If your knee pops when you're riding a bike, it is a sign of two possible conditions. One is an overuse syndrome caused by the repetitive load you place on the knee with every pedal stroke -- the constant pounding can cause the kneecap to come off its track. The other possible cause of knee popping is a ligament tear or rupture.
The overuse syndrome that makes the knee pop or click when it flexes is patellofemoral pain syndrome. This is when the patella, or knee cap, no longer tracks properly on the groove in the femur, the bone in your thigh. As a result, each time you flex your knee while cycling, you force your knee cap to rub against the underside of the patella, thus creating the popping or clicking sound.
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While your kneecap might be misaligned and causing a pop or click as you roll along, it might not cause any pain. However, the longer you leave the condition untreated, the more likely it is that pain will result. The pain may occur gradually and feel like it is coming from behind or around the knee cap. If you leave it untreated, the condition can lead to chondromalacia, the break down of the cartilage beneath your kneecap.
Patellofemoral Pain Syndrome Treatment
Treatment for patellofemoral pain syndrome depends on the advancement of the condition. If you have knee pain, rest the knee and ice it for relief. If the pain subsides, you can begin a physical therapy regimen to strengthen and stretch the muscles that support the knee. The main focus of therapy will likely be the quadriceps in the front of your thigh, which stabilize the knee cap and allow you to straighten your leg. Therapists might also focus on the hamstrings in the back of your thigh, which allow your leg to bend.
If the audible pop in your knee while riding your bike is followed immediately by sudden pain and swelling, you probably have a tear or rupture of the anterior cruciate ligament in your knee. Consult your doctor right away if this occurs. Treatment might require you to rest and ice your knee and take anti-inflammatory medication until the swelling goes down. In serious cases, surgery to repair or replace the ligament might be necessary.