Dehydration has clear negative effects on exercise performance. During prolonged endurance training dehydration may cause just as much fatigue as fuel depletion; but the negative effects are even more pronounced in hot environments and during repeated anaerobic work (weightlifting). Exercise performance is impaired by dehydration as low as 2% of total body weight; water loss in excess of 5% has been shown to decrease the capacity for work by about 30% and greatly increase the risk for heat illnesses such as heat cramps, heat exhaustion and heat stroke. The effects of dehydration on strength and power activities (lasting >30 sec, but <120 sec) is poorly understood but believed to reduce performance by 2%-3%, while clinical dehydration (2.5% of body weight) can reduce high-intensity interval sprinting performance by about 45%. Significant and dangerous dehydration (7% of BW) has been shown to even reduce the total time to exhaustion during tasks as simple as walking.
Physiological effects that occur during dehydration which have an adverse effect on exercise performance: