Don't be surprised if you feel a little extra sluggish when temperatures soar. When it's hot outside, the brain tells the muscles to relax in order to keep the body's core temperature low, and the process of sweating to stay cool diverts blood away from the muscles. Add allergies, dehydration, and longer daylight hours to the equation, and it's tempting to take a vacation from training until cooler temperatures prevail. But your performance doesn't have to suffer just because the mercury's on the rise. Here are Runner's World's best tips for beating the heat.