Why does the knee hurt when straightening the leg?
Fully straightening the knee is not a neutral movement for the structures around it. At end of extension, the patellar tendon and the quadriceps tendon come under active tension if you contract to finish the movement, Hoffa’s fat pad (just under the kneecap) can get compressed between the tibia and the femur if you lock the joint, and a damaged fragment of meniscus can get mechanically caught at that precise point in the range. These mechanisms are very different from those that trigger pain in flexion, which is why it is worth identifying exactly when in the movement your pain appears.
As with flexion, the good news is that the precise location of the pain and the exact movement that triggers it give good clues about the likely cause. That is the purpose of this page: to help you spot your pain profile to identify the most likely lead, then point you to the dedicated page for that injury so you can start a suitable rehab programme.
Does your pain appear more when bending the knee?
This guide focuses on pain triggered when straightening the leg. If your pain appears more when you bend your knee (squats, landings, stairs), this guide on pain in flexion will be more relevant for you.
Table: identify the likely cause of your pain
Use this table as a starting point. It does not replace a diagnosis, but it helps you spot the most likely lead based on the exact location of your pain and the movement that triggers it in extension.
This table only covers the broad trends: several causes can coexist, especially after a knee injury. If in doubt, the following sections detail the most common causes.
Physio insight
pain that appears only when you actively contract to straighten (against resistance) points more towards a tendon. Pain that appears when passively locking the knee, with no contraction, points more towards Hoffa’s fat pad or the joint itself.
Physio tip
note whether your pain appears when you contract to straighten (active extension) or only when the knee reaches the end of its range, locked (passive extension). This nuance, more than the intensity of the pain, goes a long way towards guiding the diagnosis.
Pain under the kneecap with the leg straight: Hoffa’s disease
This is the cause most directly associated with full extension. The infrapatellar fat pad, known as “Hoffa’s fat pad”, is a cushion of fatty tissue located just under the kneecap, between the patellar tendon and the joint. It is richly innervated and very sensitive to compression: when the knee locks into hyperextension or rests directly on the ground (kneeling), this tissue gets pinched between the tibia and the femur, which triggers the pain.[1]
Unlike tendinopathies, the pain of Hoffa’s disease does not need an active contraction to appear: simply locking the knee passively, with the leg straight and relaxed, is enough to trigger it. This criterion (painful passive extension, with no particular muscular effort) is what distinguishes it most clearly from the other causes on this page.
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Pain at a precise point in resisted extension: knee tendinopathies
If your pain appears mainly when you contract to straighten your leg against resistance (kicking a ball, finishing a squat on the way up, pushing off in a sprint), the lead shifts towards a tendon rather than Hoffa’s fat pad. Two tendons are involved depending on the exact location:
- Patellar tendinopathy: precisely under the kneecap, at the tendon insertion, painful when the quadriceps contracts to finish the extension.
- Quadriceps tendinopathy: just above the kneecap, with the same type of trigger in resisted extension (loaded leg extension, sprinting, landing).
In both cases, the pain often fades during the warm-up and comes back after exercise, a classic sign of tendinopathy that leads many athletes to carry on without adjusting their load.
Blocked or incomplete extension: meniscal injury
Extension that never quite reaches the end, with the feeling of a true mechanical latch rather than simple stiffness, combined with pain along the medial or lateral joint line, points to a medial meniscus injury or a lateral meniscus injury. It is a purely mechanical process: a detached fragment of meniscus (often called a “bucket handle” tear in the most pronounced forms) can literally get wedged between the joint surfaces and physically stop the knee from straightening fully.[2]
This is an important difference from the other causes on this page, which are mostly pains triggered by a movement. Here, it is the range of motion itself that is mechanically limited, regardless of pain. A true block short of full extension is a sign to take seriously (see below).
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Other possible causes
Three other situations can explain pain or limitation when straightening the knee, less common but easy to identify if you know their profile:
- Patellofemoral pain syndrome and patellofemoral chondropathy: pain around or behind the kneecap, better known for being triggered in flexion, but which can also get worse at end of extension because of the prolonged contact between the kneecap and the femur at that precise point in the movement. A clearly perceptible crack points more towards chondropathy, more diffuse pain towards patellofemoral pain syndrome.
- Knee osteoarthritis: a progressive extension deficit is one of the most characteristic signs of osteoarthritis, often accompanied by morning stiffness that eases with movement. The knee simply never straightens fully any more, without a sharp block as in a meniscal injury, but with a limitation that sets in over time. If this diagnosis applies to you, discover the movements to avoid and to favour in everyday life.
- Popliteal cyst: tightness or a lump at the back of the knee that can limit or hurt at the very end of extension, most often secondary to an underlying meniscal injury.
When to seek urgent care before starting a programme
In the vast majority of cases, knee pain when straightening the leg can be managed directly with a targeted rehab programme, without waiting for further tests. Some signs, however, should prompt you to seek medical advice quickly to get imaging before starting anything:
- A block short of full extension: impossible to straighten the knee fully, a meniscal sign you must not ignore
- Significant, sudden swelling that appeared within a few hours after an impact or a twisting movement
- A total inability to straighten the leg fully after an injury (direct impact, fall, high-speed twist)
- A clear feeling of instability, as if the knee was “giving way” or buckling during extension
These signs suggest a more serious injury (displaced meniscal tear, ligament injury, fracture) that requires imaging before knowing which rehab programme suits your situation. Outside these cases, you can start working today.
F.A.Q
Why does my knee only hurt when I straighten it fully?
Because full extension puts certain structures under tension or compression in a different way from flexion: the patellar or quadriceps tendon contracts to finish the movement, Hoffa’s fat pad can get compressed at the end of the range, and a fragment of meniscus can get caught at precisely that moment. The exact location of the pain and what triggers it (active contraction or passive locking) point to the likely cause, as summarised in the table above.
I can no longer straighten my knee fully, is it serious?
It depends on the context. A little stiffness that eases after a few movements is not alarming. But a true mechanical block, where the knee stays stuck in slight flexion and cannot straighten fully, especially after a twisting movement, suggests a meniscal tear and calls for prompt medical advice rather than an immediate rehab programme.
Should I stop sport if my knee hurts when I straighten my leg?
In most cases, no: it is better to temporarily adapt the movements that clearly reproduce the pain (resisted extension, sprinting, kicking) while starting a targeted strengthening programme, rather than stopping activity altogether.
When should I worry about knee pain in extension?
You should seek medical advice quickly if your knee locks short of full extension after an injury, if significant swelling appears within a few hours, or if you are completely unable to straighten your leg after an impact or a twist. These signs require imaging before starting any rehab programme.
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