They call him "Diesel" because running back John Riggins powers his way down the football field, often with defending linemen clinging to his 6-foot-2, 240-pound body.

But after 13 seasons, 2,741 carries, 10,675 yards and 108 touchdowns, the back of the Washington Redskins' leading ball carrier is showing signs of the punishment it has suffered -- specifically pain from a lower back sprain that sometimes shoots down to the knees.

Riggins' back became the topic of great speculation this season after the 35-year-old Redskin checked into Sibley Memorial Hospital several times for traction and bed rest. After Sunday's season-ending loss, the running back's back figures prominently in discussions of whether he will play next season.

"If I know that next year will be like this year," Riggins told reporters just before Christmas, "I wouldn't go through it again."

The back is one of the body's most injury-prone areas. After the knees and ankles, it's the part most likely to get hurt in sports.

"All you have to do is look at some slow-motion movies of football, and imagine the force involved," says Dr. Augustus A. White III, chief of orthopedic surgery at Boston's Beth Israel Hospital and author of "Your Aching Back: A Doctor's Guide to Relief." "It's a combination of the player's acceleration, the weight of his body, twisting motions as he passes through the line, and then the forces pounding on the body as opposing players crash into him."

The result: "Trauma, pure and simple."

The back can suffer three major kinds of sports-related injuries: strains and sprains of the ligaments and muscles, ruptured or slipped disks, and spondylolisthesis, a displacement of the vertebrae.

In a sprain, fibers in a muscle, tendon (which attaches the muscle to the bone) or ligament (a fibrous structure that holds bones together) are severed. The primary treatment is rest. The chances of a sprain can be reduced by warming up slowly, and by practicing conditioning and flexibility exercises.

When a disk ruptures, the tough outer covering tears, allowing the jelly-like center to ooze out of position between the vertebrae and press on the nerves in the spinal cord. The disk is much like a jelly doughnut -- tough on the outside and soft in the middle -- while the spinal column is much like a stack of soup cans with jelly doughnuts between each can, White says. With age, the disk becomes less flexible, increasing the chances that the cover will tear.

"When a disk is involved," White says, "you have leg pain." That pain is often called sciatica because it radiates from the point where the disk presses on the spinal cord down the leg's sciatic nerve.

Disk problems are treated with rest, medications and, less frequently, surgery. There is no way to prevent a ruptured disk.

Spondylolisthesis (vertebral displacement) occurs when the vertebrae themselves are damaged or deformed and no longer move smoothly against each other. This injury is commmon, White says, "in football linemen who do a lot of lifting with their back when they are blocking and tackling." The problem can be treated with braces and exercise, or, if it's extremely severe, by surgically fusing the affected vertebrae together.

"The pro football players that I have seen have had an incredible number of problems on the X-rays," says Dr. Robert P. Nirschl, orthopedic surgeon and medical director of the Virginia Sports Medicine Institute in Arlington. "You see little breaks. It would not be implausible to suggest that Riggins has a smattering of several problems . He may not have any of them with full intensity, so he can still function."

Few of these back injuries cause permanent damage, and most heal with time. But the competitive pressures on professional athletes seldom allow enough healing time during the season.

Riggins tried a short cut: traction, in which the person lies on his back for hours, or even days, while weights attached to his feet or hips dangle off the end of the bed.

"Traction is helpful because it opens up the disk space," Nirschl says. "It is good for nerve pinch. The disk will sag back into its normal position and take the pressure off the nerve."

But not all traction works, Nirschl says, and some physicians argue that the main purpose of traction is to keep someone in bed.

"My opinion is that there is a traditional tendency to say 'back pain: bed rest, traction,' " White says. "The most important is the bed rest. The traction itself is not necessarily that much of a therapeutic factor."