Heat's Effect On The Body During Running

When temperatures rise past 55 F, a runner’s performance begins to be affected. Athletes generally run slower and feel worse than in cooler conditions. This is because the body’s core temperature rises greatly during the high-intensity exertion it takes to run. Rising temperatures affect runners in two main ways: dehydration and overheating.

Heat and Pace

Studies show the ideal temperature for running is about 55 F. For every 5 degrees hotter, runners’ performances degrade about 2 percent, according to Jeff Galloway, running coach and author. A runner who usually runs a 10-minute mile at 55 degrees or less would run a 10:06-minute mile at temperatures between 56 and 60 degrees. Humidity higher than 60 percent further slows the pace, according to Galloway.

What Happens

When runners compete or train in temperatures above 55 F, the body builds up heat that it can’t remove quickly enough to regulate the temperature. Blood travels to the skin’s surface trying to release the heat through sweat, which limits the amount of blood and fluids that can nourish the muscles. This especially affects the legs, which are working hard to carry you on the run.

Dehydration

When you run in hot temperatures, you lose fluids from the body more rapidly through sweat. This causes you to lose electrolytes – chemicals such as sodium and potassium – which help the body’s cells generate energy. Runners can lose 3 to 4 lbs. of water on a warm day, according to hall of fame runner Pete Pfitzinger. The hotter it is when you are running, the more quickly you could become dehydrated. Symptoms of dehydration include muscle cramps or spasms.

Tips

Run before the sun rises or after it sets on hot or humid days. Drink plenty of ice-cold water before you run to get fluid to your cells, and continue to drink water in small doses during long runs. Dress appropriately for the weather; avoid cotton, which tends to hold moisture, opting instead for lightweight technical gear that wicks moisture. Dump cold or cool water over your head and clothing as you run, and take walk breaks as necessary. If temperatures top 85 F during training, move your run inside.

Warnings

Heat exhaustion and heat stroke are serious conditions that can result from running when temperatures are high. Monitor yourself and fellow runners for symptoms of exhaustion, including light-headedness, nausea, headaches, confusion and profuse sweating. A drop in blood pressure or spike in body temperature can indicate heat disease. Seek medical attention immediately if you cease sweating or your body temperature rises above 106 F, symptoms of heat stroke, which can be deadly.