The patellar reflex, or knee jerk, is a stretch reflex in which a tap on the patellar tendon makes the quadriceps contract and the knee straighten. It tests the L3 and L4 roots and the femoral nerve. It is the best known tendon reflex.
| Nerve root | L3-L4 (L2-L4) |
|---|---|
| Area | Patellar tendon, between the lower edge of the patella and the tibial tuberosity |
| How to test | With the knee bent and the leg hanging freely or supported, the examiner strikes the patellar tendon and watches for quadriceps contraction and knee extension. |
| Muscles | |
| Nerve Supply | Femoral nerve |
Confirms the integrity of the L3-L4 reflex arc and helps maintain upright posture by resisting knee buckling.
A reduced knee jerk on one side suggests L3 or L4 radiculopathy or femoral neuropathy, while brisk knee jerks with spread to the other leg or clonus suggest an upper motor neuron lesion. Reinforcement, such as clenching the teeth or pulling the hands apart (Jendrassik manoeuvre), helps bring out a weak reflex. Tendon reflexes are graded 0 (absent) to 4+ (clonus), with 2+ normal; symmetry matters more than the absolute grade.
Root compression in the upper lumbar spine reduces the patellar reflex, with anterior thigh pain and quadriceps weakness.
Damage to the femoral nerve abolishes the knee jerk and weakens knee extension.
Damage above the reflex arc, such as cervical myelopathy, stroke or multiple sclerosis, removes inhibition and makes the reflex brisk, sometimes with spread to nearby muscles.
This website uses cookies to enhance your experience. Some are essential for site functionality, while others help us analyze and improve your usage experience. Please review your options and make your choice.
If you are under 16 years old, please ensure that you have received consent from your parent or guardian for any non-essential cookies.
Your privacy is important to us. You can adjust your cookie settings at any time. For more information about how we use data, please read our privacy policy. You may change your preferences at any time by clicking on the settings button below.
Note that if you choose to disable some types of cookies, it may impact your experience of the site and the services we are able to offer.
Some required resources have been blocked, which can affect third-party services and may cause the site to not function properly.
This website uses cookies to enhance your browsing experience and ensure the site functions properly. By continuing to use this site, you acknowledge and accept our use of cookies.
