BIGARD, A-X., H. SANCHEZ, G. CLAVEYROLAS, S. MARTIN, B. THIMONIER, and M. J. ARNAUD. Effects of dehydration and rehydration on EMG changes during fatiguing contractions. Med. Sci. Sports Exerc., Vol. 33, No. 10, 2001, pp. 1694-1700. Purpose: This study measured the effects of sauna-induced dehydration (Dhy) and the effectiveness of rapid rehydration on muscle performance and EMG frequency spectrum changes associated with fatigue during isometric contractions. Methods: Knee extensor muscle strength during isometric maximal voluntary contraction (MVC) and endurance time at 25% and 70% of MVC (ET25 and ET70, respectively) were measured three times in 11 healthy male subjects, under euhydration conditions (Eu), after Dhy, and after rehydration following Dhy (Rhy). Results: Dhy led to a decrease in body weight by 2.95 ± 0.05%. No significant effect of the hydration status was shown on MVC values. A 23% decrease in ET25 was recorded during Dhy (P < 0.01), whereas ET70 only tended to decrease (-13%, P = 0.06). ET25 was higher during Rhy than Dhy (8%, P < 0.05) but remained lower than during Eu (-17%, P < 0.05). The EMG root mean square (RMS) increased earlier during Dhy than Eu. Opposite changes were shown for the mean power frequency (MPF) of EMG, and Dhy resulted in an accelerated fall in MPF. However, because ET25 decreased with dehydration, RMS and MPF changes were similar during Eu and Dhy when reported to normalized contraction time, exhaustion was thus associated with similar values of RMS and MPF for all conditions. RMS and MPF changes during Rhy showed an intermediate pattern between Eu and Dhy. Conclusions: Dhy induced an increase in muscle fatigue, associated with early changes in EMG spectral parameters. It is not clear whether these alterations could be attributed to biochemical modifications, and the role of increased perception of effort when subjects were dehydrated should be clarified.