4 Possible Reasons Your Knee Hurts When You Straighten Your Knee

A female road runner holding her injured knee
A knee injury such as "runner's knee" may cause pain when the leg is straightened.
Image Credit: lzf/iStock/Getty Images

It's an athlete's greatest fear: You're training consistently each week, moving closer and closer to your personal goals. The pain in your knee begins as a little itch but becomes more difficult to ignore with each workout you finish. Before long, you're in the doctor's office because your knee hurts every time you straighten your leg.

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Unfortunately, knee pain is a common issue among all kinds of athletes and gym-goers and can happen through either overuse or direct trauma. Whether your knee pain is dull or sharp, or it hurts when you bend and straighten it, consult a doctor before continuing your training schedule.

Causes of Knee Pain When Straightening Your Leg

Knee pain can come from a variety of direct causes, whether it's sprinting downhill or even kicking a ball. However, the most common cause of knee pain is usually doing "too much too soon," says Tyler Nightingale, DPT, a physical therapist with Bespoke Treatments in New York City. A rapid increase in activity or repetitive performance of a new movement can result in tissue overload, causing swelling or pain.

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If you're feeling pain while straightening the knee, it's probably due to cartilage or joint surface damage between your larger leg bones (the femur and tibia), says Nightingale. When you extend your knee, contact between these bones increases, which can cause painful pressure. This damage is usually the result of an injury.

Here, we'll break down the most common injuries that cause knee pain when straightening the leg, which can be categorized as either overuse injuries or traumatic injuries.

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Knee Overuse Injuries

1. Runner's Knee

Tendons are strong bands of fibrous tissue that connect muscle to bone. Pain in the patellar tendon (patellofemoral pain syndrome) — which attaches the bottom of the kneecap to the top of the shinbone — is common.

It can occur from repetitive knee movements, especially jumping and climbing stairs, according to the American Academy of Orthopaedic Surgeons (AAOS).

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A common form of this syndrome is runner's knee, which is caused by overuse of the tendons through the repetitive motion of running.

2. Muscle Imbalance

Overuse knee pain can also be caused by a muscular imbalance in the quadriceps muscles, according to the AAOS.

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When the knee bends, it relies on the quad muscles and quadriceps tendon to keep the kneecap within the trochlear groove (think of this like the pathway the knee should follow). If your quads are weak or imbalanced, that might cause the knee to track abnormally through this groove, resulting in pain.

The severity of the pain determines the best treatment method for either injury, but it is typically advised to begin knee pain treatment with the RICE method, according to the AAOS. RICE stands for rest, ice, compression and elevation.

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However, if pain persists after you've iced your knee several times a day, rested properly and taken other precautions, it's a good idea to visit your doctor. He or she will most likely recommend physical therapy or orthotics if the pain can be treated non-surgically.

Traumatic Knee Injuries

1. Meniscus Tear

One common form of traumatic knee injury is a meniscus tear. Each knee contains two small pieces of rubbery cartilage called menisci located deep within the joint, according to the AAOS. Your menisci act as shock absorbers between the two main bones of the knee — the femur (or thighbone) and the tibia (or shinbone).

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Meniscus tears are common injuries that typically result from exercise or other physical activity. Sometimes this injury can feel like a "pop" in the knee and cause stiffness when straightening. Knee swelling may occur, especially with tears caused by an acute injury. And part of the torn meniscus sometimes catches between the bones, causing locking of the knee.

2. ACL Tear

Tears of the anterior cruciate ligament (ACL) are another traumatic knee injury that may be the source of pain while straightening the knee, according to Nightingale. Often, these injuries are sustained by athletes who play high-intensity sports like soccer, football or basketball where the player rapidly changes direction, stops and starts frequently or experiences direct collision, according to the AAOS.

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Unlike many overuse injuries, an ACL tear cannot heal without surgery, according to the AAOS. Typically, a surgeon must rebuild the ligament and stitch it back together in order to restore stability to the knee. After surgery, patients typically follow a physical therapy program to help regain strength and motion of the knee before returning to any physical activities.

3. PCL Tear

A posterior cruciate ligament (PCL) tear is another common injury that may cause pain in the back of your knee when you straighten your leg, according to Manhattan Orthopedic Care. Posterior knee pain can happen when you overstrain and tear the ligaments in the back of your knee, often from quick changes in direction (as in a lot of sports).

Like with ACL tears, PCL injuries usually require the help of a physician, so if you feel any sharp pain in the back of your knee when you straighten it, it's best to consult your doctor.

When to See a Doctor

If you are experiencing sharp, shooting or throbbing knee pain that prevents you from putting full weight onto your leg, it's best to see your doctor right away, says Nightingale — especially if you sustained the injury traumatically through a fall or collision.

Symptoms like locking or catching of the knee are also common signs of a more serious injury and a red flag that you should consult a doctor ASAP. Also consult the help of a professional if your knee hurts when you bend it and straighten it, as this may signal a more serious injury, like an ACL or meniscus tear.

Any chronic knee pain lasting more than a few days should be assessed as well.

Decode Your Knee Pain

To figure out what's causing your knee pain, it can be helpful to think through a few things: type and intensity, onset, location and activities.

  • Type and Intensity: ​Is the pain sharp? Dull? Throbbing? Sharp pain, as well as anything that's intense enough to change the way you move or do daily activities, is cause to talk to your doctor.
  • Onset: ​Did the pain come on all of the sudden, or has it gradually been getting worse over time? Sudden knee pain is usually a sign of an acute injury, whereas slowly developing pain tends to be due to overuse.
  • Location: ​The more centralized your pain is to the joint itself — rather than the surrounding muscles — the more likely you're dealing with an injury as opposed to a muscle ache. Pay attention to if it's in your kneecap or the back of your knee and pass on that info to your doctor or physical therapist.
  • Activities:​ Do you feel knee pain every time you straighten your knee? Or just when climbing the stairs or running? Narrow down the exact activities that worsen the pain and try to avoid them as you recover.

Relieve Your Knee Pain

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