(CNN) -- President Clinton is expected to recover fully from knee surgery, although he will need a brace and crutches for several weeks, and he'll have to undergo careful rehabilitation.

The 50-year-old Clinton caught his heel on a step at the Florida estate of Australian golfer Greg Norman early Friday, tearing the quadriceps tendon connecting the kneecap to the muscle of the upper thigh in his right leg.

The injury is common, especially in sports, and about 7,000 people a year in the United States are hospitalized by it.

Doctors who examined the president say more than 50 percent of his tendon was torn, but it was not completely severed.

'He heard a very loud pop'

Four major muscles that descend from the thigh come together at the quadriceps tendon, which connects to the kneecap then extends below that to the top of the lower leg.

The function of the muscles and tendon is to straighten out the leg with each stride, and they are essential to normal walking or jogging. When going down steps, as the president was doing, the muscles may be subjected to force equivalent to three times a person's body weight, doctors say.

When Clinton stumbled, the sudden muscle contraction as he tried to catch himself likely overloaded the tendon. "He heard a very loud pop," one aide said.

Surgery time can vary

Doctors said the surgery to repair such a tendon tear is common and straightforward.

"A good surgeon can do the job in just a half hour or so," said Dr. Kenneth Fine, director of the George Washington University sports medicine center. "It is not very complicated."

"A number of sutures allow it to heal," orthopedic surgeon Dr. Drew Miller added. In the president's case, the surgery lasted two hours.

Clinton had a "regional" anesthetic that numbed him approximately from the mid-section down, so that he was never rendered unconscious, White House spokesman Mike McCurry said. Doctors said he was alert most of the time.

Knee injuries by the numbers

Almost 5 million people visit orthopedic surgeons in the United States each year because of knee problems. More than 3 million of the visits are injury-related; the rest are due to arthritis or other illnesses. Another 1.4 million people go to the hospital emergency room for knee problems. About 7,000 hospital visits are recorded each year for torn quadriceps tendon, and 25 percent of those patients were 45-64 years of age. In 1994, there were more than 300,000 knee procedures done in the United States. Source: American Academy of Orthopedic Surgeons

The tendon is repaired by drilling holes into the top of the kneecap and attaching the tendon to the bone with thread or wire anchored in the holes. The leg is then put into a cast or brace to keep it perfectly straight, to put the least amount of pressure on the tendon.

Clinton should be out of the hospital in a few days and will be able to get around using crutches and a knee brace. Rehabilitation will begin with range-of-motion exercises.

Strength exercises will be slowly added to gradually rebuild muscle strength. Middle-age people typically recover fully, although in older patients it is not uncommon for there to be a residual stiffness in the knee.

"It could be 12 weeks before a person is back jogging again," Fine said.

Correspondent Andrew Holtz contributed to this report.