Hypohydration Decreases Neuromuscular Performance before and after Intermittent Exercise in the Heat

To examine the effect of hydration status on neuromuscular performance before and after intermittent exercise in the heat.

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 and muscle thickness), landing error scoring system, 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). After this, participants completed 45-min of intermittent exercise in the heat (33ºC, 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 the trial; HYP performed a 24-h fluid restriction before the trial. Before and after exercise, urine specific gravity, urine osmolality, and body mass loss were measured. A linear mixed effect model with Tukey post hoc was utilized to assess differences between trials.

Cross-sectional area of the rectus femoris was smaller in HYP pre (13.3 ± 2.4 cm 2 ) compared with EUH pre (14.0 ± 2.6 cm 2 ), and in HYP post (12.7 ± 2.4 cm 2 ) compared with 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 with EUH (2.45 ± 0.2 cm, P < 0.05). Furthermore, landing error scoring system scores were higher in HYP (4.1 ± 2.0) compared with EUH (2.9 ± 2.0, P < 0.05). RTD 30, 50, and 100 were all lower in HYP (326 ± 138 Nm·s -1 ; 441 ± 207 Nm·s -1 ; 648 ± 302 Nm·s -1 ) compared with EUH (427 ± 175 Nm·s -1 ; 584 ± 282 Nm·s -1 ; 796 ± 392 Nm·s -1 , P < 0.05), respectively. Additionally, urine specific gravity, urine osmolality, and body mass loss were all higher in HYP compared with EUH (P < 0.05).

Hypohydration can negatively impact neuromuscular performance before and after intermittent exercise in the heat.