Knee Pain When Straight: 8 Causes and How to Treat Them

Written By on October 26, 2021 — Medically Reviewed By Kristopher Ceniza

Written by on October 26, 2021 — Medically Reviewed By: Kristopher Cenzia

There are several knee injuries that cause pain with the knee straight. The exact diagnosis will depend on other symptoms as well as your lifestyle, medical history, and age among other factors.

For example, athletes are more prone to jumper’s knees and runner’s knees. But, arthritis also gets more likely the older you get.

In any case, here are 8 potential causes of knee pain with the knee straight. Each cause has different symptoms, so pay attention to them.

8 causes of knee pain while straight

1) Patellofemoral pain syndrome (PFPS)

Patellofemoral pain syndrome (PFPS) is a common injury in running athletes. Thus, why it’s also known as “runner’s knee.” (1)

It’s also a diagnosis by exclusion. Meaning, you probably have PFPS if your doctor or therapist has ruled out other knee problems.

Here, your knee will feel sore if you straighten it. It can also hurt if you put pressure on your leg. The pain is often felt around the kneecap. Symptoms worsen when you do any physical activity that requires bending the knee, like (1):

  • Walking.
  • Running.
  • Standing up or sitting down.
  • Climbing or going down stairs.
  • Squatting, doing lunges, and similar exercises.

Further reading: Why and how to treat knee stiffness while bending?

2) Patellar tendinitis

Patellar tendinitis is also known as “jumper’s knee.” It happens when there’s damage and inflammation to your quadriceps tendon, leading to pain around the kneecap.

The repetitive jumping and running in certain sports are the most common causes of this injury. Some common examples include basketball, volleyball, and skiing. (2)

The pain also tends to get worse when jumping or running. But, it can also feel worse when you’re sitting for a long time. You may feel more pain with weight-bearing, and it will stop once you stop loading the leg.

Knee Force Knee Sleeve

3) Meniscus tear

Meniscus tears often happen after a sudden twist of the knee. They’re common injuries in running sports with constant changes of direction. Like soccer or tennis. (3)

Having said that, a meniscus is a cartilage that acts as a shock absorber. We have two menisci in each knee joint:

You may feel a pop at the moment of injury. Apart from pain while knee straightening, other symptoms of a torn meniscus include (3):

  • Knee locking.
  • Clicking or popping that wasn’t there before the injury.
  • Pain and swelling start 24-48 hours after the initial injury.
  • Knee instability.

4) Overworked muscles

This is known as “delayed onset muscle soreness,” or DOMS. It’s a minor injury that improves 2-3 days after intense activity. The pain in your knees can worsen with repeated bending.

DOMS can happen after:

  • A sudden bout of physical activity. Like moving to a new place, going on vacation, or walking more than you’re used to.
  • An increase in workout intensity. For example, working your quadriceps muscles too hard will lead to pain while straightening your knee.

DOMS can be a warning sign and your body could be telling you that you did too much. It doesn’t always mean you had a good workout. You don’t need DOMS for your muscles to grow.

This might also feel similar to cramps.

5) Anterior cruciate ligament tear

The anterior cruciate ligament (ACL) is one of four important knee ligaments. They connect our bones together and keep our joints stable.

An ACL tear (or any other ligament tear) happens when the ligament stretches beyond its capacity.

Most ACL tears are traumatic injuries. Meaning, you will likely be able to tell how and when it happened. Common causes include (4):

  • Twisting the knee,
  • Sudden changes of direction, or
  • After a direct hit.

This makes ACL tears a common injury in running sports.

You may feel a popping sound and the knee giving out at the moment of injury. Pain and swelling often appear during the first 24 hours. Your knee can feel unstable. It can be hard to walk as well. (4)

6) Dislocated kneecap

A kneecap dislocation happens when the patella slips out of its position. It’s commonly caused after a direct trauma on the knee. People with hypermobility are prone to patellar dislocation. (5)

In a dislocated kneecap, you may not be able to fully extend your knee. It can be painful to do so. Your knee will look deformed. Most of the time, the joint will swell up fast.

Most cases improve with physical therapy and knee bracing.

But, this knee injury also has a 15-60% recurrence rate. So, it’s key to treat them properly the first time. (5)

7) Knee osteoarthritis

This is the wear and tear of the knee joint cartilage. It happens with aging. But, some people are prone to have it early in life. For example, people with (6):

  • A BMI >25.
  • Rheumatoid arthritis.
  • A previous knee injury or surgery.
  • Physically demanding occupations.

Wear and tear can affect other knee structures as well. This makes people with osteoarthritis prone to other knee injuries. Like meniscus tears or ligament sprains.

At the beginning of this condition, you may feel pain when the knee bends. As it progresses, pain can be present with other movements. There can be joint stiffness and swelling as well.

8) Knee bursitis

A bursa is a fluid-filled sac between tendons, bones, muscles, and/or skin. It reduces friction between those structures. Knee bursitis happens when any of the knee bursae get swollen.

Knee Force Knee Sleeve

Prolonged kneeling is the most common cause of knee bursitis. The main symptom of this condition is a lump on the affected bursa. In cases of infection, there can be severe pain, redness, and fever as well.

Several types of bursitis can cause knee pain when the joint is straight. It’s key to treat them as soon as possible to prevent them from worsening.

Related: 5 Causes of knee pain while lying on your side.

Knee pain treatment

Home care

Ice your knee during the first 72 hours after injury. Do this 2-3 times per day, a few minutes at a time. This will reduce pain and swelling. Elevate the injured knee while doing this for better results.

It’s key to rest as well. Your knee needs time to heal. Avoid movements that hurt. Also, let your symptoms guide you on how much you should move. If it hurts, stop. If it doesn’t hurt, move your knee a little.

Physical therapy

Physical therapists are your best allies when it comes to recovery. This is regardless of your knee injury. A therapist will help you get back to normal as fast as possible.

A PT will also address any muscle weakness you may have. This will treat the current injury and prevent future ones.

You can also find a physical therapist that specializes in your needs. For example, there are sports PTs for athletes. Or geriatric therapists for the elderly.

Anti-inflammatory medications

These drugs can reduce pain and inflammation. But, depending on the injury, they may hinder recovery if you take them excessively.

So, for fresh injuries, it’s best to reduce the swelling as naturally as possible. For more persistent injuries like osteoarthritis, talk to your doctor about the best medications for you.

Wear a knee brace

Knee braces can help in a variety of knee conditions.

For example, some designs can reduce swelling and keep your knee stable. This can promote the healing of a torn meniscus, a ligament sprain, or a jumper’s knee.

Your doctor or therapist will let you know if you need one. They’ll also help you find the right knee brace for you.

Knee surgery

Most knee problems will improve with the treatments above. But, others may need surgery to get better. Keep in mind that this is a case-by-case decision.

This is a common scenario in sports. Some athletes may need surgery after a severe meniscus or ligament tear. This could help them get back into the field safely.

Some others may need total replacement of the joint to reduce their symptoms. Also, to improve their quality of life.


What does it mean if it hurts to straighten your leg?

It could mean several things. Most of them can heal at home or with some physical therapy. Go to the doctor to identify the cause to treat it.

How do I know if my knee pain is serious?

Please seek medical attention if you have any of the following. They could mean your knee pain is serious:

– Can’t straighten your knee.
– Can’t bear weight on the injured leg.
– The pain and swelling aren’t improving after 72 hours.
– There’s knee clicking and/or popping that wasn’t before the injury.
– Unexplained weight loss.
– Can’t sleep due to knee pain.
– You have a visual deformity on the knee joint.
– Fever or any other sign of infection.

What is the fastest way to relieve knee pain?

The fastest way is treating the root cause of knee pain. A physical therapist can help you with this. They will identify the root cause and treat it in the fastest way possible.

Conclusion: My knee hurts when straight

Now you know 8 possible reasons why your knee hurts when it’s straight. They often depend on your lifestyle, age, and occupation.

Finally, remember it’s best to go to the doctor to get your knee checked. This is key to having proper treatment and getting rid of your pain as fast as possible.


  1. Bump JM, Lewis L. Patellofemoral Syndrome. [Updated 2021 May 8]. Statpearls. From:
  2. Santana JA, Mabrouk A, Sherman Al. Jumpers Knee. [Updated 2021 Aug 6]. Statpearls. From:
  3. Raj MA, Bubnis MA. Knee Meniscal Tears. [Updated 2021 Jul 21]. Statpearls. From:
  4. Evans J, Nielson J l. Anterior Cruciate Ligament Knee Injuries. Statpearls. From:
  5. Hayat Z, El Bitar Y, Case J L. Patella Dislocation. [Updated 2021 Jul 25]. Statpearls. From:
  6. Hsu H, Siwiec R M. Knee Osteoarthritis. [Updated 2021 Jul 25]. Statpearls. From:
Mitch Torres (PT)
Mitch is a physical therapist, personal trainer, and nutrition coach. Fascinated with the knee joint, Mitch poured that passion into writing about knee pain and how to overcome it with movement. His goal is to teach you how to apply this knowledge into your daily life, so you can keep knee pain away for good.