Effects of dehydration and rehydration on EMG changes during fatiguing contractions

Purpose: This study measured the effects of sauna-induced dehydration (Dhy) and the effectiveness of rapid rehydration on muscle performance and EMG f requency spectrum changes associated with fatigue during isometric contract ions. Methods: Knee extensor muscle strength during isometric maximal volun tary contraction (MVC) and endurance time at 25% and 70% of MVC (ET25 and E T70, respectively) were measured three times in 11 healthy male subjects, u nder euhydration conditions (Eu), after Dhy, and after rehydration followin g 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 r oot mean square (RMS) increased earlier during Dhy than Eu. Opposite change s were shown for the mean power frequency (MPF) of EMG, and Dhy resulted in an accelerated fall in MPF. However, because ET25 decreased with dehydrati on, RMS and MPF changes were similar during Eu and Dhy when reported to nor malized contraction time, exhaustion was thus associated with similar value s 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 inc rease in muscle fatigue, associated with early changes in EMG spectral para meters. It is not clear whether these alterations could be attributed to bi ochemical modifications, and the role of increased perception of effort whe n subjects were dehydrated should be clarified.