After you have played a long tennis match on a hot summer day, you feel weaker and less accurate with your shots. The fatigue, muscle weakness, tired aching feeling and decreased coordination that you get in any sport lasting several hours is caused by low levels of fluids, salt or calories.

There are no early warning signals. By the time you feel hungry, you have already run low on calories and are ready to crash. By the time you feel thirsty, you are already severely dehydrated and feel weak and tired. By the time you are low on salt, you already have tired, aching or burning muscles; feel weak, tired and dizzy; and may already have muscle cramps.