What Are Those Knee Pops I Hear When I Exercise?

What those sounds you here in your knees when you exercise?It’s a bit disturbing when you’re doing a squat and your knees make strange sounds like popcorn popping on the stove or like nuts cracking. Some people also hear strange knee sounds when they’re working out on an elliptical machine. What do these knee pops and “catches” really mean?

What Causes Your Knees to Pop or Crack?

Those strange popping and crackling sounds have a name. The medical term for them is “crepitus.” Some people who have crepitus can feel it when they place their hand over their knee and flex and extend it repeatedly. Instead of feeling smooth, there’s cracking and grinding. There are several reasons you might have this symptom.

Your knee joint is protected by a layer of cartilage. This cartilage layer acts as a cushion to protect the knee joint during movement of the knee. Think of how many times you flex and extend your knees over a lifetime without discomfort, and you’ll see it does a pretty good job. Over time, this protective cartilaginous layer can become “worn” so that movement of the knee joint is less smooth and fluid. This can produce the popping or crackling sound known as crepitus. Occasionally, a small piece of cartilage can break off and move around in the joint when you flex and extend your knee, causing a distinct grinding, cracking or popping.

Another Reason for Knee Popping and Crepitus

Another reason some people have popping in their knee when they do certain exercises is their knee cap doesn’t track properly through the groove it moves through when you flex and extend your knee. Your quadriceps is made up of four different muscles. The two most important for keeping the knee cap tracking properly are the vastus medialis that runs along your inner thigh and the vastus lateralis, running along the outside of your thigh. If one of these muscles is stronger than the other, it can cause the knee cap not to track properly. Usually, the vastus medialis muscle is the weaker one, causing the knee cap to track too far to the outside.

If this muscle imbalance continues over time, it can damage cartilage, leading to a condition called chondromalacia patella. This condition, most commonly seen in young people, is often painful, especially when squatting, going up and down the stairs, kneeling or standing from a sitting position. Fortunately, abnormal patellar tracking can often be corrected by strengthening the weaker portion of the quadriceps so the imbalance is corrected. Quadriceps strengthening and flexibility exercises help, but it’s best to avoid jumping, running and squatting until you’re completely pain-free. Bracing your knee and wearing good, supportive footwear can also reduce the risk of future problems.

When Cracking or Popping is a More Serious Sign

Simple crepitus that you hear occasionally when you exercise is usually not serious as long as you’re not having pain or swelling. When you have popping with pain or swelling, especially if you’ve recently twisted your knee, it can be a sign of a torn meniscus in your knee. A meniscus is a piece of cartilaginous tissue that reduces friction when you move your knee. Small tears can cause the knee to “catch” or “pop” while larger tears can cause the knee to lock up. This is a problem that needs medical attention.

The Bottom Line?

Knee popping or crepitus with exercise is a common symptom. If your knee is completely pain-free and you have no other knee symptoms, it’s usually not something to worry about. On the other hand, knee pain or swelling needs evaluation. Even if you’re not having pain, it’s a good idea to strengthen your quadriceps. Having strong quadriceps muscles helps to prevent further cartilage deterioration.



Science Daily. “Stronger Leg Muscles Can Protect Against Knee Osteoarthritis”

Mercier LR (2008). Practical Orthopedics, 6th ed.


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When Your Knees “Crack” and “Pop” What Does It Mean?

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