Knee jerk
The reflex tested by tapping just below the knee causing the lower leg to suddenly jerk forward. What is tapped to elicit this reaction is the patellar tendon, the tendon that runs down from the quadriceps muscle in the front of the thigh, over the kneecap (the patella), down to the lower leg. And what happens is that the quadriceps contracts and abruptly brings the lower leg forward. This reaction is involuntary since it occurs without the person willing it to happen. The knee jerk is best tested by giving a smart tap on the patellar tendon while the lower leg hangs loosely at a right angle with the thigh. The test is part of the clinical neurologic examination. The knee jerk is a deep tendon reflex (DTR). (The ankle (Achilles tendon) jerk is another.) DTRs can be graded: {{}}zero absent 1+ hypoactive (underactive) 2+ "normal" 3+ hyperactive (overactive) without clonus (extra jerks) 4+ hyperactive with unsustained clonus (just 1 or 2 extra jerks) 5+ hyperactive with sustained clonus (continued jerking) The normal knee-jerk reflex may range from hypoactive (1+) to brisk (3+). An asymmetric reflex in which one knee jerk on one side is different than that on the other side is abnormal. So is the spread of tendon reflexes (tapping one knee and observing contraction of muscles elsewhere). And sustained clonus is also distinctly abnormal. Absence of the knee jerk can be due to an abnormality in the "reflex arc" required for the reflex to occur (the muscle "spindles" or the nerve fibers going from the patellar tendon to the spinal cord and returning from the spinal cord to the quadriceps). With a stroke, the knee-jerk reflex may at first be underactive, then recover and become hyperactive within a day or two. The knee jerk has been so often tested and become so familiar that it has given rise to the adjective "knee-jerk" as in a knee-jerk reaction. Knee-jerk in this figurative sense means "readily predictable to the point of being automatic." It often has a negative connotation and conveys the idea of an all-too-hasty, impulsive, irrational response based on a preset idea. For example, a dictator's knee-jerk response to a democratic movement is to suppress it. The knee-jerk reflex is also medically called the patellar reflex. It is less often referred to as the knee phenomenon, the knee reflex, the patellar tendon reflex, the quadriceps reflex, or the patellar tendon reflex.

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knee jerk n an involuntary forward jerk or kick produced by a light blow or sudden strain upon the patellar ligament of the knee that causes a reflex contraction of the quadriceps muscle called also patellar reflex

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patellar reflex.

Medical dictionary. 2011.

Look at other dictionaries:

  • knee-jerk — is a popularized technicality taken from the physical meaning ‘a sudden involuntary kick caused by a blow on the tendon just below the knee when the leg is hanging loose’. It is now popular among politicians, broadcasters, etc., especially in the …   Modern English usage

  • Knee jerk — (Physiol.) A jerk or forward kick of the lower part of the leg produced by a blow or sudden strain upon the patellar tendon of the knee, which causes a sudden contraction of the quadriceps muscle; the patellar reflex. [Webster 1913 Suppl.] …   The Collaborative International Dictionary of English

  • knee-jerk — adj [only before noun] a knee jerk reaction, answer etc is what you feel or say about a situation from habit, without thinking about it = ↑automatic ▪ A victim s knee jerk reaction to the crime is often revenge …   Dictionary of contemporary English

  • knee-jerk — knee ,jerk adjective only before noun a knee jerk reaction is immediate and not carefully considered: Calling for longer prison sentences is just a knee jerk reaction to a rise in crime …   Usage of the words and phrases in modern English

  • Knee jerk — Knee jerk, n. emotional and predictable; of certain people and their reactions to events. [PJC] …   The Collaborative International Dictionary of English

  • knee-jerk — knee′ jerk adj. cvb Informal. reacting in an automatic, habitual manner; unthinking: a knee jerk liberal[/ex] • Etymology: 1895–1900 …   From formal English to slang

  • knee-jerk — ☆ knee jerk [nē′jʉrk΄ ] adj. [< KNEE JERK] Informal designating, characterized by, or reacting with an automatic, predictable response …   English World dictionary

  • knee-jerk — ► NOUN ▪ an involuntary reflex kick caused by a blow on the tendon just below the knee. ► ADJECTIVE ▪ automatic and unthinking: a kneejerk reaction …   English terms dictionary

  • knee-jerk — patellar reflex, neurological phenomenon discovered and named 1876; the figurative use appeared soon after the phrase was coined …   Etymology dictionary

  • knee jerk — knee′ jerk n. phl a reflex extension of the leg resulting from a sharp tap on the patellar tendon • Etymology: 1875–80 …   From formal English to slang

  • knee jerk — n. PATELLAR REFLEX …   English World dictionary

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