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Supinator jerk- Explained

It is a deep tendon reflex with localising value
Method of elicitation
  • Forearm is semipronated and the elbow is flexed. 
  • Hold the patient's hand at wrist and
  •  a sharp tap is applied on the styloid process  (ie on the radius about 5 cm above the wrist)
  • You can observe 2 responses
  • contraction of brachioradialis and flexion of the elbow.
As this muscle was previously called supinator longus. This  reflex is known as supinator Jerk (also
referred as radial jerk)

Innervation  of Supinator Jerk
C5 -6th cervical segment
Nerve involved is radial nerve
Inverted Supinator Jerk is characterised by the following
  • Absent supinator jerk
  • An  exaggerted triceps
  • An exaggerated flexion of the fingers
Indicate a lesion at C6
When here is a lesion in the spinal cord at the  C5 segment, there is hyperexcitability of the anterior horn cells below this level. Hence during elicitation of supinator Jerk, there is no flexion response at elbow but only a brisk flexion of the fingers (as C7.8 take upperhand) occur. This occurs because there is exaggeration of reflexes sub served by segments below the fifth and sixth cervical segments. A tap on the styloid process stimulates both the contraction of the brachioradialis (which is served by C5-6 and hence abolished) and long flexors of the fingers (served by C7-8 and hence brisk).
Inversion of a jerk localises the level of lesion in the spinal cord. Usually inversion ol supinator and biceps jerks are seen together.