TENDINITIS

Tendinitis is the problem behind many common overuse injuries. The suffix -itis means inflammation (characterized by pain, swelling, warmth, and redness). Tendons -- the fibrous cords that anchor muscles to bones -- are vulnerable, since the force of muscle contractions is transmitted through them. People who exercise regularly are especially at risk because of the strong forces produced by their well-conditioned muscles. These increase tension on the tendons, which can then rub against bones, ligaments, and other tendons, causing irritation.

Tendinitis is deceptive: The pain can be severe when you start exercising, then diminish as you continue -- only to return sharply once you've stopped. Perhaps the most common form of tendinitis is tennis elbow, which affects not only tennis players but also rowers, carpenters, gardeners, and anyone else who repeatedly bends his or her arm forcefully. In sports and activities that involve running and jumping, tendinitis is most likely to develop in the knee, foot, and the Achilles tendon (attaching the calf muscle to the foot) at the back of ankle. For cyclists, knees are most vulnerable. Shoulder tendinitis (involving the rotator cuff tendons) can develop from pitching a ball, swinging a golf club, or raising the arms in an overhead motion when playing tennis or swimming.

TIP: Stretching and strengthening routines can help prevent tendinitis, but equipment and technique may be equally important. For example, an improperly executed backhand is often the cause of tennis elbow, and running shoes with worn-down heels contribute to Achilles tendinitis. Also, alternate the types of workouts you do to avoid repetitive stress injuries. Instead of running every day, for instance, do low-impact activities such as swimming on alternate days.

STRESS FRACTURES

Stress fractures are microscopic breaks in bone, usually in the foot, shin, or thigh. Common among long-distance runners and basketball players, the fractures are brought on by the repeated impact of running or jumping. Often the pain is mild at first, occurring during or right after exercising. Continuing to exercise makes it gradually worse, but for the first few weeks, such fractures are usually too small to be detected, even by X-ray. Fortunately, the fractures rarely break through the bone, so they don't require splints or casts to heal, only rest.

TIP: Prevent stress fractures by increasing the intensity of your workouts gradually, not dramatically. Try to minimize impact on your legs: Run and jump on soft or resilient surfaces -- grass, carpet, mats, or suspended wooden gym floors -- instead of concrete. Wear well-cushioned exercise shoes.