Babinski reflex

How it works

The test is simple. A blunt object is drawn along the outer edge of the sole, from the heel toward the little toe, and then across toward the big toe. In a healthy adult the toes curl downward. This is the normal plantar reflex¹.

In a baby the opposite happens. The big toe rises, and the other toes spread apart like a fan. This is the Babinski sign¹. Both responses are reflexes that run through the spinal cord. The difference is what the brain is doing to them.

In an adult, a bundle of nerve fibres called the corticospinal tract runs from the brain down the spinal cord. It carries orders for movement, and it also holds the older spinal reflexes in check. One of the reflexes it suppresses is the toe-lifting response¹⁴. In a baby this tract is not finished. Its fibres have not yet grown their fatty insulation, so the brain cannot fully control the spinal cord, and the old reflex shows through. The sign is normal in infants up to about 12 to 24 months¹. It fades as the tract matures.

How common it is

The sign is present in most healthy newborns. In one study of 256 healthy infants, 73.8% showed it¹. It is one of the primitive reflexes catalogued in early paediatric neurology, and later studies of infant development confirmed that it fades as the corticospinal tract matures, usually by 12 months¹. In an adult with a healthy nervous system, a positive sign is uncommon.

Is it a problem?

In a baby, no. It is expected and doctors check for it.

In an older child or an adult, a Babinski sign means that the corticospinal tract is not doing its job somewhere in the brain or spinal cord. Causes include stroke, multiple sclerosis, traumatic brain injury, and amyotrophic lateral sclerosis³. This is why neurologists test it in almost every examination. Sometimes it is the first and only sign of a serious disease¹. A single test can be hard to read, so doctors combine it with other checks. The sign does not say what the cause is, only where to look.

History and culture

The French neurologist Joseph Babinski described the sign in 1896². He was looking for a way to tell apart weakness caused by nerve damage from weakness caused by hysteria, and he found that the toe gave the answer⁵. Doctors also compare the sign with the palmar grasp and other primitive reflexes that babies are born with and lose over their first year⁴.