Article 1:
Contralateral effects of neuromuscular fatigue: neurophysiological mechanisms and kinetics of onset
Summary
Objectives: The objective of this study was to analyze the kinetics of appearance of the neurophysiological mechanisms involved in neuromuscular fatigue in the dominant and non-dominant limbs after maximal isometric knee extension exercises of the dominant limb (for example, the phenomenon of ” cross-over”).
Method: Fifteen healthy subjects, all male, performed sequences of neuromuscular tests and fatigue exercises on an isokinetic ergometer. To quantify the onset of fatigue, neuromuscular testing sequences were performed before (PRE) and after (MID) the first fatigue exercise, as well as at the end (POST) of the second fatigue exercise. The fatigue exercises were performed unilaterally on the dominant limb, while the neuromuscular testing sequences were performed on the ipsilateral and contralateral limbs, evaluated randomly in each subject. In each fatigue exercise, subjects had to maintain an isometric maximal voluntary contraction (MIVC) of knee extension for 100 seconds. The neuromuscular testing sequences included: i) CVMI together with the collection of electromyographic (EMG) activity of the vastus medialis, vastus lateralis, rectus femoris and semitendinosus muscles, and ii) contractions evoked by percutaneous nerve stimulation of the femoral nerve, which allow quantification the level of muscle activation and the contractile properties of the knee extensor muscles.
Results: A significant decrease in CVMI was observed in the dominant limb after each fatigue exercise, while this decrease was only evident in the non-dominant limb after the second fatigue exercise. A decrease in activation levels was obtained, regardless of the limb evaluated, reflecting the presence of fatigue of central origin. Finally, peripheral fatigue was also observed in the dominant limb.
Conclusion: The experimental protocol used allowed us to demonstrate the “cross-over” phenomenon, since the CVMI of the knee extensor muscles of the dominant and non-dominant limbs was reduced. The underlying neurophysiological mechanisms and kinetics of fatigue onset differ between dominant and non-dominant limbs.
