No matter what your high school coach told you, your workout should not be an exercise in masochism. "Exercise only hurts if something's wrong," says Lewis Maharam, M.D., a sports medicine specialist and president of the American College of Sports Medicine's New York chapter. So what's up with those snaps, crackles and pops that crop up during your workout? Should you cut your bike ride short and call it a day, go to the emergency room, or just chalk it up to old age? Here, Dr. Maharam cuts to the chase:

Cracking knees: If your knees make odd noises during your workouts, or even when you're walking down the street, it's probably nothing to worry about. Some researchers think the popping and cracking are caused by gas in the joints; others speculate that the noises are caused by a piece of loose cartilage settling during movement. If your popping knees are painful, however, or if they swell after activity, you should see a physician to rule out something more serious. Likewise, if the noise is more akin to a creaking or grating sound, it may mean that the cartilage that covers the joint has worn away, leaving the roughened areas to rub together. "Generally, noise isn't the issue," says Dr. Maharam. "The question is: Does it give way, or feel like it's going to give way? And does it hurt?" If the answer to either is yes, see your doctor.

Side stitches: This nuisance pain has several causes including exercising too soon after eating, an overstretched tendon or muscle cramp in the diaphragm from heavy breathing, gas, dehydration or, in some cases, exercise-induced asthma. Side stitches usually cease as you get more fit. If your workouts are frequently interrupted by side pain (or you have a hard time breathing during your workout), talk to your physician.

Muscle cramps: Do you get cramps during your workout or on the way home in the car? If it's a cramp that forces you to hop off the treadmill or put down the weight you're lifting, it means that the muscle is fatigued and you need to slow down or stop what you're doing, says Dr. Maharam. (Interestingly, cramps can only occur in a muscle when it's being shortened—as opposed to being extended.) Trying to tough it out and exercise through the pain could invite injury. On the other hand, if the cramp comes on after a workout, it means you've irritated some of the fibers in the muscle. Though a natural consequence of strenuous exercise, the pain shouldn't last more than a few seconds. A little stretching can immediately alleviate the symptoms.

Popping shoulder: If you play a sport like baseball, volleyball or tennis, or are a swimmer—any activity that involves raising your arms overhead—you've probably heard or felt your shoulders popping. "It's probably an impingement syndrome, a disorder in which the soft tissues on the top of the shoulder joint (the rotator cuff muscles and tendons, the subacromial bursa and biceps tendon) catch on the underside of the shoulder blade," explains Dr. Maharam. Repetitive motion, sometimes made worse by weak shoulder muscles and bad form, are the culprits. It could also be caused by trauma, such as a torn muscle. To be safe, ask a sports physician, physical therapist or qualified fitness trainer to suggest some safe shoulder-strengthening exercises.