Hydration

An estimated 3-4 pounds of water per hour is lost when running on a warm day. By this measure, a two-hour running schedule will cause a 150-pound runner to lose 6-8 pounds. Put in laymen terms, this implies a 4-5% loss in bodyweight and 10-15% fall in performance.

Dehydration severely tightens the muscle, leading to a cramp and results in conditions such as heat cramps, heat exhaustion and heat stroke. Exercise-related dehydration reduces endurance, increases body temperature and heart rate. The condition also takes a toll on the psychological levels of the runner as it makes the regular run seem much harder than usual.

It is worth mentioning here that each of the damaging effects of dehydration get even more aggravated in hot weather. The amount you sweat is directly related to how warm the weather is, which in turn leads to a decrease in the blood volume. Hence, a lesser amount of blood returns to your heart, making it pump less blood per contraction.

Consequently, in order to pump the same amount of blood, the heart rate must increase which means that the runner is not able to maintain an equally fast pace.