Fatigue and motor redundancy: adaptive increase in finger force variance in multi-finger tasks

Abstract

We studied the effects of fatigue of the index finger on indices of force variability in discrete and rhythmic accurate force production tasks performed by the index finger and by all four fingers pressing in parallel. An increase in the variance of the force produced by the fatigued index finger was expected. We hypothesized that the other fingers would also show increased variance of their forces, which would be accompanied by co-variation among the finger forces resulting in relatively preserved accuracy of performance. The subjects performed isometric tasks including maximal voluntary contraction (MVC) and accurate force production before and after a 1-min MVC fatiguing exercise by the index finger. During fatigue, there was a significant increase in the root mean square index of force variability during accurate force production by the index finger. In the four-finger tasks, the variance of the individual finger force increased for all four fingers, while the total force variance showed only a modest change. We quantified two components of variance in the space of hypothetical commands to fingers, finger modes. There was a large increase in the variance component that did not affect total force and a much smaller increase in the component that did. The results suggest an adaptive increase in force variance by nonfatigued elements as a strategy to attenuate effects of fatigue on accuracy of multi-element performance. These effects were unlikely to originate at the level of synchronization of motor units across muscle compartments but rather involved higher control levels.

Fig. 1.

Experimental setup with the feedback for the discrete experiment and for the rhythmic experiment. Examples of typical subject performance (single trials, 4-finger tasks, before fatigue) are shown together with the presentation of the task on the screen. Note that the discrete task involved several episodes of force ramp changes within a trial. - - - and — (in the rhythmic task), the target force values and the permissible error margins.

Fig. 2.

Maximal voluntary contraction force before (☐) and during fatigue (■). *, significant differences (P < 0.05) between the prefatigue and during-fatigue conditions. Mean data across the subjects are shown with SE bars. I, index; M, middle; R, ring; L, little finger.

Fig. 3.

Indices of performance (root mean square, RMS) error in the index (I) finger and 4-finger (IMRL) tasks, discrete and rhythmic, before (☐) and during fatigue (■). Note the significant increase in force RMS during fatigue for the I tasks but not for the IMRL tasks.

Fig. 4.

Force variance [normalized to squared maximal voluntary contraction (MVC) of individual fingers] of individual fingers, averaged over the task interval, for the discrete (A) and rhythmic (B) tasks. Note that fatigue induced approximately equal changes in the force variance in all 4 fingers (☐, before fatigue; ■, during fatigue).

Fig. 5.

Two components of variance in the finger mode space computed over the discrete and rhythmic 4-finger (IMRL) tasks. A: V_UCM reflects the amount of variance that did not affect total force. B: V_ORT reflects the amount of variance that affected total force. Note the larger increase in V_UCM during fatigue. Note the 10-fold difference in the scales of the y axes in A and B.

Fig. 6.

Z-transformed index (Z_ΔV) of the force-stabilizing synergy before (□) and during fatigue (■) for the discrete and rhythmic 4-finger task. Note the increase in Z_ΔV during fatigue.

Fig. 7.

An illustration of hypothetical data point distributions for the task of accurate total force production with 2 fingers. A: before fatigue, there is a strong negative co-variation between the 2 finger forces corresponding to a force-stabilizing synergy (the ellipse of data point distribution is elongated along the uncontrolled manifold, UCM). B: the fatigued finger (F₁) shows increased force variance (V_F1). If the other finger shows an unchanged force variance (white ellipse), the projections of force variance on the direction orthogonal to the UCM (V_ORT) is expected to increase thus leading to larger variance in the total force. To avoid this effect, variance of the 2nd finger force (V_F2) also has to increase with a simultaneous increase in the index of force co-variation (the darker ellipse).