If you're experiencing pain in the back of your knee or leg, it might be one of these four serious injuries.

By Jenna Autuori-Dedić
October 09, 2019
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As a runner, knee pain is a real pain in the butt (OK, not really). But if you've ever gotten an achy-behind-the-knee feeling or swelling and stiffness behind your knee, you know it may prevent you from crushing that next 5K or meeting up with friends for your weekend run in the park. Posterior knee pain means you have a bigger issue to deal with—and ignoring it now won't help you later. Read on to discover some of the most common causes of chronic pain behind the knee joint and how to fix it so you can get back out there in no time. (Related: 3 Things to Consider Before Running a Race with an Injury)

Hamstring Tendonitis

The condition: If you experience sudden stiffness or back-of-knee pain at the start of your run that eventually subsides a few minutes into your workout, then you may have tendonitis in your hamstring. This typically occurs with runners who log longer distances and participate in road races (like a half-marathon or marathon) and eventually don't get enough hip flexion from fatigue and improper technique. "As you run, your hamstring fatigues and can no longer decelerate the foot from going forward in the same repetitive motion so you'll start to experience pain behind your knee," says Jessica Greaux, D.C., a biomechanist and founder of Press Play Performance Lab and Innersport Chiropractic in Berkeley, California. "The overuse occurs when the lower leg swings forward like a pendulum and thus puts a strain on the hamstrings over and over during a long run."

The treatment: See a sports doctor who will figure out what's aggravating your injury by doing a gait analysis. "This will pinpoint which muscles are not firing properly and paint a picture of your running form and technique," Greaux says. Often weak butt muscles are to blame for back-of-knee pain, but strengthening pelvic stabilizers (like the hip flexors) is also important. Try the clam opener move from Michele Olson, Ph.D., professor of exercise physiology at Auburn University at Montgomery, who regularly treats patients with this type of injury. Lie on your back and place a resistance band loop around your bent knees; open your legs by pushing against the band (an abduction). Do three sets of 25 reps.

Baker's Cyst

The condition: If you feel pain in the back of your knee when bending and straightening your leg, it might be a Baker's cyst. As you bend and straighten, you'll feel a swollen lump that feels like a water balloon and pain behind your knee. Since runners do one thing—run—legs move in a very repetitive motion. "Too much running without variations in speed or distance can cause excessive rubbing of the cartilages in and around the knee, which can irritate the soft tissues and posterior surface of the cap," says Olsen. (Related: How an Injury Taught Me That There's Nothing Wrong with Running a Shorter Distance)

The treatment: This same repetitive knee bending and straightening action from excessive overuse leads to an accumulation of fluid, which results in a cyst behind the knee. While you can take time off from training, the best way to treat this is to get an injection or have the spot aspirated to draw out the fluid. However, Greaux suggests manual therapy or Active Release Techniques (ART), a special kind of massage to treat injuries. It can break up scar tissue in the hamstring and relieve pain behind the knee. While this will help the symptoms subside, a Baker's cyst can reoccur if you have arthritis or a meniscus tear.

Torn Meniscus

The condition: A sudden fall or twisting of the knee—or just plain wear and tear—can result in a tear to the meniscus that sits between the upper and lower leg bones (this cartilage is shaped like a horseshoe). This cartilage keeps your knee steady, but it is easily worn down with age. A small tear will result in minor swelling that will get better over two to three weeks. A more moderate tear is the type to cause pain when you bend your knee. While it may go away after a few weeks, it can easily be reinjured with overuse. The most severe cases of a meniscus tear will make it challenging to walk. Your knee may feel wobbly, or it may lock up or abruptly give out during any kind of activity. (Related: 6 Reasons Why Your Knees Hurt When Running—Plus How to Make 'Em Ache Less)

The treatment: An MRI will be able to tell you how severe your injury is, but repeated icing and rest can help a meniscus tear feel better. Doctors may suggest physical therapy. You may be given certain moves to work on, such as quad sets to strengthen the knee joint and hip strengthening exercises such as clam openers. However, if pain and swelling persist, surgery is your best bet to repair and remove the damaged cartilage. While you may be off your feet for a couple of months, you'll be thrilled when you can ditch the crutches and start logging miles again.

Gastrocnemius Tendonitis

The condition: This is the calf muscle that crosses behind the knee and attaches above the knee joint. The "Gastroc" tendons can become strained when the knee is extended while the toes point upward, therefore pulling on the tendons. "This is seen more in cyclists than runners due to improper fit or fatigued muscles elsewhere in the body," says Greaux. If a bike seat is too high or too far back, your calf muscles will be put in a vulnerable position. They can become strained if the butt muscles are not doing their job, especially with heavy climbing or uphill riding or running.

The treatment: Follow the RICE treatment (rest, ice, compression, elevation) to reduce swelling and try wearing a calf sleeve for extra compression when you're out for a run. A sleeve, like 2XU Compression Calf Guard (Buy It, $45, zappos.com), will support the muscle and reduce the strain as it begins to heal. If this occurs during riding, get a professional bike fit by a qualified bike professional or medical practitioner, says Greaux.

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