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How to elicit plantar reflex?

Plantar response is a local reflex arc modified by the pyramidal tract.
Position of the patient
All the leg muscles should be visible and in a relaxed state. It can be achieved by positioning the patient in a way that the knee is slightly flexed and the thigh is externally rotated. The patient should be warned that the sole is going to be scratched and ask him to try to let his limb remain as floppy as possible. You should not be touched the toes.
Many factors can influence the clinical response including the site of stimulation, the intensity of stimulation, and even the object used for stimulation . Any part of the leg can be stimulated, but the best technique is to stimulate the lateral plantar surface and the transverse arch in a single movement upto the middle metatarsophalangeal joint with a firm applicator lasting 5 to 6 seconds.
Method of elicitation of elicitation of plantar reflex.
  • The patient lies supine with extended legs and there should be relaxation of the muscles of the Iower limb.
  • Hold the leg firmly above the ankle joint with examiners  left hand 
  • The outer border of the sole is scratched gently by a key (preferable). stick of the hammer or blunt needle. 
  • Starting from the heel, go along the lateral border towards the little toe and then turn medially across the metatarsus upto the head ol 2nd metatarsus in a semicircular fashion. 
  • You should never touch the ball of the great toe and the flexor creases of the toes.
  • The stimulus should not produce injury but it should be of noxious character as  this is a ‘nociceptive reflex'. 
  • You should stop stimulating the sole as soon as you get the first movement of the great toe.
  • Now do the test on the other side. 
What are the precautions taken before eliciting plantar response ?
  • You should assess the following before checking the reflex
  • Assess the thickness of the sole—Thick sole may be responsible for absent response .
  • Look for any deformity of great toe—Move the great toe. and confirm that it is moving freely and
  • It is not rigid.
  • Knee must be extended.
  • Sole should be made warm (eg. by rubbing with your palm, specially in winter seasons
  • Leg should lie straight  don't allow rotation of thigh.
  • One may assess the power of extensor hullucis longus before doing the test (optional).

What is the root value of this reflex ?
Why the lateral aspect of the sole is stimulated ?
1.Lateral aspect of the sole is preferred because the receptors for extensor plantar response are present there in abundance and hence,even. In minimal UMN lesion, extensor plantar response is obtained  by stimulating the lateral aspect of the sole.
2. If the  medial aspect of the sole is stimulated, it may elicit flexion (even in the presence of UMN
lesion) of all the toes as part of the grasp relex,which is a different phenomenon.