Glossary
Quadriceps lag (extensor lag)
Quick answer
Quadriceps lag, or extensor lag, is when you cannot actively straighten your knee fully even though it can be straightened by someone else. It shows that the thigh muscle is weak, switched off by swelling, or that the tendon mechanism is damaged, explains Doha knee surgeon Dr. Pablo Gelber at Aman Hospital.
How it is tested
You lie on your back and lift the straight leg off the couch, or sit and straighten the knee as far as you can. The surgeon measures the angle you reach and then gently lifts the heel to see how much further the knee can go passively. The gap between the two is the lag.
It often feels as if the leg is heavy or the muscle simply will not respond, especially after surgery. That can be frustrating, but it is common in the early days.
What it tells the surgeon
After surgery or injury, swelling sends signals that inhibit the quadriceps, so a lag is expected at first and usually improves with swelling control and exercises. A lag that persists may mean weakness that needs targeted rehabilitation.
A sudden inability to lift the straight leg after a fall, with a gap felt above or below the kneecap, is different. It suggests a ruptured quadriceps or patellar tendon, or a kneecap fracture, and needs prompt assessment because these injuries are usually repaired surgically within days to weeks.
Frequently asked questions
How long does quad lag last after knee surgery?
It often improves over the first days to weeks as swelling settles and exercises begin. Your physiotherapist tracks it closely and adjusts the programme if it is slow.
Is not being able to lift my leg after a fall serious?
It can be. If you cannot raise the straight leg at all after a fall onto the knee, see a knee surgeon promptly to rule out a tendon rupture or fracture.
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