Babinski sign
/ buh-BIN-skee /
Run a blunt point firmly up the outer edge of the sole of someone's foot, from heel toward the toes. In an adult with intact pathways, the toes curl down and away from the touch. But in a person whose upper motor neuron pathway is damaged, the big toe instead bends up toward the head while the other toes fan apart. That upward-curling big toe is the Babinski sign — named after the neurologist Joseph Babinski — and it is one of the oldest and most reliable bedside clues that the corticospinal tract has been injured.
The response is actually a primitive withdrawal reflex that everyone is born with: babies show the toe-up pattern, because the descending pathways that will later suppress it have not yet matured. As a child grows and the corticospinal tract takes control, the reflex flips to the normal toe-down response. When the corticospinal tract is later damaged by stroke, spinal-cord injury, or other disease, that mature suppression is lost and the infantile toe-up pattern re-emerges. So the sign is not a new disease appearing — it is an old reflex being released from control.
In rehabilitation the Babinski sign is a quick, equipment-free confirmation that a problem is upper motor neuron in origin, which fits it into the same family as spasticity, hyperreflexia, and clonus. It does not measure severity and it does not, by itself, dictate treatment, but it helps a clinician place the lesion and make sense of the whole pattern — a weak, stiff, hyperreflexic limb with an up-going toe is a coherent upper-neuron picture rather than a muscle or peripheral-nerve problem.
A doctor strokes the sole of a newly admitted stroke patient's affected foot; the big toe rises and the toes fan. Combined with the man's weak, stiff arm and brisk reflexes, the up-going toe confirms an upper motor neuron lesion and helps the team plan rehabilitation rather than chase a peripheral cause.
An up-going big toe on stroking the sole signals corticospinal tract injury — a released infant reflex.
The toe-up response is normal in infants under about one year and can be hard to interpret in ticklish or guarding adults. It locates the lesion but says nothing about how severe or treatable the spasticity is.