Knee Pain When Straightening Leg

Several studies offer valuable information about the problem of knee pain that accompanies you straightening your leg.

The Clinical Journal of Sports Medicine, the Journal of Orthopedic & Sports Physical Therapy, and the federal government site, Medicine Plus, proved the most helpful in understanding the causes of this problem and available treatments.

Health Report Live reviewed the most up-to-date and authoritative sources to provide meaningful physical therapy and knee pain information.

What Is Knee Pain?

It is challenging to diagnose knee pain because there are many possible causes correctly. Tight muscles around the knee can cause or aggravate knee pain, and this problem is quickly addressed with stretching. 

However, when knee pain persists to the point the problem is chronic, it is essential to consult an orthopedic specialist who has extensive experience treating and diagnosing knee injuries and conditions.

Dr. Lauren Elson, an instructor in physical medicine at Harvard Medical School, notes that knee pain is often not caused by the knee. Dr. Elson says. “The knee is often an innocent witness between the hips and the feet.”

A weak hip muscle can cause additional strain on your knees, causing more pain. Dr. Elson suggests strengthening the hip joint muscles to relieve the pain. Sometimes, the knee joint will not move properly if the muscles are not flexible. 

Other times, chronic knee pain indicates problems that can only be identified through diagnostic imaging and an educated process of elimination.

Various Symptoms of Knee Pain When Straightening Leg

Knee Pain in General

Millions of Americans seek treatment every year for their knee pain. Whether the knee pain is caused by a sports injury or simply wear and tear, it ranks among the top reasons for medical visits.

The type of injury or disorder caused by knee pain plays a significant role in medical advice and treatment. The most common causes of knee pain are sudden injuries, overuse, or degenerative conditions. The most common symptoms include stiffness, swelling, and pain.

Pain With Joint Locking

Whether it is from excessive strain, injury, or calcium loss, people can become more comfortable with the condition known as the locked knee. A locked knee is a condition that makes it impossible for a person to move their joint due to inflammation, injury, or some other chronic diseases.

Most commonly, knee locking is caused by damage to the bone and cartilage, tendons, or ligaments. The most common reason for locked knees is damage to the meniscus.

Pain Sensation Behind the Kneecap

We tend to think of the sides and front of knee pain. However, pain from the back of the knee is just as expected.

There are many causes of back-of-the-knee pain. Sometimes such pain arises suddenly, and the occurrence is gradual for others. Some people may experience swelling or inflammation, while others might not notice any. It is difficult to extend one’s leg fully due to this.

Jolting Pain Behind the Knee

This could be due to:

  • Cartilage tear – The meniscus, which provides cushioning for the knees, and the chondral, which provides a smooth coating for bones, are two common areas that can be injured. A tear in either one of these at the backside of your knee can often be evident to the touch. Cartilage damage can also be caused by wear and tear from long distances.
  • Osteoarthritis – is a condition where your cartilage begins to wear down. It can occur after an injury or as a result of age. Osteoarthritis can lead to a flexion contraction, which is a condition where the knee becomes more difficult to straighten or extend.
  • Knee sprain – can be caused by a fall, sporting injury, or a strain to the knee ligaments. Instability, swelling, and pain can occur – sometimes in the back of the knee. The Posterior Cruciate Ligament, which runs in a cross-shaped at the rear end of the joint, can be a possible culprit.
  •  Hyperextension – is a condition where the knee joint bends backward. This can usually be caused by a sporting injury or a shortened knee. Hyperextension can cause instability, pain, or discomfort at the back of the knee.
  • Baker’s cyst – is the most common cause of rear knee pain. A popliteal cyst, also known as fluid leakage from the joint, often happens due to injury or arthritis and then collects in a lump. Larger cysts can put pressure on nerves and muscles. These cysts are often seen as a window to the knee by doctors because they can indicate something else going on.
  • Hamstring issues – most people experience a hamstring problem at one time or another. This could be after a poorly judged football lunge or when sprinting. Tendonitis can develop in the hamstring. This is a condition where small tears cause inflammation and pain. A hamstring injury can cause severe pain.
  • Calf strain – Just as problems can be caused by upper leg muscles such as the hamstring, so can lower leg tissues. The soleus and gastrocnemius (calf muscles). It can become tired, strained, or torn and cause posterior knee pain. Another potential problem is tendonitis of the gastrocnemius.

Pain on the Outside of the Knee

Although there are many causes of pain around the knees, the most common is Iliotibial Band Syndrome, also known as IT band syndrome. According to Cedars-Sinai Medical Center medical researchers, anyone can get iliotibial syndrome, but it is pretty common among distance runners.

What are the symptoms of iliotibial syndrome?

The Iliotibial Band Syndrome causes pain on the outside of the knee. This could affect either one or both of the knees. It can cause a burning sensation and ache in the legs.

This pain may only be felt when a person is running or exercising and is most noticeable right after the foot strikes the ground. This pain might not be felt until finishing one’s workout and grows worse over time. Going up and down stairs often worsens this pain.

Pain on the Inside of the Knee

The medial or inner knee refers to the area closest to the other. This area can be affected by a variety of injuries or conditions. These include:

  • Injury – Inner knee pain may be caused by an injury sustained during a sport, like running, which puts strains on the knee joint.
  • Trauma – such as if someone has been injured or falls on their knees.
  • Bursitis – is a condition that results from inflammation of the bursa, a part of the tissue found in the knee. Bursae are tiny, fluid-filled sacs that keep bones, muscles, and tendons from rubbing together. Bursitis pain can be felt in the inner knee, 2 to 3 inches below the Knee joint.
  • Sprain or tear – An injury to the medial collateral ligament, or MCL, can cause inner knee pain. This may happen when the outer side of the knee is struck by a force or blow. It can also occur when the inner side of the knee is injured. This injury causes almost immediate swelling and pain.
  • Torn meniscus – The meniscus, which is the protective cartilage around the knee joint, can be torn. Inner knee pain can be caused by a tear in the medial meniscus. Also, the meniscus can wear down and cause pain when someone moves their knee.
  • Knee osteoarthritis – Osteoarthritis can cause the protective cartilage in the knees to wear down. This is especially true for the knees. This condition can cause pain in the knees, especially early in the morning.
  • Medial plica syndrome – can cause inflammation of the plica, which may also be present in the knee. Overuse injuries occur most often after an individual has increased their level of activity.
  • Rheumatoid Arthritis – is an autoimmune disorder that causes inflammation. Rheumatoid arthritis can also cause severe knee pain. It often is more severe in the morning, but it gets better throughout the day.

Popping Sound

Crepitus is a sound that is similar to popping your knuckles. This happens when carbon dioxide builds up in the synovial fluid of the joint. It is released as a gas bubble and bursts when it adjusts quickly.

Crepitus is a common condition that affects many people throughout their lives. However, these joint noises are of concern when they are accompanied by discomfort, pain, or swelling. This could indicate that medical attention is required.

Loss of Strength

One may feel weak or have their knees give out. This can happen suddenly or slowly. You may feel this as the only symptom, but you might also experience other symptoms such as popping, pain, or swelling in the knee.

Tears, injuries to the knees, and underlying inflammation conditions could be causes of knee weakness. Locking symptoms can occur either immediately following injury or, more often, after the initial severe, acute phase of injury has passed.

These symptoms can be a sign of a mechanical block, most commonly to the knee extension. However, the instability may also be due to ligamentous disease, which also causes giving way.

Another reason for locking or giving away is a patellofemoral disease, which can include patellofemoral wear, degeneration, or mal-tracking. Although this is not a mechanical locking sensation, the patient feels the locking sensation after rising from a seated position for a prolonged period of time or when squatting.

Common Causes of Knee Pain When Straightening Leg

Cartilage Injury

Patients with prior knee injuries often have cartilage injuries. This is a common source of pain and significant morbidity. Routine knee arthroscopy is performed on most patients with evidence of a cartilage defect.

Chondral injuries are common in the knee. A study that reviewed over 30,000 arthroscopic procedures found that approximately 60% of patients had high-grade cartilaginous defects. These lesion depths were 50% or greater.

Ligament Injury

The knee ligaments are the connective tissue, which is a short band of strong and flexible connective tissue that holds the knee together. Trauma, such as a collision with a vehicle, can cause knee ligament injuries. Sports injuries can also cause knee ligament injuries. A twisting injury to the knee in skiing or basketball is an example.

The femur (thighbone), tibia, and tibia (shinbone) are connected by four main ligaments in your knee, any of which might be injured.

  • Anterior Cruciate ligament (ACL) – This ligament is located in the middle of the knee and controls the forward movement and rotation of the tibia (shinbone).
  • Posterior Cruciate ligament (PCL) – This ligament is located in the middle of the knee and controls the backward movement (shin bone) of the tibia.
  • Medial collateral ligament (MCL) – This ligament provides stability for the inner knee.
  • Lateral collateral ligament (LCL) – This ligament provides stability for the outer knee.

Meniscus Injury

Common meniscal injuries to the knee are frequent. Acute meniscal tears are most common from twisting injuries. Chronic degenerative tears, which occur in older patients, can be caused by minimal twisting and stress. 

Tears can be classified as either partial or complex, anterior, posterior, lateral or posterior, traumatic, degenerative, horizontal, vertical and radial, “parrot beak,” and “bucket-hand.”

If left untreated, complex tears in the meniscal ligament can cause problems with the smooth movement of the knee and joint effusions. This could lead to osteoarthritis. Meniscal injuries may occur alone or in combination with collateral and cruciate ligament tear.

Tendon Injury

Like other tendons in the body, the patellar tendon attaches muscle to bone. The patellar tendon runs over the kneecap and connects with the quadriceps (thigh muscle) to straighten the legs. 

Active people who are involved in sports experience the most knee tendon pain. However, repetitive knee use can also cause knee-tendon problems.

The severity of a knee tendon injury will determine the treatment needed.

First, rest your knee tendon pain if it is due to inflammation. Over-the-counter NSAIDs like ibuprofen or aspirin may be helpful in relieving inflammation. Icing the affected area can also help. 

A physical therapist is the best treatment for knee tendonitis. They can help strengthen the muscles surrounding the tendon. Surgery may be necessary in rare cases of chronic tendonitis.

Knee Tendon Tear

If the patient is careful, a partial tear might be possible to heal. Your doctor may recommend a patella stabilizer knee brace or a cast. A physical therapist can help you with knee exercises for partial and full knee tendon injuries.

A complete tear of the patellar tendon almost always makes surgery necessary as the patient may not be able to walk without it.


The Vastus Lateralis, which is the outer edge of the thigh muscle, and the Vastus Medilis on the inside serve a similar function. Although the Vastus Lateralis is larger than the Vastus Medilis, both should be equally developed.

When the Vastus Lateralis muscle, which is the outer muscle, becomes so dominant and powerful that the Vastus Medilis inner muscle begins to be employed less, an imbalance can occur. 

A feedback loop continues to reinforce the imbalance, with the Vastus Lateralis taking over from the Vastus Medialis and the weaker muscle becoming neglected and atrophying. Because the Vastus Medilis is not strong enough to stabilize it, the Vastus Lateralis pulls the kneecap outwards.

Nerve Injury

Directly addressing the pain nerves that supply the knee can be a good option for patients suffering from chronic knee pain. There are three main knee pain nerves that are responsible for feeling in and around the knee: the medial and lateral genicular, infrapatellar, and medial genicular. 

These nerves can cause pain depending on their location and severity. Nerve pain (neuralgia) can be described as sharp, painful, burning, electric, and pins/needles sensations.

Bone Injury

A patella (kneecap) fracture is a broken bone at the knee joint’s front. A sesamoid bone is the patella. It is around bone embedded within a tendon that protects and shields a joint. The patella is attached to the quadriceps muscles and the shinbone (tibia) by ligaments.

Referred pain

The onset of knee pain is the most common sign that a hip problem has begun. Referred knee pain in arthritis patients can delay the diagnosis of a hip problem. This can make it difficult to diagnose a problem if the provider is not an orthopedic specialist.

Not only are hip problems a possible cause of referred knee pain, but so is the foot. Our knee joints can be affected by how we walk, especially over time. If the feet are rolled too far inward or outward when walking, it can cause the lower leg to move in a way that places additional strain on the knee.

Strained Muscles

A common injury is a knee strain. This involves a torn tendon or muscle around the knee. Sports injuries are often the cause of strains.

A 2012 analysis found that knee sprains or strains account for 42.1% of all cases of knee injuries treated in American emergency rooms.

Common Conditions That Cause Knee Pain When Straightening Leg

Causes of a knee strain or sprain can be sports injuries, trauma, or falls that overstretch the joints and soft tissue that surround the knee. Other causes include overuse, muscle weakness, reduced flexibility, and bad running form.

Here are the types of conditions that cause knee pain seen most by physicians.

Knee Arthritis

When the cartilage cushioning in the joints wears down, the result is knee arthritis. This can make the knee stiff or painful when you perform certain movements.


Bursitis refers to inflammation or irritation of the bursa sac. This is usually caused by excessive pressure or repetitive pressure on an area.

Ligament Injuries

The knee ligaments are the connective tissue, which is a short band of strong and flexible connective tissue that holds the knee together. Trauma, such as a collision with a vehicle, can cause knee ligament injuries. Sports injuries can also cause knee ligament injuries.

Jumper’s Knee

Jumper’s knee, also called patellar tendonitis or patellar tendonitis, is a condition that causes inflammation of the patellar tendon.

Meniscus Injuries

Meniscal tears can be caused by hyperflexion or twisting of the joint. These tears may also occur as a result of normal aging-related degenerative processes. Sometimes, these tears can occur at the same time as an ACL Injury.

Baker’s Cyst

A Baker’s Cyst is a fluid-filled cyst behind your knee that can cause a bulge or tightening sensation.

Runner’s Knee

The common name for runner’s knee refers to any of the conditions that cause pain around your kneecap (also known as the patella).

 ACL Tears

An ACL injury is a tear or strain of the anterior cruciate (KROO’she-ate) ligament (ACL). This strong band of tissue connects your thigh bone (femur) to your shinbones (tibia).


Chondromalacia patella, also known as “runner’s leg,” is a common condition that causes pain in the kneecap.

Ehlers-Danlos Syndrome (EDS)

Ehlers Danlos syndromes (EDS), a group of inherited connective tissue disorders, are caused by abnormalities with the structure, production, and/or processing of collagen. EDS symptoms vary depending on the type and can range from mildly loosening joints to more serious complications.

Iliotibial Band Syndrome (ITB)

Iliotibial syndrome refers to a condition where the tendon, also known as the iliotibial bands, becomes irritated from rubbing against your knee or hip bones.

LCL Sprain

A strain or tear to the lateral collateral ligament (LCL) is known as an LCL injury.

Potential Treatments for Knee Pain When Straightening Leg

Physical Therapy

Non-invasive treatment for knee pain is possible with physical therapy. It can provide both short-term and long-term benefits. These techniques can improve your knee strength, mobility, balance, and strength.

This exercise increases hip flexibility and strengthens the lower back. As with straight-leg raises, the recommendation is usually two sets of 10 reps but consult with a skilled physical therapist for the appropriate amount of exercise in your case.

Steroid Injections

Injections of medicine into the knee joint are called therapeutic knee injections. They are used to treat knee arthritis symptoms such as pain and swelling.

There are a few injection options available to help relieve knee pain, including corticosteroids, hyaluronic acid (viscosupplementation), platelet-rich plasma (PRP), dextrose prolotherapy, and saline prolotherapy. 

Different types of knee pain can be treated with different injections depending on their cause and age.

Although knee injections are safe and there is little risk of complications, there may be side effects. To decide on the best treatment plan for your condition, it is important that you discuss all options for knee injections with your healthcare provider.

However, the American Academy of Orthopedic Surgery states that evidence supporting PRP and cortisone effectiveness is inconclusive. Stem cells and PRP are more recent and therefore have less evidence. Strong evidence suggests that viscosupplementation is not effective.


Arthroscopic Surgery

This type of surgery involves the surgeon inserting a thin tube that has a tiny camera at the end through small holes in the skin. It allows the surgeon to view inside the knee. 

The surgeon will then be able to remove the damaged cartilage, which is the protective covering around the bones of the joint. To remove any cartilage or bone fragments that might cause pain, the knee can be cleaned.

Most people are able to return to their normal activities within a few days. Most people recover quickly and with no pain.

Arthroscopic surgery can provide temporary relief from pain and may delay more complicated surgery.

Knee Replacement Surgery

Your doctor may recommend knee replacement if you have exhausted all options for osteoarthritis treatment and still experience knee pain. This can reduce pain and improve mobility.

A knee replacement procedure involves the removal of all or part of the knee joint and the replacement with an artificial one. These can be made from metals or plastics. Robotic surgery is often possible in certain cases.

If you are suffering from knee pain, it may be time for knee replacement surgery if:

  • You have severe knee pain that restricts your daily activities
  • Day or night, moderate or severe knee pain when you rest
  • Chronic knee swelling and inflammation that persists beyond the time you take medication or rest
  • Bowing in or out of your legs
  • NSAIDs do not provide pain relief or are not tolerated by the patient

Regenerative Options

The growth of regenerative medicines offers promising treatment options. The most promising options are platelet-rich plasma (PRP) and mesenchymal stem cell (MSCs) injections. These can be used to relieve pain, restore functional capacity, and possibly repair tissue.

How Do I Know if My Knee Pain Is Serious?

Swelling is a key sign to watch out for. Serious injuries can cause swelling in the knee. This is particularly true if the swelling occurs quickly. It is not possible for someone to be arthritic and then have some swelling. 

However, if you notice rapid swelling that makes it difficult to see your kneecap, this is a sign that something is wrong with your knee.

When to See a Doctor

  • If you are unsure, call your doctor.
  • You cannot support your weight on your knees, or your knees feel unstable.
  • You have noticeable knee swelling.
  • You cannot extend your knees or flex your ankles fully.
  • Observe a deformity in your knee or leg.
  • A fever is accompanied by redness, pain, swelling, and swelling of the knee.

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Sean Byers, MD

Sean Byers, MD

Sean Byers is currently a Resident in the Internal Medicine program at UTMB. He studied at the University of Queensland School of Medicine as well as received his Master’s in Public Health with a focus in epidemiology and biostatistics at the University of Southern California. His background is in biology, computer science, public health, and internal medicine.

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