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Top Causes of Back of Knee Hurts When Straightened

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Last updated December 9, 2022

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Back of the knee pain, which is usually worse when you straighten it, can be caused by a Baker’s cyst, tendonitis, arthritis, or even muscle strains. It can often be treated with rest and other at-home treatments.

Why does the back of the knee hurt when straightened?

There are several reasons the back of the knee may hurt when you straighten it. One of the most common causes is a Baker’s cyst, a pocket of fluid that forms behind the knee.

Problems with the kneecap, like instability of the kneecaps or arthritis, can cause pain that shoots from the front of the knee to the back of the knee. People who have pinched nerves or sciatica may get shooting pain at the back of the knee that travels to the foot.

You may also have muscle tightness or tendonitis, which can cause knee pain and make it difficult to fully straighten the knee.

Pro Tip

Many patients worry that pain behind the knee means that they have a blood clot. While behind the knee pain can be a sign of a blood clot in the leg, there are other more common, less dangerous causes. —Dr. Ben Schwartz

What it feels like

Depending on the cause of the knee pain, it may be a sharp, stabbing sensation or a dull ache. Pain is usually worse when you try to fully straighten the knee, which can cause a pulling sensation on the inside or outside of the knee.

Baker’s cysts cause a sense of fullness in the center of the back of the knee. It’s more noticeable when the knee is fully bent.

Dull, aching pain from kneecap problems is often worse when climbing stairs or sitting for long periods of time with the knee bent.

Knee pain from nerve problems can cause a burning or electrical sensation with numbness or tingling in the foot.


If the pain in the back of the knee is mild, you can treat it at home. Treatment for just about all causes includes:

  • Rest
  • Applying ice and heat
  • Taking antiinflammatory drugs (NSAIDs) like ibuprofen (Advil, Motrin IB) or naproxen (Aleve)
  • Changing your activities to avoid irritating your knee
  • Doing exercises that focus on stretching the hamstring tendons at the back of the knee and strengthening the quadriceps muscle at the front of the thigh.

If your symptoms don’t go away with at-home treatment in 4–6 weeks, see a healthcare provider. They may recommend physical therapy. Cortisone injections may also be used to help shrink a Baker’s cyst and reduce inflammation in the knee.

1. Baker’s (popliteal) cyst

A Baker’s cyst (also called a popliteal cyst) is from fluid behind the knee. These cysts are always caused by some other condition, like arthritis or a meniscus tear. You’ll probably notice a fullness or tightness in the center of the back of the knee that is felt even more when the knee is bent. In rare cases, the cyst may rupture (pop) and fluid will leak down the back of the knee.

Common: Baker’s cysts occur in as many as 26% of people who have knee pain [Source: Reumatismo].

Other symptoms

  • Fullness at the back of the knee
  • A visible lump behind the knee
  • Knee stiffness
  • Calf pain and swelling (ruptured cyst)

Treatment and urgency: Treatment for Baker’s cysts includes rest, ice, keeping your knee elevated, and NSAIDs. The body often absorbs the fluid, causing the cyst to shrink. The cysts may be drained or cortisone injections may be recommended to help them. Baker’s cysts may come back if the underlying cause isn’t treated.

Dr. Rx

A thorough physical exam is critical to diagnosing the cause of behind the knee pain.  Baker’s cysts may be palpable or visible behind the knee. Inflamed tendons are often tender to the touch, while meniscus tears may cause popping or catching in the knee during certain physical exam tests. —Dr. Schwartz

2. Hamstring tendonitis

Your hamstrings are a group of muscles at the back of your thigh that help bend your knee.  Tendons attach the hamstrings to your pelvis and the knee. These tendons can become inflamed by activities that require repetitive knee bending or may get injured during sports. The tendons may also become stiff if you develop knee arthritis.

The inflamed or tight hamstrings can prevent you from fully straightening the knee and can pull on the knee, causing pain. Pain may be sharp and is often located toward the outer side of the knee where one of the hamstring tendons attaches. The area may be sore to the touch or painful when you straighten your leg.

Uncommon: Hamstring tendonitis may be more common in people who have knee arthritis.

Other symptoms

  • Muscle spasms
  • Difficulty fully straightening the knee
  • A pulling sensation at the back of the knee
  • Pain with knee bending

Treatment: Hamstring tendonitis is usually treated with rest, and avoiding certain movements. Physical therapy can be helpful.

3. Calf strain

The calf muscles run down the back of your lower leg. A calf strain can happen when doing a sport that has you pushing off or going from a standing to running position quickly. Pain can happen suddenly and may be worse the next day. In some cases, the calf muscle may become partially torn, causing bruising and swelling.

Common: Calf strains are a common sports injury.

Other symptoms

  • Swelling or bruising of the calf muscle
  • Pain when pointing the toes downward
  • Pain when pushing off with the foot (like when climbing stairs)

Treatment: Calf strain is usually treated with rest, changing your activities, using ice and heat, taking NSAIDs like ibuprofen (Advil, Motrin), and stretching exercises. If symptoms don’t improve after several weeks, physical therapy may be recommended.

4. Meniscus tear

The meniscus is a ring of cartilage that sits between the two leg bones that make up your knee joint—the femur (thigh) and the tibia (lower leg). Each knee has a meniscus on the inside (back of the knee) and outside (front) of the knee. The most common area for a meniscus tear is at the back of the knee. The meniscus may tear from a twisting injury or wear and tear. Meniscus tears often cause a sharp stabbing pain in the knee.

Common: About 12-14% of people have had a meniscus tear. Degenerative (wear and tear) meniscus tears most often occur in men ages 40–60 [Source: American Academy of Physical Medicine & Rehabilitation].

Other symptoms

  • Swelling in the knee
  • Catching, locking, or giving way of the leg
  • Clicking of the knee with movement

Treatment: Treatment starts with rest, ice, and NSAIDs. Physical therapy may also help with recovery. If the knee is locked in a bent position (called a locked bucket handle tear) or if symptoms don’t improve after doing these treatments, surgery may be recommended.

Pro Tip

Many causes of pain behind the knee may be made worse by deep bending of the knee, like squatting, crouching, or lunging.  Some patients first experience symptoms after starting a new exercise routine or class that does these types of exercises. —Dr. Schwartz

5. Sciatica

Pain in the back of the knee can be from the sciatic nerve, which runs down the back of your thigh, becoming irritated, inflamed, or pinched. It can be pinched by muscles, nerves, discs, or bone spurs. Sciatica pain often shoots from the buttock down the back of the thigh and to the back of the knee. Pain is often described as burning or electrical sensation. You may also feel numbness, tingling, or pins and needles in the leg and foot. Straightening the knee may cause pain by placing tension on the sciatic nerve.

Common: About 40% of Americans experience sciatica at some point.

Other symptoms:

  • Numbness and tingling in the foot
  • Pain at the back of the leg

Treatment: Sciatica is very common and most cases go away on their own with time and home treatments. These may include rest, ice, NSAIDs, and changing your activities. Physical therapy can be helpful if your symptoms last more than a few weeks.

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The stories shared below are not written by Buoy employees. Buoy does not endorse any of the information in these stories. Whenever you have questions or concerns about a medical condition, you should always contact your doctor or a healthcare provider.
Dr. Schwartz is a board-certified Orthopedic Surgeon and Member of the Buoy Medical Advisory Board. He graduated Magna Cum Laude from the College of William and Mary (1998) with a B.S. in Biology, then obtained his medical degree from the Medical College of Virginia (2002) where he was elected to the Alpha Omega Alpha Medical Honor Society. After completing his Orthopedic Surgery Residency at Bost...
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