Swelling of the knee can occur post-running, but it’s important to differentiate between swelling that occurs upon cessation of activity and swelling that occurs much later.
Acute knee trauma such as anterior cruciate ligament strains or ruptures, medial collateral ligament sprains, and meniscus injuries, can all result in an almost immediate onset of knee swelling. In such instances, the swelling can be very pronounced (like a ‘football’ at times). Such a swelling response is due to tissue damage or internal derangement of a knee structure. Pronounced and rapid onset of knee swelling is due to bleeding within the joint, known as a haemarthrosis.
In comparison, many athletes, particularly masters athletes, will experience more mild swelling hours after running. Such swelling is typically the result of an excess secretion of synovial fluid from the knee joint’s synovial membrane. Athletes with symptomatic knee arthropathy (OA) may often experience post-running knee swelling or effusion. Likewise athletes with anterior knee or patello-femoral pain may also experience some mild to moderate effusion around the knee after running.
If swelling persists for more than 24hrs after the run, it may indicate that the prior session exceeded the joint’s tolerance. If it’s mild to moderate swelling, then you should rest up before running again, and take note as to when the swelling occurs in relation to the length of session completed and cut as needed.
In short, in the case of significant knee swelling, cease running and seek advice from a sports physio until an accurate diagnosis is made. In cases where knee swelling is mild to moderate, it’s generally safe to continue running, but do keep an eye on potential triggers and rest accordingly.
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