To determine whether moderate water loss (,1.5–2% of body mass (BM)) represents a significant impairment to soccer match-play and the related fitness variables. 11 moderately active male soccer players (mean (SD) age 24.4 (3) years, BM 74.03 (10.5) kg,peak oxygen consumption 50.91 (4.0) ml/kg/min) volunteered to participate. The experimental procedure comprised: (1) a 45 min pre-match period of cycle ergometry exercise (90% of individual ventilatory threshold); (2) the completion of a 45 min soccer match; and (3) the immediate post-match performance of sport-specific and mental concentration tests. The subjects completed the procedure on three occasions each in a different experimental condition (fluid intake (FL), no fluid (NF) and mouth rinse) in an individually randomised order. Core temperature (Tc), heart rates, plasma and urine osmolalities, BM, sweat rates and heat storage were all measured. The only condition-dependent difference during the match-play element of the protocol was a significantly increased Tc in the NF condition compared with the FL condition (39.28°C (0.35°C) and 38.8°C (0.47°C), respectively; p,0.05). The immediate post-match performance of a sport-specific fitness test was significantly impaired where FL had been denied (p,0.01). The post-test evaluation of rating of perceived exertion and thirst indicated that the NF condition was perceived to be the most challenging (p,0.05). The condition-dependent differences in match-play and post-match tests demonstrate that moderate dehydration is detrimental to soccer performance. However, it remains unclear whether this could be attributable to water loss in itself or the negative psychological associations derived from a greater perception of effort in that condition.
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