WEST PALM BEACH, Fla. (AP) _ President Clinton tore a tendon in his knee early today after stumbling on the stairs at golfer Greg Norman's home and required surgery.

Doctors described the damage to his right knee as substantial.

Clinton left for Bethesda Naval Hospital where the surgery was scheduled for late this afternoon.

As he left Florida, Clinton flashed a thumbs up and shouted ``I feel good'' above the roar of Air Force One engines.

His white cast peeking out from the leg of a black jogging suit, Clinton was flanked by two Secret Service agents who helped him onto the plane using a wheelchair ramp.

About 1:20 a.m., Clinton stumbled on a step as he left Norman's home and tore his quadriceps tendon, which connects the upper thigh to the kneecap.

``He remembers his right knee buckling out. He heard a very loud pop,'' said Navy Capt. Connie Mariano, the president's personal physician.

The president suffered a 50 percent tear of his tendon, requiring surgery to repair.

``It's technically not that difficult to do,'' said Dr. Joel Cohen, vice president at St. Mary's Hospital. ``It's not a major operation, but it's not a minor one.''

``You literally drill a hole in the knee cap,'' Cohen said, to reattach the tendon to the kneecap. If there are no complications, the surgery generally takes about 90 minutes.

The president will be given an epidural or a local anesthetic during the surgery and will be conscious, the doctors said.

There are no plans to turn presidential powers over to Vice President Al Gore.

Clinton will be in a leg brace and use crutches for from two to six weeks, the doctors said. Physical therapy will begin in three to six weeks and could take up to six months.

The president was given an anti-inflammatory, non-narcotic pain killer called Toradol to ease the painful injury, his doctor said. His leg was put in a temporary cast running from his thigh to his ankle. He was transported to Air Force One in a wheelchair.

Press secretary Mary Ellen Glynn said the president was in good spirits. ``He asked for a deck of cards and was playing hearts'' with confidante Bruce Lindsey.

Clinton is due to travel to Helsinki to meet with Russian President Boris Yeltsin next Wednesday and Thursday. ``It will go on,'' Clinton said as he left Florida. ``You bet.''

He said he told Norman, ``My handicap was going up by the minute.''

First lady Hillary Rodham Clinton is to leave Saturday morning on a two-week trip to Africa with their daughter, Chelsea. Aides said she probably still would go but delay her departure a few hours to be with the president. ``I think a lot depends on her conversations with the president,'' spokesman Neel Lattimore said.

The president was in Hobe Sound, Fla., to play in a two-day tournament sponsored by Norman. Clinton was spending the night at Norman's 80-acre oceanside estate.

The two chatted late into the night at Norman's home. Clinton left about 1:20 a.m. to go to a separate cottage on the estate where he was to sleep. Mariano said Clinton caught his heel on a short flight of stairs and his right knee buckled out. Norman apparently broke his fall. She denied that alcohol was a factor.

Paramedics and a physician were at the Norman estate, and rushed to the president's side. Parademic Richard Wilde said Clinton was lying on the ground, with Norman and a Secret Service agent assisting him.

``If it wasn't for Greg Norman, he probably would have been hurt a lot more. Those were concrete steps,'' Wilde said.

The paramedics checked the president's vital signs, put ice on his leg and a splint and put him into an ambulance.

After arriving on a stretcher with his leg immobilized, Clinton underwent a magnetic resonance image, or MRI scan.

Hillary Clinton, who was at the White House, was told about the president's injury.

At 50, Clinton's health has been generally good. Doctors removed a precancerous lesion from his nose during the course of his physical exam last summer, and a benign cyst near his left ear last September.

The president jogs frequently. He had a skiing accident 17 years ago in which he strained a ligament in the same knee.

A torn quadriceps tendon is a painful injury. The tendon attaches four muscles from the top of the leg to the joint at the knee cap. The tendon's job is to straighten out the leg during the forward motion of a stride.

Typical surgery often includes drilling holes into the knee cap to make attachment points. The tendon is hooked to the knee cap with surgical thread or wire. Afterward, the leg is kept straight and immobile, usually in a cast that runs from upper thigh to ankle.