Paradoxical influence of encouragement on muscle fatigue.

A comparison between the fatigue developed during encouraged and non-encouraged maximum voluntary contractions (MVC) was carried out. Six subjects performed 150 isometric leg extension MVCs with the right and the left leg separately, and seven subjects performed 150 isometric MVCs with both legs simultaneously. Force and rectified smoothed electromyography (rsEMG) were followed during the contraction series. No significant difference was found in the initial force between the encouraged and the non-encouraged contraction series. Irrespective of the type of exercise performed, the relative strength in the encouraged contractions decreased more than the relative strength in the non-encouraged contractions. In all subjects a decrease in rsEMG was seen, and the ratio between rsEMG and strength remained constant throughout the 150 MVCs. The results suggest that part of the fatigue produced during repeated MVCs originates within the central nervous system, and is influenced by the presence and the encouragement given by the experimenters.