The warm-up
Think of this segment of your workout as a way of priming the body for what it's about to do. Just like you'd let your car warm up for a few minutes before hitting the highway on a bitterly cold morning, you don't want to take your body -- the muscles, joints, heart and lungs -- from zero to 80 without a little preparation. The warm-up period (it doesn't need to be more than about five to 10 minutes) will help the circulatory system, respiratory system, muscles and joints get ready for a more intense workout. In fact, a proper warm-up will actually enable you to work out harder. "If you ease in, start getting oxygen to your cells, you will be able to exercise at a higher intensity before any lactic acid builds up and causes muscle cramping and fatigue," explains Millar.

A second benefit of gearing up gradually is that you're less likely to wind up with a sidelining injury. Just imagine going out to the local track and immediately doing some sprints, or hopping on a bike in the middle of an intense spin class. Making cold muscles ramp up to high intensity is a recipe for tears, sprains and strains. "Doing some dynamic movements will help bring oxygen and blood to the muscles," says Arnold Nelson, Ph.D., associate professor of kinesiology at Louisiana State University. "It literally wakes the muscles up."

What you do to warm up should depend mostly on what you intend to do for your workout. "The best warm-up for any exercise is to do that exercise at a very low intensity -- like start off walking, then a light jog, then start running," says Michael Bracko, Ed.D., sports physiologist at the Institute for Hockey Research in Calgary, Alberta. Of course, some activities don't lend themselves to a gentle warm-up (like a tennis match or basketball game). In that case, the warm-up should consist of some light aerobic activity to prime your heart and lungs, plus some sports-specific range-of-motion moves. "If you're going to play tennis, for example, do some arm movements that mimic the motions of the game -- like swinging the arms, doing shoulder circles, and overhead swings," suggests Millar.

The cool-down
When the high-intensity portion of your workout is done, it's not a good idea to immediately sit down and rest. Just as the body responds best when you ramp it up slowly, it also prefers to take a few minutes to ramp back down and get your respiratory and circulatory systems back to normal. This is especially important for anyone with any type of heart disease (or anyone who is out of shape and just starting to exercise). "If you stop exercising without cooling down, it puts you at a higher risk for heart arrhythmias," says Millar.

And everyone's muscles can benefit from a little post-workout TLC. After you slowly transition from full-speed exercise to a cool-down walk and your heart rate has returned to normal (this usually takes about five minutes), do some stretching to improve range of motion and relax the muscles that just worked. "Let your body be your guide, and stretch whatever muscles feel tight from your workout," says Bracko. After a run, that will probably mean hips, quads and calves. After a tennis game, you might need to stretch your arms, shoulders and upper back in addition to your legs. Don't expect the stretching to miraculously eliminate soreness, but it can improve your range of motion so that your body can function better. "Your quads might still be sore, but if you've done some targeted stretches, you'll have the range of motion to still climb a flight of stairs," says Millar.