A sudden involuntary forward movement of the lower leg that can be produced by a firm tap to the tendon located just below the kneecap.
Note: The term is loosely applied to any response or belief that is automatic rather than thoughtful. In this sense, it is usually a term of reproach: “Smith is a knee-jerk liberal.”
sudden kicking movement of the lower leg in response to a sharp tap on the patellar tendon, which lies just below the kneecap. One of the several positions that a subject may take for the test is to sit with knees bent and with one leg crossed over the other so that the upper foot hangs clear of the floor. The sharp tap on the tendon slightly stretches the quadriceps, the complex of muscles at the front of the upper leg. In reaction these muscles contract, and the contraction tends to straighten the leg in a kicking motion. Exaggeration or absence of the reaction suggests that there may be damage to the central nervous system. The knee jerk can also be helpful in recognizing thyroid disease.
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