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

Achilles jerk

SEE: Achilles tendon reflex.

ankle jerk

SEE: ankle clonus reflex

biceps jerk

SEE: biceps reflex

elbow jerk

SEE: Triceps reflex.

jaw jerk

SEE: Chin reflex.

knee jerk

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 jerk

The contraction of a muscle after tapping its tendon.

triceps surae jerk

SEE: Achilles tendon reflex.