Dehydration is a common cause for cramping due to low levels of calcium, magnesium, and fluids in the body. Heat exhaustion can lead to dehydration and cramping in the muscles of the hand and other extremities like your legs.
Using the opposite hand, push back all four fingers and thumb on the cramping hand, very lightly, and massage the tight muscle. Heat A heat wrap can also help tame tight muscles, but don't worry if you don't have one at the ready — simply run your handsunder warm water instead.