The reason is that because the air is already very saturated with water, sweat can't evaporate. Sweat that beads up and rolls off doesn't function in the cooling process. As dehydration progresses cooling becomes more difficult. Performance drops and heat injury becomes a real threat. Often the border between them is blurred into a continuous spectrum.
Heat cramps are due to muscle spasms and often occur in the arms, legs, or abdomen.
They are thought to be caused by dehydration and loss of salt and other electrolytes. Heat exhaustion is due to more profound loss of water and electrolytes. Both can usually be treated by moving out of the sun, drinking fluids, and eating salty food. Heatstroke is a life threatening condition and represents severe dehydration, high body temperature, and a shut-down of the cooling mechanisms. The athlete may be delirious or comatose, and half of the victims have stopped sweating.