Effects of fatigue on synergies in a hierarchical system

Abstract

We investigated the effect of fatigue produced by timed maximal voluntary contraction (MVC) of the index finger of one of the hands on performance in MVC and accurate cyclic force production tasks in right-handed subjects. Based on earlier studies, we hypothesized that fatigue would produce an increase in the indices of force-stabilizing synergies in both hands as well as between the hands in two-hand tasks. Synergies were defined as co-varied adjustments of commands to fingers (modes) across cycles that stabilized total force. Fatigue caused a significant reduction in the MVC of the exercised as well as the non-exercised hand. Indices of finger enslaving (lack of individuation) increased with fatigue in both hands, although the increase was significant in the exercised hand only. In contrast to the significant effects of fatigue on MVC forces performed by the non-exercised hand, there were no comparable transfer effects on the root mean square errors during accurate force production. During one-hand tasks, both hands showed high indices of force-stabilizing synergies. These indices were larger in the left hand. Fatigue led to a general increase in synergy indices. Exercise by the left hand had stronger effects on synergy indices seen in both hands. Exercise by the right hand showed ipsilateral effects only. Smaller effects of fatigue were observed on accuracy of performance of the force-down segments of the force cycles compared to the force-up segments. For the bimanual tasks, synergies were analyzed at two hierarchical levels, two-hand (four-finger) and within-a-hand (two-finger). An increase in the synergy index with fatigue was observed at the lower (two-finger) level of the hierarchy only. We interpret the lack of effects of fatigue at the upper (two-hand) level as a consequence of a trade-off between synergies at different levels of the hierarchy. The differences between the hands are discussed within the dynamic dominance hypothesis.

Prehension Synergies During Fatigue of a Single Digit: Adaptations in Control With Referent Configurations

“…We expected the adaptations to fatigue to be consistent with our previous studies where both V UCM and V ORT increased in accurate force production tasks during fatigue with V UCM showing a larger increase (Park et al, 2012; Singh, SKM, Zatsiorsky, & Latash, 2010a; Singh et al, 2010b; Singh et al, 2012). Therefore, our second hypothesis was that indices of force stabilizing synergies would increase at both levels of the hierarchy.…”

“…Some studies have shown that in motor tasks that involve a redundant set of elements, the goal-relevant features of performance are relatively preserved during fatigue (Gates & Dingwell, 2008; Huffenus, Amarantini, & Forestier, 2006). We have also shown this to be true in accurate force production tasks (Park, Singh, Zatsiorsky, & Latash, 2012; Singh et al, 2010b; Singh, Zatsiorsky, & Latash, 2012). In the current study, we investigated the effects of exercise-induced fatigue of a single digit on motor variability and digit coordination in a static prehension task.…”

The nature of constant and cyclic force production: unintentional force-drift characteristics

“…First, we did not explicitly test the effect of fatigue on the force production. However, the force levels observed in this study are lower than those required to produce measurable fatigue (Singh et al 2012). Second, we utilized a 5-s rest period during the discontinuous tasks.…”

The CAM test: a novel tool to quantify the decline in vertical upper limb pointing movements with ageing

“…Regarding the differences found according to the side tested (dominance), studies on motor units of voluntary muscles have identified lower average firing rates, lower recruitment thresholds, and greater firing rate/force delay in the dominant hand as compared to the non-dominant hand [19]. In addition, muscle synergies and predictive mechanisms of movement provide an advantage to the dominant side [20,21]. These findings can contribute to the better performance on the dominant side in our subjects.…”