simultaneous force production (enslaving) by the other fingers. D, a typical set of data recorded during a mini-session. For each test, the force of each finger is reported at the time of MVC. Numbers in bold represent the forces produced by the master fingers; other numbers are the forces produced by the slave fingers. The figure illustrates the definitions of enslaving, force deficit and sharing (see text for more details).

Figure 3. Drop in MVC induced by the fatiguing exercise

Averaged values across subjects are presented for each task, exercise site and test site of force production. Error bars show s.e. For abbreviations see Fig. 2B.

Figure 4. Force deficit in the four-finger tests

Averaged values across subjects are presented for each exercise site and test site of force production. Error bars show s.e. For abbreviations see Fig. 2B.

Figure 5. 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 very small changes across all experimental conditions. For abbreviations see Fig. 2B.