Why does the back of my knee hurt when I straighten my leg?

What gets stretched when the knee straightens

When your knee locks straight, the structures at the back, including the joint capsule, the hamstring and calf tendons and any Baker's cyst, are pulled tight. A cyst gives a feeling of a tight ball behind the knee. Tight or strained hamstrings or calf muscles give a pulling pain that also appears when you lean forward to touch your toes or stretch your calf.

If the knee stops short of straight, with a springy block rather than just tightness, something inside the joint may be in the way, such as a torn piece of meniscus. Swelling inside the knee from any cause also limits straightening. After knee surgery, scar tissue at the back of the joint can do the same.

What you can do

• Gentle calf and hamstring stretches, held for about 30 seconds without bouncing
• Lie on your back with the heel on a rolled towel and let the knee relax towards straight for a few minutes
• Ice after activity if the back of the knee feels full
• Reduce running, jumping and deep squatting while it settles

Red flags

Get urgent same-day care if the calf is swollen, warm or tender, particularly after a long flight, surgery or a period of immobility, as this can be a blood clot. See a knee surgeon promptly if the knee is locked and will not straighten, if it swelled quickly after an injury, or if it gives way.

Assessment in Doha

Losing full straightening matters, because walking with a slightly bent knee strains the rest of the leg. Dr. Gelber at Aman Hospital measures the range, checks the meniscus, ligaments and back of the knee, and arranges an ultrasound or MRI if a cyst or tear is suspected. Treatment ranges from targeted physiotherapy to arthroscopic surgery for a tear that is blocking the knee.