Why Does My Knee Hurt When I Bend It? Common Causes, Diagnosis & Relief

Knees are incredibly complex joints that bear the weight of your body while walking, climbing stairs, running, squatting, or simply bending. If you’ve ever found yourself asking, “Why does my knee hurt when I bend it?”, you’re not alone.

Knee pain while bending is one of the most common orthopedic complaints, ranging from mild discomfort to sharp, debilitating pain. Whether it’s a twinge going downstairs, stiffness during yoga, or soreness after a long day, pain during bending often signals underlying stress, injury, or inflammation in the joint.

This guide will help you explore the top reasons your knee might hurt when you bend it, identify specific symptoms, and consider treatment options.

Quick Answer: Top 6 Reasons Your Knee Hurts When You Bend It

1. Patellofemoral Pain Syndrome (Runner’s Knee)

2. Meniscus Tear

3. Ligament Injuries (ACL/MCL/LCL)

4. Knee Bursitis

5. Tendinitis (Patellar or Quadriceps)

6. Osteoarthritis or Degenerative Joint Disease

Let’s break each one down to better understand the “why” behind your pain.

1. Patellofemoral Pain Syndrome (PFPS)

Also called “runner’s knee,” PFPS occurs when the cartilage under your kneecap becomes irritated, typically due to overuse, misalignment, or muscle imbalances.

Symptoms:

• Dull, aching pain around or behind the kneecap

• Worsens when going up/down stairs, squatting, or sitting for long periods

• A “clicking” or grinding sensation while bending the knee

Common in:

• Runners, athletes, and people with poor foot/knee alignment

Treatment:

• Rest and ice

• Physical therapy to strengthen the surrounding muscles

• Proper footwear or orthotics

2. Meniscus Tear

The meniscus is a rubbery cartilage that cushions your knee joint. Twisting injuries, especially while the foot is planted, can tear it.

Symptoms:

• Sharp pain during bending or squatting

• Swelling, stiffness, or locking of the knee

• Pain on one side of the knee joint

Who’s at risk:

• Athletes, weekend warriors, or individuals over 40 with degenerative tears

Treatment:

• Mild tears: RICE (Rest, Ice, Compression, Elevation)

• Severe tears: Physical therapy or arthroscopic surgery

3. Ligament Injuries (ACL, MCL, LCL)

Your knee is held together by four major ligaments. Injuries—especially to the Anterior Cruciate Ligament (ACL) or Medial Collateral Ligament (MCL)—can make bending extremely painful.

Symptoms:

• Sudden, sharp pain at the time of injury

• Swelling within 24 hours

• Instability or buckling of the knee

• Difficulty bending or straightening fully

Common causes:

• Sports injuries (football, soccer, basketball)

• Sudden stops, changes in direction, or collisions

Treatment:

• Bracing and physical therapy

• Surgery in severe cases

4. Knee Bursitis

The bursa is a fluid-filled sac that reduces friction between tissues. When inflamed, it can cause localized pain when bending or kneeling.

Symptoms:

• Tenderness or swelling on the front, side, or back of the knee

• Pain worsens with prolonged kneeling or repeated motion

• Warmth or redness in severe cases
  

Often seen in:

• Gardeners, carpenters, athletes, or those with repetitive knee pressure

Treatment:

• Ice and anti-inflammatory medication

• Avoiding activities that aggravate the area

• Aspiration or corticosteroid injections (in severe cases)

5. Tendinitis (Patellar or Quadriceps Tendon)

Tendinitis is inflammation of the tendons that connect your muscles to your bones. The patellar tendon, located just below the kneecap, is often affected in people who jump or run frequently.

Symptoms:

• Pain during bending, jumping, or landing

• Tenderness directly over the tendon

• Stiffness or burning sensation below the kneecap

Treatment:

• RICE protocol

• Eccentric strengthening exercises

• Stretching tight hamstrings/quads

6. Osteoarthritis (OA)

For those over 40, one of the most common causes of knee pain while bending is osteoarthritis—a degenerative condition where cartilage wears down over time.

Symptoms:

• Pain or stiffness, especially in the morning or after inactivity

• Crunching or grinding sounds (crepitus)

• Swelling, joint tenderness, reduced range of motion

Treatment:

• Weight management and low-impact exercise

• Anti-inflammatory medications

• Joint injections (hyaluronic acid or steroids)

• Knee replacement in advanced cases

Other Possible Causes

• IT Band Syndrome (pain on outer knee)

• Plica Syndrome (irritated knee folds)

• Osgood-Schlatter Disease (in adolescents)

• Referred pain from the hip or lower back

Always remember: accurate diagnosis is key. Don’t self-diagnose based on one symptom. What feels like tendonitis could be a meniscus tear—or vice versa.

When Should You See a Doctor?

You should consult a physician or orthopedic specialist if:

• Pain persists for more than 1–2 weeks

• You experience swelling, locking, or buckling

• There was a traumatic injury

• You’re unable to fully bend or straighten your knee

• Home treatments aren’t helping

Early diagnosis can prevent a minor injury from becoming chronic.

How to Get Relief from Knee Pain When Bending

Here are general strategies to ease knee pain at home, before you see a doctor:

RICE Protocol:

• Rest: Avoid activities that worsen pain

• Ice: Apply 15–20 minutes every 2–3 hours

• Compression: Use a wrap or brace

• Elevation: Raise leg above heart level

Over-the-Counter Meds:

• Ibuprofen or naproxen for inflammation

• Acetaminophen for general pain

Gentle Stretches & Strengthening:

• Quadriceps and hamstring stretches

• Low-impact activities like swimming or cycling

• Physical therapy for chronic or recurring pain

Preventing Knee Pain in the Future

• Warm up properly before workouts

• Wear supportive shoes

• Strengthen surrounding muscles (especially quads, glutes, and hamstrings)

• Maintain a healthy weight to reduce joint load

• Use proper technique when squatting, lifting, or running

Final Thoughts: Listen to Your Knees

Knee pain when bending is a warning sign, not something to ignore. Whether it stems from overuse, injury, or age-related wear and tear, the key is to identify the cause and treat it early.

So if you find yourself wondering, “Why does my knee hurt when I bend it?”—know that relief is possible. From targeted exercises to medical treatment, you have options.

Still in pain? Don’t guess—consult a doctor or physical therapist for a personalized diagnosis and care plan.