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a sudden reflex or involuntary movement.
Achilles jerk (ankle jerk) plantar extension of the foot elicited by a tap on the achilles tendon, while the patient is seated on a bed or chair, with feet hanging freely; called also Achilles reflex and triceps surae jerk or reflex.
biceps jerk biceps reflex.
elbow jerk involuntary flexion of the elbow on striking the tendon of the biceps or triceps muscle.
jaw jerk jaw-jerk reflex.
knee jerk see knee jerk.
tendon jerk tendon reflex.
triceps surae jerk ankle jerk.


1. A sudden pull.
2. Synonym(s): deep reflex


(jurk) a sudden reflex or involuntary movement.jer´ky
Achilles jerk , ankle jerk triceps surae reflex.
biceps jerk  see under reflex.
elbow jerk  triceps reflex.
jaw jerk  see under reflex.
knee jerk , quadriceps jerk patellar reflex.
tendon jerk  see under reflex.
triceps surae jerk  see under reflex.


1 a sudden abrupt motion such as a thrust, yank, push, or pull.
2 a quick muscular contraction induced when a tendon over a bone is tapped. See also spasm.


1. A sudden pull.
2. Synonym(s): deep reflex.


1. A sudden involuntary movement, usually of the head or a limb.
2. A reflex muscle or muscle group contraction in response to a sudden stretching by briskly tapping the tendon. A tendon reflex.


the rate of change of acceleration with respect to time.


involuntary and largely unconscious reaction in response to a peripheral stimulus; detected by an affector organ (e.g. tendon stretch receptors; heat-sensitive nerve endings within the skin), transmitted along afferent nerve fibres to central nervous system centres, and thence to an effector organ via efferent nerve fibres, e.g. skeletal muscle, causing distal limb movement, or sweat gland tissue, causing sweat flow and resultant cooling; see Table 1
  • Achilles reflex; ankle reflex deep tendon reflex triggered by percussion of Achilles tendon 5cm proximal to its insertion into posterior calcaneum; induces contraction of posterior muscle group (gastrocnemius and soleus) and brief plantarflexion of foot at ankle joint; demonstrates normal S1 function

  • Babinski's reflex see response, extensor plantar

  • Chaddock reflex see response, Chaddock

  • deep tendon reflex; DTR involuntary contraction of skeletal muscle induced by sudden and brief lengthening or percussion of subserving tendon; magnitude increased in upper motor neurone lesions, and absent/greatly reduced in any type of lower motor neurone lesion, peripheral sensory neuropathy and the elderly; enhanced by isometric contraction (e.g. hand-clenching) at time of test

  • Gordon reflex see response, Gordon

  • knee jerk reflex see reflex, patellar

  • motor reflex see reflex, deep tendon

  • muscle stretch reflex see reflex, deep tendon

  • Oppenheim reflex see response, Oppenheim

  • patellar reflex; knee-jerk reflex; quadriceps reflex sudden contraction of anterior-thigh muscle group (quadriceps) and resultant lower-limb ‘kick’ action due to brief knee joint extension; induced by transitory lengthening/percussion of patellar tendon at midpoint, 2 cm inferior to inferior border of patella when leg is relaxed and hanging pendulously from 90° flexed knee; demonstrates normal L4 function

  • pilomotor reflex contraction of smooth muscle within skin; caused by cold-induced contraction of arrector piles muscles; induces appearance of ‘gooseflesh’

  • plantar reflex plantar flexion of toes at metatarsophalangeal joints and foot withdrawal, away from stimulus; induced by a firmly moving stimulus across the plantar surface; normal response to plantar stimulation (contrast with response, extensor plantar)

  • postural reflex reflex contraction of trunk and other postural muscles; induced by excursion of the centre of gravity, to maintain upright stance (see reflex, spinal)

  • quadriceps reflex see reflex, patellar

  • spinal reflex response of longitudinal muscles surrounding the spine to sudden and imposed stretch; unexpected flexion (forward) or extension (backward) movements induce reflex contraction of the opposing spinal muscle group, maintaining the erect posture

  • tendon reflex see reflex, deep tendon

Table 1: Percussion responses
Tendon stretch excited by percussionSpinal nerve roots stimulatedReflex response elicited
Biceps brachialisC5–C6Flexion of forearm at elbow
Triceps brachialisC7–C8Extension of forearm at elbow
PatellarL3–L4Knee joint extension
AchillesS1–S2Ankle joint plantarflexion


A sudden pull.


a sudden reflex or involuntary movement.

ankle jerk
biceps jerk
called also elbow jerk; see biceps reflex.
elbow jerk
see biceps jerk (above).
jerk nystagmus
see vestibular nystagmus.
patellar jerk
see patellar reflex.
tendon jerk
see tendon reflex.