According to the Centers for Disease Control and Prevention, 18 percent of people who exercise suffer from knee pain during any given 30 day period. Knee pain and swelling typically go together and can be caused by a host of injuries related to exercise. Anything that vigorously works the joint could be the culprit, including running, jogging, jumping rope and tennis.
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Before you can understand the causes of swelling behind the knee after exercise, you first need to understand the anatomy of the knee itself. The knee is the joint that connects your upper and lower leg bones. The knee cap, or patella, covers the cartilage that connects these bones, according to "Medicine & Science in Sports & Exercise." Meniscus, or pads, cushion the bones within the knee. Often, swelling behind the knee occurs when these pads get compressed or the ligaments and cartilage of the knee are pulled, strained or torn.
A common cause of swelling behind the knee following a workout is osteoarthritis. This condition typically occurs as you age and is the result of the thinning of the cartilage within the knee. This is very painful and can cause swelling if the joint is particularly aggravated. Being overweight and over the age of 50 increases your risk of developing this condition.
Another reason the area behind your knee might get swollen after exercise is a condition called patellofemoral pain. Following rigorous activity, the kneecap could become compressed. If the kneecap has moved, you may hear a grinding noise upon movement, along with swelling around its sides.
If the area behind your knee swells up after exercise or it is painful, you should see a doctor right away. You may have torn a ligament or have compressed cartilage, which will only get worse over time. In the mean time, rest the joint by elevating your leg and icing the area. Gentle stretches and range of motion exercises can also be completed before returning to your typical exercise regimen.