Experiencing pain in the back of your leg behind the knee could mean a lot of different things. Here are a few of the potential causes and what to do about it.
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Whether you're a runner or not, knees and joints can get creaky and achy from time to time, but something about a back-of-knee pain just hits different. While you might be tempted to keep running through it or foam roll until you're convinced it's better, ignoring pain behind the knee now won't help you later. Read on to discover some of the most common causes of chronic pain in the back of the knee and how to fix it so you can get back out there in no time.

Common Reasons for Back of Knee Pain

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 (such as 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 pain in the back of your knee by doing a gait analysis. "This will pinpoint which muscles are not firing properly and paint a picture of your running form and technique," says Greaux. Often weak butt muscles are to blame for back-of-knee pain, but strengthening pelvic stabilizers (such as 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.

What Is the Proper Running Form, Anyway?

Baker's Cyst

The condition: If you feel pain in the back of your knee when bending as well as pain in the back of your knee when 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. If you're an avid runner, all that mileage could be to blame. "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: While you can take time off from training, the best way to treat this pain in the back of your knee while straightening and bending the leg is to get an injection or have the spot aspirated to draw out the fluid. Greaux also 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, so speak with your doctor to find the best course of action for you.

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 in your knee when bending. 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 knee pain from a meniscus tear at least 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 back-of-knee pain and swelling persist, surgery is your best bet to repair and remove the damaged cartilage. You may be off your feet for a couple of months, but it will be worth it to be rid of this knee pain for good.

Gastrocnemius Tendonitis

The condition: The gastrocnemius 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 aren't 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 to limit back-of-knee pain. A sleeve, such as 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.