Dehydration decreases endurance performance and can lead to life-threatening heat injuries that include heat syncope (fainting), heat exhaustion, and heatstroke. Curiously, most studies show that dehydration has little effect on muscle strength and power.
Lawrence Hayes and Christopher Morse, from Manchester Metropolitan University in Great Britain, found that progressive levels of dehydration induced through jogging had no effect on power as measured by vertical jump and high-speed knee extensions, but decreased muscle strength as measured by slow-speed knee extensions. The decrease in strength would likely cause a significant decrease in performance in athletes such as wrestlers and weightlifters who compete in weight classes. (European Journal of Applied Physiology, 108: 701-707)