Fatigue- and Training-Related Differences in Muscle Activation and Coordination.

Acute adaptations to fatiguing conditions may be altered concerning the training background. Muscle activation and coordination are affected by fatigue, although differences between subjects with different training status remain unclear. This study evaluated 28 individuals, investigating differences between strength-trained and untrained individuals in leg-press isometric maximum voluntary contractions before and after a back-squat fatiguing protocol. The peak force, rate of force development, electromyographic amplitude of seven lower-limb muscles, rate of electromyographic rise of agonist muscles and intermuscular coherence between synergist or antagonist pairs of muscles were evaluated. Strength-trained individuals exhibited a greater peak force, maximal rate of force development and rate of force development at 150-200 ms. All force-related variables decreased with fatigue in both groups, and the peak force decreased to a greater extent in strength-trained individuals. The electromyographic amplitudes of the vastus medialis (p=0.005) and rectus femoris (p=0.039) increased in both groups, and the rate of electromyographic rise of the rectus femoris increased in strength-trained individuals but decreased in untrained individuals (time x group interactions: 0.006<p<0.025). Additionally, coherence analysis revealed greater coherence in the 15-35 Hz band between the rectus femoris and the vastus medialis in untrained individuals than in strength-trained individuals, while fatigue affected coherence across the bands of interest differently concerning the functional relationship between the paired muscles. Different training status imply different acute responses to fatigue relying on changes in the activation of agonist muscles as well as coordination between the pairs of synergist and/or antagonist muscles.