How many times have you exercised vigorously over the weekend only to wake up Monday morning with an annoying pain in the back?

Some recreational sports, such as racquetball, tennis, running and weightlifting, are especially irritating to the back because they involve constant twisting, abrupt motions and a lot of reaching up, out, down and around, squatting and bending at the back and waist. In addition, racquetball and tennis can be dangerous because players often step onto the court without warm-up or preparatory stretching exercises.

Back injuries sustained in running are mainly an overuse problem, says Jay Irrgang, 29, director of sports medicine at the National Hospital for Orthopaedics and Rehabilitation. Irrgang estimates that as much as 5 percent of all running injuries are related to the back. "With running, every time your foot hits the ground, there is a force running up the leg into the back, from 11/2 to three times your body weight per step," Irrgang explained. "It's different than in contact sports, where the injury is related to trauma, one direct blow."

Often athletes who lift weights as a sport or as a strength workout for another sport injure the back because "people don't use good biomechanics using Nautilus (equipment) and free weights," explains Katherine Braun, a physical therapist at the Sports Medicine Center in Chevy Chase. "They are often pushing heavier weights, too heavy for their body and trunk to stabilize. Use of proper biomechanics can lessen problems." When using the bench press, for example, bend the knees and flatten out the back, because strain occurs when you arch the back.

Recent medical research suggests, however, that sports often takes the blame for injuries that are sustained elsewhere -- that a majority of athletes have back injuries from non-athletic circumstances, such as moving boxes at home or being involved in car accidents. Most back injuries are caused by overuse, usually from lifting objects without bending the knees or reaching and twisting to pull or hit objects.

"The spine is not the most stable structure," explains Irrgang. "Basically it's a stack of bones held together by ligaments and muscles that don't do a real good job of controlling the bones in the back. Maybe the idea that man should be on all fours is true."

Diagnosing back injuries is complicated because of the different components that compose the back -- bones, vertebrae, disks between vertebrae, nerves and muscles. The spine is a flexuous and flexible column formed of a series of bones called vertebrae (from vertere, to turn). The 33 vertebrae, connected by a series of disks, are classified according to position, from top to bottom: cervical (7), drsal (12), lumbar (5), sacral (5) and coccygeal (4).

The vertebrae are piled one upon the other, forming a strong pillar for the support of the head and trunk. Between each pair of vertebrae, apertures exist through which the spinal nerves pass.

Problems in the back can arise when a bone slips forward or when there is a crack in the bone, a pinched nerve or a slipped disk. Sports medicine doctor Gabe Mirkin stresses that "most of the time it isn't one of those major injuries, it's usually a strain of a muscle or a ligament that holds the bone together . . . "

The most difficult part of back rehabilitation is giving the sore back the proper time to heal. "It sounds extreme when the doctor says to lie on your back for three days, but that's the only way to rest your back," says Braun. Most people, though, choose to keep going about their daily lives, even though, says Braun, "you can't totally rest your back like you can with an arm injury." And, Irrgang observes, "You can't take a runner out of running, or he'll go crazy."

Many experts recommend bicycling and swimming as a way to maintain carvascular fitness while nursing the injury back to health. "Bicycling is best," Mirkin points out. "You don't use the back muscles when bicycling. Pedal if it doesn't hurt." But Mirkin advises against swimming, because he says that the lifting motion of the arm out of the water arches the back backward. Instead, he recommends exercising on a rowing machine, which strengthens the back without irritation if not overused.

While many back injuries, such as those resulting from contact sports, cannot be prevented, the risk of injury can be minimized through proper preparation and stretching. Tennis and racquetball players often stretch out their arms but forget to warm up the back. Runners do lower leg stretching, but ignore the back. Take the time to do trunk rotations and side bends, and to lie on your back and hold both knees to your chest. An injury to that part of the body could put you back on your back.