10 reasons for back of knee pain when straightening your leg

Home » 10 reasons for back of knee pain when straightening your leg

Do you have leg pain in the back of your knee?

From simple muscle soreness to sciatica or blood vessel concerns and beyond, I will discuss for you 10 common reasons why this might be the case when you straighten your leg.

The 10 reasons for back of the knee leg pain are:


Delayed onset muscle soreness is the most common type of posterior knee pain we hear about – especially after those new year’s resolutions kick in!

It can be caused by overuse, such as too much running without proper stretching and warm-up exercises. The pain is usually felt 12 to 24 hours after the physical activity that caused it.

DOMS generally goes away within a few days with rest, gentle massage, and foam rolling. I usually suggest a high-quality magnesium supplement as well.

Calf Strain

A common lower leg injury, especially in runners!

  • Usually caused by overuse or sudden calf muscle contraction, such as when you push off to take a step.
  • The calf is actually made up of two muscles: the Gastrocnemius, and the Soleus. These muscles work together to point your foot downward (plantar flexion).
  • The gastrocnemius is the larger, more superficial muscle that creates the “bump” on the back of your calf. It crosses both the knee and ankle joints to attach to the heel.
  • The soleus is a smaller muscle that lies underneath

So, when your calf muscle is strained, it means that one or both of these muscles have been overstretched or torn. The good news is that calf strains are usually minor injuries that heal with time and conservative physical therapy treatment or at-home care.

doctor checking the back of a patient's leg for knee pain
A physical therapist uses a range of orthopedic tests for knee pain

Hamstring Injury

The hamstrings are a group of three muscles that run along the back of your thigh. They attach at the hip and knee and help you bend your leg at the knee (knee flexion). Hamstring injuries are common in athletes, especially those who participate in sports that require sprinting or sudden stopping, like soccer, football, or tennis.

They range from a mild hamstring strain to a more serious hamstring tendon rupture.

Treatment for a hamstring injury depends on the severity of the injury.

Can tight hamstrings cause knee pain?

They can! Physical activity, but also lack of it, can cause “tight” hamstrings.

When the hamstrings are tight, they can pull on their attachment points behind the knee, causing back of the knee pain (unlike the Quadriceps causing front of the knee pain, like with patellar tendonitis). But they are not the key player in back-of-knee pain…

popliteus causing back of the knee leg pain

What is the Muscle behind the knee?

The popliteus is the muscle located behind the knee. It originates from the femur (thighbone) and the back part of the lateral meniscus and connects to the tibia (shinbone).

It helps to rotate and bend the knee and initiates the unlocking of a straight leg. It also may have an anti-shearing, protective effect on the knee joint.

Popliteus syndrome

Popliteus syndrome is a type of tendonitis that affects the popliteus tendon in the back of the knee. This condition is often seen in athletes, such as runners or skiers, who may overuse their leg muscles.

It is less common than most of the lower-leg complaints in athletes [1].

Symptoms of Popliteal Syndrome

  • Knee pain at the back of the leg
  • Swelling at the outside or at the back of the knee
  • Pain when straightening the knee
  • Sensation of weakness or giving way of the knee, when weight-bearing on the affected leg, whilst walking.

Popliteus tendonitis is treated with rest, strengthening and soft tissue release, such as Active Release Technique (ART).

Baker’s cyst

Baker’s cysts are fluid-filled sacs that form behind the knee. They are also called popliteal cysts or synovial cysts.

A Baker’s cyst is not a true cyst because it is not lined with epithelial cells (cells that line most body cavities).

Rather, Baker’s cysts are filled with synovial fluid. Synovial fluid is the lubricating fluid that surrounds joints and tendons.

A baker’s cyst can cause leg pain behind the knee when straightening your leg
[image: msk.org.au]

Baker’s cysts usually form as a result of another condition, such as arthritis or meniscus tears, but are a common cause of knee pain without injury. The excess synovial fluid builds up in the back of the knee and forms a sac (cyst).

Baker’s cysts are usually painless unless they rupture and the synovial fluid leaks into the calf muscle. This can cause pain, swelling, and stiffness in the back of the leg, especially when straightening the leg.

Baker’s cysts are treated with rest, ice, and anti-inflammatory medication. If the cyst ruptures, you may need to have it drained by a doctor.

Read about the best type of doctor to seek for knee pain here


Sciatica is characterized by pain in the lower back, buttock, thigh, or leg, arising from irritation to the sciatic nerve. This can be from muscular compression or tension upon this large nerve, which runs from the lower back, down through the buttock, into the back of your leg.

It may be isolated to just one, or multiple of these areas, sometimes making it a tricky diagnosis.

Sciatica can be caused by a variety of things, such as:

sciatica nerve back of knee pain diagram

Symptoms of sciatica include:

  • Pain in the lower back or buttock
  • Pain that radiates down the leg and into the foot
  • Numbness or tingling in the leg or foot
  • Weakness in the leg or foot

Treatment for sciatica depends on what is causing the nerve irritation/tension. A suspected spinal problem such as a disc issue should be assessed by a Chiropractor. Learn all about disc bulges here.

If it is piriformis syndrome, you may need to do stretches and physical therapy to release the muscle. Dry needling and acupuncture may also be beneficial.

Case Study:

“My knee hurts when I straighten it!”

In this case, your injury is likely to be either

  • Muscle strain in the calf or hamstring
  • Sciatica, or
  • Meniscus tear causing locking and pain going from bent to straight

The inside of my knee hurts when I straighten it

In this case, your injury is likely to be either

  • Medial collateral ligament sprain or strain
  • Medial hamstring tendinopathy
  • Medial Meniscus injury

Make sure you find the underlying cause, whether it be repetitive motions, or excess sports training – common in young adults.

Should you be walking on a sprained knee? Find out here.

Cruciate ligament tears

The cruciate ligaments are strong, fibrous bands of tissue that cross each other in the knee joint, hence the name. They help to keep the knee stable by preventing forward and backward movement of the shinbone (tibia) on the thighbone (femur).

The cruciate ligaments also provide rotational stability to the knee.

There are two cruciate ligaments in the knee: the anterior cruciate ligament (ACL) and the posterior cruciate ligament (PCL). The ACL is located in the front of the knee, while the PCL is located in the back.

The ACL and PCL
The ACL and PCL

A PCL ligament strain will cause pain in the back of the knee when straightening the leg.

Cruciate ligament tears are a common knee injury, especially in athletes who participate in contact sports, such as football and basketball. The ACL is the most commonly injured cruciate ligament.

Symptoms of a cruciate ligament tear include:

– Pain and swelling in the knee

– Joint instability

– Feeling of the knee “giving way”

– Knee pain when walking

Cruciate ligament tears can often be managed non-surgically with an effective exercise plan to strengthen the stability of the knee joint.

Some people even present to a doctor with an ACL tear that they were unaware of at the time of injury!

Cruciate ligament tears may be treated with surgery to reconstruct the ligament, especially with a complete tear.

Meniscus tear

The meniscus is a shock-absorbing C-shaped disc of cartilage that sits inside the knee. The meniscus acts as a “cushion” between these bones and helps to lubricate and protect the knee joint.

A meniscus tear can cause knee pain when straightening the leg
[Image: Andrewmeyerson, via Wikimedia commons]

A meniscus tear is a common knee injury, especially in athletes. Meniscus tears can be caused by a sudden twisting motion of the knee, especially under load, or by gradual wear and tear over time.

Symptoms of a meniscus tear include:

– Pain in the knee, often when bending it or kneeling

– Swelling in the knee

– Stiffness in the knee

– Joint instability

– Feeling of the knee “giving way”

Meniscus tears are often treated with rest, ice packs, and anti-inflammatory medication on initial diagnosis. When the acute phase is over, it becomes clearer what you should do. The answer is, it depends!

It depends on things like your level of pain, clicking, and dysfunction in the knee, and how much it locks or gives way to you. It depends on your sporting status and goals.

The more severe tears may require surgery to repair or remove the damaged cartilage, partially or completely.

Popliteal artery entrapment syndrome

Popliteal artery entrapment syndrome (PAES) is a condition caused by the compression of the popliteal artery, which is the main blood vessel that supplies blood to the lower leg, via the back of your knee. PAES can occur at any age but is more common in young adults and athletes.

The most common symptom of PAES is a pain in the back of the knee, which is made worse by exercise. Other symptoms include:

  • Swelling in the back of the knee
  • Lower leg cramping, and skin color changes
  • Numbness or tingling in the leg or foot
  • Weakness in the leg or foot

PAES is diagnosed with a combination of physical examination and imaging tests, such as an ultrasound, MRI, or angiogram. Compartment syndrome and stress fractures are other causes of these symptoms, so urgent attention is necessary to rule these all out.

Treatment for PAES depends on the severity of the condition and may include:

– Rest and ice packs to reduce inflammation

– Wearing a knee brace or support to offload the artery

– Physical therapy to improve range of motion and strength

– Surgery to release the trapped artery

If you have any of the above symptoms, it is important to seek medical attention so that the cause can be accurately diagnosed and treated. Left untreated, PAES can lead to serious complications such as tissue death. Early diagnosis and treatment are therefore essential.

doctor speaks with patient about pain behind the knee

Blood clot/Deep vein thrombosis

Deep vein thrombosis (DVT) is a blood clot that forms in the deep veins of the body, usually in the leg. DVT can occur after long periods of immobility, such as during a long plane flight or hospital stay. It can also be caused by injury to the veins, such as from surgery.

DVT is a serious condition because the blood clot can break off and travel to the lungs, causing a pulmonary embolism (PE). A PE is a life-threatening emergency.

Symptoms of DVT include:

– Pain or tenderness in the leg

– Swelling in the leg or foot

– Red or discolored skin on the leg or foot

– Warmth in the affected area

-Increased calf muscle pain when flexing the foot ( Homan’s sign)

If you have these symptoms with pain in the back of your knee when straightening your leg, seek medical attention immediately. DVT is diagnosed with a combination of physical examination and imaging tests, such as an ultrasound or MRI.

Hopefully you now have a better understanding of what might be causing your discomfort.

It’s not an exhaustive list, but a good bunch to rule out for starters. All are situations we have encountered in the practice – and each requires nuanced treatment protocols with your physical therapist or health care provider.

Which niggles have you been battling, and what have you tried for relief? Let us know in the comments below.

Home » 10 reasons for back of knee pain when straightening your leg

Author Bio

Dr Jason Whealing headshot

Dr. Jason Whealing is a Chiropractor with extensive experience across the UK and Australia. He is passionate about family care and injury management. The cases Jason works with daily include back pain, neck pain, jaw pain, sciatica, knee pain, shoulder pain, headaches and migraine.

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  1. Mohile N, Perez J, Rizzo M, Emerson CP, Foremny G, Allegra P, Greditzer HG 4th, Jose J. Chronic Lower Leg Pain in Athletes: Overview of Presentation and Management. HSS J. 2020

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