Babinski phenomenon: A neurologic reflex that constitutes an important medical examination based upon what the big toe does when the sole of the foot is stroked. If the big toe goes up, that may mean trouble.
The Babinski reflex is obtained by stimulating the external portion (the outside) of the sole. The examiner begins the stimulation back at the heel and goes forward to the base of the toes. There are diverse ways to elicit the Babinski response. A useful way that requires no special equipment is with firm pressure from the examiner's thumb. Just stroke the sole firmly with the thumb from back to front along the outside edge.
Too vigorous stimulation may cause withdrawal of the foot or toe, which can be mistaken as a Babinski response.
Most newborn babies are not neurologically mature so they normally give a Babinski response. Upon stimulation of the sole, they extend the great toe. Many young infants do this, too, and it is perfectly normal. However, in time during infancy the Babinski response vanishes and, under normal circumstances, should never return.
Asymmetry of the Babinski response -- when it is present on one side but not the other -- is abnormal. It is a sign not merely of trouble but helps to lateralize that trouble (tell which side of the CNS is involved).
The Babinski reflex is known by a number of other names: the plantar response (because the sole is the plantar surface of the foot), the toe or big toe sign or phenomenon, and the Babinski sign. (It is wrong to say that the Babinski sign is positive or negative; it is present or absent).
Babinski, despite the Slavic sound of the name, was French: Joseph Francois Felix Babinski (1857-1932). His name will not be soon forgotten in medicine, thanks to the Babinski phenomenon.