Swollen Knee After Running? How to Treat and Prevent It

A swollen knee can be concerning, and may warrant a trip to the doctor.  Swollen knees with running should not be considered normal-despite warnings about running being “bad for the knees,” studies of both club-level runners and recreational runners have shown that running is not detrimental to the knees on a long-term basis.  On a more acute basis, increased knee swelling after running has not been seen in many studies.  So, if knee swelling is present, further investigation is warranted.  To figure out why it’s swollen, it’s important to consider if the swelling occurred acutely, or if it is a more chronic, longer-term issue.

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Acute (short term) swelling of the knee

Swelling that occurs soon after an activity (within about four hours) is much more serious than swelling that shows up, say, the next day.  It is a sign of bleeding within the knee, or hemarthrosis, which is suspicious of serious structural damage.  Basically, something has been torn or broken. Chances are that a specific injury occurred, usually involving twisting or cutting, hyperextension, or a direct blow to the knee.  A pop or crack may occur, and instability may be present.  The knee may lock, or give way.  Pain is often immediate, and results in an inability to bear weight through the knee.  Although the majority of triathlon-related injuries tend to be chronic, acute knee injuries can still happen.  For example, you twist your knee trail running, trip and land on your patella (kneecap) while running on concrete, or hit the outside of your knee on the ground during a bike crash.   Ligaments (especially the ACL) are most commonly damaged in acute sports injuries involving knee effusion.  Ligament injuries can range in severity from sprains involving some stretching, to partial tears, to complete disruptions.  Other possible acute injuries include meniscus tears and patellar dislocations or fractures.  Acute swelling can also occur outside of the knee joint, but within the prepatellar bursa, a fluid-filled sac that sits on top of the kneecap, as the result of a blow to the front of the knee.     

Chronic (long term) swelling of the knee

Swelling that arrives later or is a chronic problem is generally caused by excess synovial fluid (the lubricant in joints) in the knee, much like too much oil in a car. Overuse and underlying chronic degenerative conditions are the most common causes.  Osteoarthritis (OA), which is a wearing away of the articular cartilage that serves to cushion joints, is the most common joint problem in the United States, especially with age.  As the cartilage wears away, bones rub together more closely, leading to pain and inflammation.  Knee OA can occur in the joint between the femur (thigh bone) and tibia (shin bone), and/or in the patellofemoral joint between the kneecap and the underlying femur.  Meniscus tears are another common underlying pathology that may lead to chronic knee effusion.  As previously discussed, meniscus tears can occur with an acute injury, especially in younger athletes, but they can also be a result of chronic wear and tear over time, particularly in masters athletes.  The meniscus acts as a joint stabilizer and shock absorber, so damage or prior surgery to remove a tear compromises the knee, and contributes to the development of OA.  Chronic joint conditions may cause swelling in the back of the knee, termed a Baker’s cyst.  Finally, prepatellar bursitis can also occur as a chronic condition, particularly in individuals who frequently kneel.

How to Fix Your Swollen Knee

See a doctor.

Anytime you have a swollen joint, you should see a doctor.  A physician can help shed light on the mystery, whether by physical exam, analysis of fluid drawn from the knee, or review of images such as MRIs or X-rays.  This is especially true with sudden-onset effusion, or effusion that is idiopathic.  Knee joint effusion that is accompanied by fever can indicate septic arthritis, and should be considered a medical emergency.  Other medical causes of idiopathic knee joint swelling include gout or pseudogout, inflammatory arthritis, systemic diseases such as Lyme disease, cysts, or tumors.  Proper diagnosis is important to determine if the issue is purely a localized, mechanical joint problem, or a symptom of an underlying systemic issue.  Serious orthopedic issues that involve structural damage need to be promptly diagnosed and treated, sometimes surgically.  Chronic issues such as OA also may benefit from medical intervention, including hyaluronic acid injections that help to lubricate the knee.

Employ dynamic rest.

Even if the swelling comes without pain, avoid loading the knee until the swelling subsides. Trade knee-loading exercises for upper-body and core work.  Gentle range of motion and isometric (muscle setting) exercises can help with swelling and pain associated with less severe injuries, but only after an accurate diagnosis, as movement may be contraindicated with some ligament injuries.  In those cases, immobilization may be necessary.  

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Ice it.

Apply ice to the swollen area for 15 minutes 4–6 times a day for the first two days. Elevating the knee as you ice it can also help reduce the swelling.  Compression also may be helpful.  Research has suggested that intermittent pneumatic compression treatment improves swelling and clinical outcomes in knee OA patients.

Prevent Your Knee From Swelling

Strengthen your legs.

Strong legs protect your knees. Be sure your workout regimen includes regular lower-body strength training, in addition to any running and biking that you do.  Strength training cannot change underlying structural problems in the knee, but stronger muscles and proper alignment can help absorb force and redistribute loads in order to reduce overall stress on the joint, and can help to prevent acute injuries.  Exercise is widely recommended as a treatment for knee OA, as it has been shown to decrease pain and improve function.  Although sports that involve cutting, pivoting, and jumping put athletes at higher risk of ACL tears than straight endurance sports, faulty mechanics that result in dynamic knee valgus (where the knee collapses in) are a risk factor for this injury.  Specific strength, balance, and proprioception programs to address this can help decrease risk of any knee injury, especially ACL tears.  Thankfully, lower body strength work has plenty of benefits-helping out those knees adds to the list!

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