(jĕrk )

1. A colloquial term for a reflex.
2. In physics, a sudden movement or change in the rate of acceleration of an object.

Achilles j.Achilles tendon reflex.

ankle j.

SEE: ankle clonus reflex.

biceps j.

SEE: biceps reflex.

elbow j.Triceps reflex.

jaw j.Chin reflex.

knee j.The extension of the lower leg upon striking the patellar tendon when the knee is flexed at a right angle. Knee jerk is absent in locomotor ataxia, infantile paralysis, meningitis, destructive lesions of the lower part of the spinal cord, and certain forms of paralysis. It is increased in lesions of pyramidal areas, brain tumors, spinal irritability, and cerebrospinal sclerosis.

SEE: knee-jerk reflex.

tendon j.The contraction of a muscle after tapping its tendon.

triceps surae j.Achilles tendon reflex.