An examination of the cross-over effects of fatigue with and without mirror visual feedback

Abstract

Skeletal muscle fatigue is a multifactorial process that leads to decrements in the force generating capacity of the neuromuscular system. During a maximal unilateral and bilateral contraction, complex interactions occur along the length of the neuromuscular system. These interlimb limb interactions occur with and without the presence of fatigue, however, it is unknown whether illusionary mirror visual feedback moderates the effects. The primary purpose of this study was to examine the influence of unilateral fatigue, with and without illusionary mirror visual feedback, on the maximal force of the fatigued and non-fatigued limb during unilateral and bilateral contraction. A secondary purpose was to examine the bilateral index and whether sex moderated the responses. Thirty healthy right-hand dominant participants (n = 15 males; 15 females) completed this study. After a familiarization session, the participants completed four experimental visits (no-mirror, mirror, non-dominant, and control) in a pseudo-randomized order. The fatigue protocol required the participants to perform nine, 20 second maximal unilateral handgrip contractions. This protocol was performed with (mirror) and without (no-mirror) illusionary mirror visual feedback with the dominant hand. The fatigue protocol was also performed with the non-dominant (non-dominant) hand without mirror visual feedback. Maximal strength and electromyographic (EMG) amplitude of the flexor carpi radialis and extensor carpi radialis were collected for both hands during unilateral and bilateral contractions before and after the fatigue protocol. The relative change (%Δ) in maximal strength and EMG amplitude was compared between visit, contraction, and sex. The bilateral index was also compared between hands and sex. The main findings show that: 1) bilateral force loss was significantly (p < 0.05) greater than unilateral force loss during the no-mirror and non-dominant visit, but not during the mirror visit (p > 0.05), 2) there were no significant (p > 0.05) changes in maximal force for the contralateral, non-fatigued hand during maximal unilateral contractions, yet there was for the bilateral contraction following the non-dominant visit (p < 0.05), 3) there was a significant (p < 0.05) increase in EMG amplitude for the non-fatigued flexor carpi radialis during the mirror visit, 4) there was a significant (p < 0.05) bilateral deficit and it was greater for the dominant hand compared to the non-dominant hand (p < 0.05), and 5) sex did not have a significant influence on any of the comparisons (p > 0.05). Together, these findings present several novel observations related to neuromuscular fatigue and cross-limb interactions.