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What Are The Reasons For Pain In Back Of Knee When Straightening Leg?

The biggest joint of your body is the knee, which is also the most injury-prone area made of bone, ligaments, tendons, and cartilage. The bone in the joint can fracture or move out, whereas you can experience tear and strain in tendons, ligaments and cartilage.

Some knee injury can heal with proper rest and care; however, some may require medical intervention. Sometimes pain in the knee is a sign of arthritis that can damage the knee over time. Are you experiencing pain in back of knee when straightening leg? Yes! Then this blog will help you in clearing all your doubts regarding pain in the back of your knee.

Why Does The Back Of My Knee Hurt When I Straighten My Leg?

Here are few conditions that cause sharp pain behind knee when straightening leg, and what should you do if you have them.

Jumpers’ Knee

The injury to the tendon, which is the cord that connects the kneecap to the shinbone, is called jumper’s knee or patellar tendonitis. Jumper’s knee can happen when you jump or change direction while playing games like basketball or volleyball. These sudden movements can cause swelling, tiny tear and weaken the tendon.

The jumper’s knee can cause pain just below the kneecap, and it can worsen over time. Other symptoms of jumper’s knee are:

  • Stiffness
  • Weakness
  • Trouble bending or sharp pain in back of knee when straightening leg

Leg Cramps

A cramp is nothing but tightening of the muscles. Just like a muscle in your claves and muscle in the back of your knee are likely to cramp. You can get a leg cramp during pregnancy or while exercising. Other possible reasons why leg cramps occur are:

  • Dehydration
  • Nerve problems in your legs
  • Liver disease
  • Infections, like tetanus
  • Toxins in the blood

Your muscle will contract or spasm when you have a cramp, and this pain will last for few seconds to approx. 10 minutes. Once the cramp passes, your muscle will be sore for few hours.

Hamstring Injury (Biceps Femoris Tendonitis)

The hamstring consist of three muscles that goes down to the back of your thigh and allows you to bend your knee:

  • Semitendinosus Muscle
  • Biceps Femoris Muscle
  • Semimembranosus Muscle

If one of these muscles get injured, then it is called hamstring strain or pulled hamstring. The hamstring occurs when the muscle gets stretched too far and can take months to heal. You will feel sudden pain after you have injured the hamstring muscle. The injury to the bicep femoris muscle causes pain behind knee when straightening leg, and this injury is called biceps femoris tendinopathy. Other symptoms of this injury are:

  • Bruising
  • Swelling
  • Weakness in the back of your leg

The hamstring injury is common in athletes who have to run fast in sports like basketball, track, cricket, and soccer. You should stretch these muscle before you go out to play as it will prevent this injury.

Calf Strain (Gastrocnemius Tendonitis)

Your calf is made up of soleus and gastrocnemius muscle which is the back of your lower leg.  These muscles help you in bending your knees and pointing your toes.

Sports like squash or tennis where you need to run from a standing position strain or tear your gastrocnemius muscle. After you have strained this muscle, you will see the following symptoms:

  • Sudden pain behind knee when straightening leg
  • Bruising in the calf
  • Pain and swelling in the calf
  • Problem standing on tiptoe

To heal the calf strain, you have to rest, elevate the leg, and ice the injured area.

Baker’s Cyst

The fluid-filled sac formed behind the knee is called baker’s cyst, and fluid inside the cyst is synovial fluid. This fluid acts as the lubricant for your knee joint. However, your knee will produce a large amount of synovial fluid if you have a knee injury or arthritis, and this extra fluid can form a cyst. Symptoms of baker’s cyst include:

  • swelling behind your knee
  • stiffness
  • pain in and behind your knee
  • trouble flexing your knee

You will feel a sharp pain in your knee when the cyst bursts. However, you need to take physical therapy, steroid injection or have to drain the cyst to treat the baker’s cyst.

Meniscus Tear

The meniscus, the wedge-shaped cartilage, stabilizes and cushions your knee joint. Each knee has two menisci which are present on either side of the knee. When athletes so squat then sometimes they may tear the meniscus and twist the knee.

Your meniscus will weaken and degenerates when you get older and may tear with the twisting motion. You can hear the popping sound when your meniscus tears, and your knees will pain when you walk with the injured meniscus. Symptoms of a meniscus tear include:

  • Swelling
  • Weakness
  • Stiffness in the knee
  • Locking of the knee

Elevation, icing and rest can help in easing the symptoms and heal them faster. You may need surgery to repair the meniscus tear if it doesn’t improve on its own.

Anterior Cruciate Ligament Injury

ACL is the band of tissue that runs through the front of your knee joint and connects your thighbone to the shinbone. This tissue provides movement and stabilizes your knee.

Most ACL injuries occur when you suddenly stop, slow down and change direction while running. This ligament can get tear and strain when you get hit in a contact sport or jump incorrectly.

When the injury happens, you may hear a pop sound, and then your knee will swell up. You will have trouble while moving your knee and can feel pain while walking. Taking proper rest and physical therapy helps in healing ACL strain.

Posterior Cruciate Ligament Injury

PCL is the band of tissue just like ACL, which connects your thighbone with the shinbone to support your knee. If you take a hard blow on the front of your knee, then you can injure the PCL. Even you can injure the PCL when you miss a step walking or twist the knee.

If you stretch your ligament too far, it can cause and tear it into two parts. PCL injury can cause:

  • Pain
  • Stiffness
  • Swelling of Knee
  • Weakness of Knee

To heal the PCL injury, you need icing, elevation and proper rest. You may need surgery if you have injured more than one ligament or your cartilage is damaged.


It is a degenerative disease in which the cartilage which supports and cushions the knee will wear away gradually. There are different types of arthritis that can affect your knee, and these are:

  • Rheumatoid Arthritis
  • Psoriatic Arthritis
  • Osteoarthritis
  • Lupus

To heal arthritis pain, do exercise, use pain medicines and injections.

Read related- Knee Pain During Pregnancy – Types, Causes & Prevention

Tips to Relief Pain In Back Of Knee When Straightening Leg

  • Rest your kneel till it heals
  • You must wear a compression bandage to support your knee
  • Place ice on it for about 20 minutes several times a day
  • Use cane or crutches for taking weight off your knee
  • Elevate your injured knee on the pillow
  • You can take over-the-counter non-steroidal anti-inflammatory drugs for pain relief

If the pain in back of knee when straightening leg is not relieved, you can consult Meddo doctors to treat it.

When Should You See The Doctor?

You need to treat the pain in back of knee when straightening your leg by availing of the doctor’s consultation. You must call the doctor if you have:

  • Swollen leg
  • The affected leg is red.
  • You have a fever and are in a lot of pain.
  • Have a history of the blood clot

Doctors can diagnose the cause of the pain in back of knee when straightening leg and help you treat it.