Experts recommend these exercises to strengthen the quadriceps:

For those who haven't exercised in a while or have never exercised, begin by rolling a towel up tightly. Sit on the floor or a bed, legs extended in front, and place the rolled towel under one knee. Contract the thigh muscle and push the back of the knee into the towel, straightening the leg. Hold for about six seconds, eventually working up to 10 repetitions, then more as the muscle becomes stronger.

"This is a really easy exercise -- we do these for people who have a lot of pain or swelling," says Jennifer Hootman, an epidemiologist with the Centers for Disease Control and Prevention in Atlanta and a certified athletic trainer.

For those in better shape, sit in a chair and strap light ankle weights around the ankles. Straighten one knee, starting with two to three sets of six repetitions, eventually working up to two to three sets of 12 repetitions. Repeat with the other leg.

This exercise can also be performed by wrapping the end of an elastic band or tube around one ankle and tying the other end to a leg of the chair, Ms. Hootman said.

Those who are used to exercising can also try a wall sit. Stand with your back against the wall, take about two to three steps away and slide your back down the wall to a 90-degree sitting position. (The out-of-shape should stop halfway between standing and 90 degrees.) Hold this position for about five seconds, working up to 30 seconds or longer, and toward that 90-degree angle, as the muscles get stronger.

These exercises, suggested by Lynn Millar, professor of physical therapy at Andrews University in Michigan, are an easy way to strengthen the quads without using equipment.

For the gym rat, Ms. Millar suggests squats on a Smith machine, a barbell-weight machine with an attached, sliding bar that allows less pressure to fall on the knees than a free-weight barbell. Begin with low weight and low repetitions and increase both as you progress.

People with existing knee pain or knee osteoarthritis should check with a physician before beginning any exercise program.