Hypohydration Decreases Neuromuscular Performance Prior to and Following Intermittent Exercise in the Heat

Purpose: To examine the effect of hydration status on neuromuscular performance prior to and following intermittent exercise in the heat. Methods: Eleven male soccer players (age, 20±2 years; height, 179.0±7.9 cm; body mass, 74.9±10.3 kg; Maximal Oxygen consumption, 62.4 ± 11.5 mL·kg -1 ·min -1 ) performed a familiarization trial involving ultrasound (cross-sectional area [CSA] and muscle thickness), landing error scoring system (LESS), and isometric knee extensions (rate of torque development [RTD]; time intervals of 0-30 ms (RTD 30), 0-50 ms (RTD 50), 0-100 ms (RTD 100), and 0-200 ms (RTD 200) from contraction onset). Following this, participants completed 45-minutes of intermittent exercise in the heat (33°C, 30% relative humidity) as part of familiarization. Experimental trials replicated the same protocol of pre- and post-neuromuscular testing separated by two 45-minute intermittent exercise bouts in the heat. Two experimental trials (euhydrated [EUH] and hypohydrated [HYP]) were randomly assigned; EUH, maintained euhydration throughout trial; HYP, performed a 24-hour fluid restriction before the trial. Prior to and following exercise, urine specific gravity (USG), urine osmolality, and body mass loss (BML) were measured. A linear mixed effect model with Tukey post hoc was utilized to assess differences between trials. Results: CSA of the rectus femoris was smaller in HYP pre (13.3±2.4 cm 2 ) compared to EUH pre (14.0±2.6 cm 2 ), and in HYP post (12.7±2.4 cm 2 ) compared to EUH post (14.0±2.6 cm 2 ) ( p< 0.05). Independent of time, muscle thickness was also smaller in HYP (2.27±0.2 cm) compared to EUH (2.45±0.2 cm, p <0.05). Furthermore, LESS scores were higher in HYP (4.1±2.0) compared to EUH (2.9±2.0, p <0.05). RTD30, 50, and 100 were all lower in HYP (326±138 Nm·s -1 ; 441±207 Nm·s -1 ; 648±302 Nm·s -1 ) compared to EUH (427±175 Nm·s -1 ; 584±282 Nm·s -1 ; 796±392 Nm·s -1 , p <0.05), respectively. Additionally, USG, urine osmolality, and BML, were all higher in HYP compared to EUH ( p <0.05). Conclusions: Hypohydration can negatively impact neuromuscular performance prior to and following intermittent exercise in the heat.