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You love to run. Climbing the hills on your usual route used to be a pleasant challenge. Now it hurts to even climb the stairs. What's going on?

Your knees may be telling you that it's time to take a break.

The cartilage under your kneecap (patella) is a natural shock absorber. But it doesn't come with a lifetime guarantee. Overuse, injury or other factors may lead to a condition known as chondromalacia patella — a general term that indicates damage to the cartilage under your kneecap. Chondromalacia patella is more accurately referred to as patellofemoral pain.

The most common symptom of patellofemoral pain is knee pain that increases when you walk up or down stairs. It also may hurt to kneel or squat. Simple treatments — such as rest and ice — often help, but sometimes more aggressive treatment is needed.

As the smooth cartilage under your patella rubs against the groove in the bottom of your femur (thighbone), you may feel dull pain in the front of your knee. The pain often increases when you walk up or down stairs. It also may hurt to kneel, squat or sit with your knee bent for long periods of time. You may notice a grating or grinding sensation when you extend your knee.

In adolescents and young adults, patellofemoral pain often is caused by overuse or injury. Sometimes an unusual alignment of the kneecap is responsible. For older adults, patellofemoral pain may be related to arthritis of the knee joint — which causes cartilage to lose its normal shock-absorbing ability. Weak thigh muscles or flat feet also may contribute to the pain.

Patellofemoral pain is most common in young adults, particularly runners and others who routinely do exercises involving the lower legs. The condition affects more women than men. People who've had a trauma to the kneecap — such as a dislocation or fracture — may be more likely to develop patellofemoral pain.

If the knee pain isn't severe or disabling, treat it yourself with rest and cold packs. It also may help to elevate your knee. If needed, take nonsteroidal anti-inflammatory pain relievers, such as aspirin or ibuprofen. If the pain doesn't improve within a few days, consult your doctor.

Seek medical care immediately if:

• Your knee hurts and doesn't function properly after an injury
• Your knee hurts, even when you're not putting weight on it
• You have knee pain after a popping sound or snapping sensation
• Your knee locks rigidly in one position, or your kneecap is visibly out of place
• Your knee seems unusually loose or unstable when you put weight on it
• Your knee is swollen
• The pain is associated with fever, chills or redness around the joint

Diagnosis is based primarily on your symptoms and a physical exam. Tell your doctor about your typical activities and any recent changes to your routine. To gauge your knee's strength and alignment, your doctor may ask you to walk, jump or move your knee in certain ways. He or she may detect tenderness or feel a grinding sensation below your kneecap when you extend your knee.

Your doctor may recommend X-rays or other imaging tests to help determine the cause of your knee pain. If your symptoms are severe, a minor surgical procedure known as arthroscopy may be needed to confirm the diagnosis and treat the condition.

Patellofemoral pain can lead to difficulty with routine activities, such as squatting and climbing stairs. The pain can contribute to weakness of the thigh muscles as well.

Treatment of patellofemoral pain often begins with simple measures. Rest your knee as much as possible. Avoid any activities that increase the pain, such as climbing stairs. If needed, take nonsteroidal anti-inflammatory pain relievers, such as aspirin or ibuprofen.

To speed your recovery, your doctor may recommend specific exercises or physical therapy to strengthen the muscles that support your knees, such as your quadriceps, hamstrings and the muscles around your hips. Sometimes physical therapy may include electrical stimulation to strengthen your muscles. In other cases, knee braces or arch supports are recommended.

When you exercise, choose activities that go easy on your knees, such as bicycling and swimming. Your physical therapist may show you how to tape your knee to reduce pain and enhance your ability to exercise.

If these measures aren't effective, surgery may be an option.

• Arthroscopy. During this procedure, the doctor inserts an arthroscope — a pencil-thin device equipped with a camera lens and light — into your knee through a tiny incision. Surgical instruments are passed through the arthroscope to remove fragments of damaged cartilage.
• Realignment. In more severe cases, a surgeon may need to open your knee to realign the angle of the kneecap or relieve pressure on the cartilage.

Sometimes knee pain just happens. But you can take steps to help prevent the pain.

• Lose excess pounds. If you're overweight, losing the extra weight relieves stress on your knees.
• Warm up. Before running or any other exercise, warm up with five to 10 minutes of light activity.
• Stretch. Promote flexibility with gentle stretching exercises.
• Increase intensity gradually. Avoid sudden changes in the intensity of your workouts.
• Practice shoe smarts. Make sure your shoes fit well and provide good shock absorption. If you have flat feet, consider shoe inserts.
• Think alignment and technique. Ask your doctor or physical therapist about flexibility and strength exercises to optimize your technique for jumping, running and pivoting — and to help the patella track properly in its groove.

It's also important to listen to your body. If your knee hurts, stop what you're doing. Pushing yourself may only lead to injury.