Pain at the back of the knee is usually from a problem in the joint, tendons, muscles, or blood vessels in that area, and the exact cause depends on how it started, what it feels like, and any recent injuries or illnesses you’ve had. This kind of pain can be minor (like a muscle strain) or occasionally serious (like a blood clot), so “context” really matters.

Why does the back of my knee hurt?

Below are the most common medical reasons people get pain behind the knee, plus red-flag symptoms when it might be an emergency.

This is general information, not a diagnosis. If anything feels severe, sudden, or “not right,” get seen urgently.

1. Common everyday causes

These are frequent reasons, especially if you walk, stand, run, or play sports a lot.

  • Muscle or tendon strain (calf or hamstring):
    • Often happens after sprinting, jumping, uphill walking, or suddenly increasing activity.
    • Pain is usually sharp at first, then becomes a dull ache at the back of the knee or upper calf/thigh.
    • You might feel tightness, soreness when stretching, or pain when pushing off the ground.
  • Overuse / repetitive strain:
    • Long days on your feet, new workout routines, or poor footwear can irritate the structures behind the knee.
    • Pain may build gradually, feel worse after activity, and ease with rest and ice.
  • Tendinitis (irritated tendon):
    • Aching or burning where tendon meets bone, often worse with movement like going up/down stairs, squatting, or running.
    • Sometimes mild swelling or tenderness to touch behind the knee.

2. Issues inside the knee joint

Problems inside the joint sometimes “refer” pain to the back of the knee.

  • Baker’s (popliteal) cyst:
    • A fluid-filled sac that forms behind the knee, often linked to arthritis or cartilage tears.
    • Feels like a lump, tightness, or fullness behind the knee; bending fully can feel uncomfortable.
    • If the cyst leaks or ruptures, pain and swelling can suddenly spread down the calf, which can mimic a blood clot and needs assessment.
  • Osteoarthritis or inflammatory arthritis:
    • More common as we get older, or in people with previous knee injuries or autoimmune conditions (like rheumatoid arthritis).
    • Stiffness, especially in the morning or after sitting; grinding/creaking sensations; difficulty fully bending or straightening.
    • Pain may be around the whole knee, including behind it.
  • Meniscus tear (cartilage tear):
    • Often from twisting on a planted foot (sports, turning quickly) but can also occur gradually with age.
    • Pain with twisting, squatting, or deep bending; possible catching, locking, or giving way.
    • Posterior meniscus tears can create pain mainly at the back or inner-back of the knee.
  • Posterior cruciate ligament (PCL) injury:
    • Usually from a fall directly onto the front of the shin, a car accident (dashboard injury), or a major sports trauma.
    • Pain, swelling, and a sense of instability, difficulty with bending and weight-bearing.
    • Often needs imaging and sometimes surgery, depending on severity.

3. Circulation and nerve problems (more serious)

These causes are less common but more urgent to rule out.

  • Deep vein thrombosis (DVT – blood clot):
    • Can cause deep ache or pain behind the knee or in the calf.
    • Red flags: one leg is more swollen than the other, warm to touch, red or discolored, pain worse when standing or walking, recent surgery, long travel, pregnancy, smoking, clotting disorders, or taking certain hormones.
    • This is an emergency because clots can travel to the lungs; seek urgent care or emergency services if you suspect it.
  • Nerve compression or irritation:
    • Sharp, burning, or shooting pain, sometimes with numbness or tingling down the leg.
    • May be from back issues, cysts, or swelling compressing nerves behind the knee.
    • Needs medical evaluation if persistent or associated with weakness.

4. How to think through your pain

Use these questions to narrow what might be going on (this doesn’t replace a doctor, but can guide what to do next):

  1. How did it start?
    • Sudden after a twist, sprint, or fall → think strain, ligament, or meniscus.
    • Gradual over weeks → think overuse, arthritis, Baker’s cyst.
  2. Where exactly is the pain?
    • Central directly behind the knee → Baker’s cyst, joint issues, or DVT.
    • High up toward back of thigh → hamstring strain.
    • Lower near the top of the calf → calf strain or DVT.
  3. What makes it worse or better?
    • Worse with stairs, squats, or twisting → often joint/cartilage.
    • Worse when stretching the calf or hamstring → muscle/tendon.
    • Constant, throbbing, with swelling/warmth → worry more about clot or infection.
  4. Any other symptoms?
    • Locking or “giving way” → meniscus or ligament.
    • Fever, redness, feeling ill → possible infection (urgent).
    • Chest pain, shortness of breath, or coughing blood → emergency (could be clot that moved).

5. When you should see a doctor urgently

Get same-day emergency or urgent care if you notice:

  • Sudden swelling, warmth, and redness in one leg, especially after travel, surgery, or long immobility.
  • Severe pain that came out of nowhere or after a serious injury and you can’t walk or bear weight.
  • The knee looks deformed or you heard a “pop” and it swelled immediately.
  • Fever, feeling unwell, and a hot, very tender knee.
  • New chest pain, shortness of breath, or coughing up blood (call emergency services).

These situations should not wait for a routine appointment.

6. What you can do at home (if no red flags)

If your symptoms are mild and there are no warning signs:

  • Rest and modify activity:
    • Cut back on running, jumping, or deep squats for a few days.
    • Switch to low-impact movement like walking on flat surfaces or cycling (if it doesn’t hurt).
  • Ice for recent, activity-related pain:
    • 10–15 minutes, a few times a day for the first 48–72 hours, with a cloth between ice and skin.
  • Compression and elevation:
    • A soft knee sleeve or elastic bandage (not too tight) can help swelling.
    • Elevate your leg above heart level when resting.
  • Over-the-counter pain relief:
    • Paracetamol/acetaminophen or, if you tolerate them and have no contraindications, NSAIDs (like ibuprofen) can reduce pain and inflammation.
    • Always follow package instructions and your doctor’s advice, especially if you have kidney, heart, stomach, or bleeding issues.
  • Gentle stretching and mobility (after the worst pain eases):
    • Light calf stretches (facing a wall, one leg back), hamstring stretches (lying on your back with a towel around your foot) in a pain-free range.
    • Stop if pain sharpens or feels unstable.

7. What a doctor or physio might do

If you see a professional, they may:

  • Ask detailed questions about onset, activities, past injuries, and medical history.
  • Examine how you walk, bend, and straighten the knee; press around the joint; test ligaments and muscles.
  • Order imaging:
    • X-ray for bones and joint space (arthritis, fractures).
    • Ultrasound for cysts or DVT.
    • MRI for ligaments, meniscus, and deeper structures.

Treatment can range from:

  • Targeted physiotherapy and exercise plans.
  • Bracing or activity modification.
  • Draining a Baker’s cyst or treating underlying arthritis.
  • Blood thinners for a clot (hospital-level care).
  • Surgery for severe ligament or meniscus damage.

8. Simple example scenarios

  • Weekend runner, sudden pain at the back of knee mid-sprint:
    Likely a calf or hamstring strain or PCL/meniscus issue; rest, ice, and see a clinician if walking is difficult or pain persists more than a week.

  • Middle-aged person with long-standing knee stiffness, new bulge behind knee:
    Could be a Baker’s cyst linked to arthritis; needs a proper exam, sometimes imaging, and treatment of the underlying joint problem.

  • Recent long-haul flight or surgery, now one leg is swollen, warm, and hurts behind knee:
    Possible DVT; this is an emergency, and you should seek immediate care.

9. Quick Scoop (SEO-style summary)

  • The question “why does the back of my knee hurt” is trending because many people notice this pain after workouts, long desk days, or as they get older.
  • Common causes include muscle or tendon strain, Baker’s cyst, arthritis, meniscus tears, and—less commonly—blood clots or nerve problems.
  • Online forum discussion often shows people confusing a Baker’s cyst “ball” behind the knee with a pulled muscle, or worrying about DVT after flights, which is why “latest news” style health articles now emphasize red-flag symptoms and when to go to the ER.
  • Most mild cases improve with rest, ice, and gradual stretching, but sudden severe pain, big swelling, redness, warmth, or trouble breathing need urgent medical attention.
  • If your pain lingers more than 1–2 weeks, gets worse, or affects walking, it’s worth seeing a professional to get the exact cause and a tailored treatment plan.

If you tell me a bit more (when it started, what you were doing, your age, and any other health issues), I can help narrow down the most likely causes and what next steps make sense for you.