Running back John Riggins, 35, has been almost as at home at Sibley Memorial Hospital the last two weeks as he has been at Redskin Park -- perhaps, a physical therapist says, because of a "twisting motion" Riggins uses after being hit while carrying the football.

A lower lumbar sprain and bursitis in his hips put him in traction three days last week and since Thursday this week. A lower lumbar sprain, according to John Romero, director of the Sports Medicine Center in Chevy Chase, involves the ligament tissue around the spine. The tissue is thick there and it takes a strong impact to injure it.

"There are two ways you can cause a lower lumbar sprain," said Romero, a physical therapist. "First, the ligament tissue around the small joints on the back of the spine gets tight as you extend backward. If Riggins gets hit (so that he arches) backward, he can strain the tissue. Second, he can get injured by getting forced forward or twisted around.

"If you look at what he does, you can see how he can get hurt. He tucks down before he gets hit, he bends forward and twists to the left or right. A twisting motion or excessive backward or forward motion can sprain the ligaments in the back. With him, he probably got it from twisting after he gets hit.

"But once he gets moving a bit, warms up and becomes more flexible, and the adrenaline gets flowing, his pain tolerance gets higher. After being initially irritated, the back can stop hurting. Then after he stops playing, it will start hurting again."

Romero explained that Riggins' bursitis could be caused by "a lot of friction or repeated direct blows to the hip (during his 13-season pro career). He gets a lot of friction when he drags players while he runs. Bursitis is an inflammation in the bursal sac, which is filled with an oily fluid that absorbs shock and reduces friction on the hip."

As Romero has not been involved in Riggins' case, he could not speculate if Riggins was injured by an accumulation of blows or one hit. But he said that lower lumbar sprain is not uncommon among athletes.

Diagnosing the potential long-term effects of continuing to play with such an injury is difficult because of the variables involved, such as how well muscles absorb the shock of hits. But Bruce Ammerman, a neurological surgeon at Sibley who has worked on Riggins, said he believes Riggins won't harm himself by continuing to play injured.