Sweating plays a critical role in maintain exercise performance. During moderate to intense exercise your muscles generate considerable amounts of heat. This heat is dissipated through sweating which can involve considerable fluid loss. Small losses in body fluid via sweating can have major consequences on performance as illustrated in the table below.

Knowing your sweat rate is important in
developing an optimum hydration plan. The average person sweats between 22 and
47 ounces per hour doing exercise however it can vary greatly and each person
has a different sweat rate. At 70% VO2Max exercising at 70° can product fluid
losses ranging from 22 to 47 ounces.
If you consume too little fluid during exercise your body pulls fluids from
your cells. If you consume too much water you can dilute the electrolytes in
your blood producing a condition called hyponatremia, which can be life
threatening.
Measuring Your Sweat
Rate
Fortunately determining your sweat rate is relatively easy. Weigh yourself
before and after a workout and convert your weight loss to fluid. (1 pound
weight loss equals approximately 16 ounces of fluid). If you lose 1.5
pounds in 60 minutes of exercise that's equal to 24 ounces of fluid
which according to the Sweat Table makes you a moderate sweater. Then
develop your own hydration plan. It may involve a sports drink or an
electrolyte replenisher plus water. The bottom line- achieving the right fluid
and electrolyte balance pays big dividends on performance.