Tonic Neck Reflex: Guide To Asymmetrical (ATNR) & Symmetrical (STNR)

What To Know About Tonic Neck Reflex — AKA Why Your Baby Looks Like They’re Fencing

November 17, 2020 Updated November 24, 2020

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Anna Shvets/Pexels

Babies’ little bodies can do some pretty amazing — and sometimes confusing — things. That precious smile? There’s a good chance that’s gas. And the way the baby grasps your finger in its palm may seem like you’ve instantly developed some type of deep connection, and while that might be true, it’s likely a reflex. So if you see your baby lounging on their back, appearing as though they’re about to grab a bayonet and become the fourth musketeer, you may leap to the assumption that they’re some type of fencing prodigy. (Think of those college scholarships!) We hate to be the bearers of bad news, but that’s a reflex, too. Specifically, tonic neck reflex, which comes in two varieties: asymmetrical and symmetrical. Here’s what to know about each.

What is asymmetrical tonic neck reflex?

More often than not, when you hear someone referring to tonic neck reflex, they’re talking about asymmetrical tonic neck reflex (ATNR) — also known as the “fencing position.” It involves the baby turning their head to one side, while the arm on that side of their body stretches out, and the opposite arm bends at the elbow. This type of tonic neck reflex typically lasts until the baby is between five and seven months old. So, hey, if your baby happens to have ATNR around Halloween, you’re already halfway to an adorable Robin Hood costume.

Why does tonic neck reflex happen? One theory is that this reflex helps the baby travel down through the birth canal more smoothly during labor and delivery. Then, once they’re out and about in the world, ATNR can assist babies with things like finding their hands and developing hand-eye coordination.

If you’d like your baby to demonstrate tonic neck reflex on command, put them on their back, and ever so gently, turn their head to one side. If ATNR happens, the arm on the same side of their face that is turned will reach out, and the other arm will look like it’s flexing beside their head.

What is symmetrical tonic neck reflex?

While the asymmetrical tonic neck reflex (ATNR) develops in utero, the symmetrical tonic neck reflex (STNR) doesn’t usually kick in until a baby is between six and nine months old, and helps them get onto their hands and knees. (For this reason, you’ll also hear it called the “crawling reflex.”) STNR involves a baby bending and stretching out their arms and legs, but it’s more complicated than that. Here’s what happens:

  • When a baby moves their chin toward their chest, and their legs straighten and their arms bend.
  • When a baby moves backward — away from their chest — and their legs bend and their arms straighten.

Not only is it super cute to watch, but STNR is also an important developmental milestone for a baby, as they learn how to move the top and bottom halves of their body independently. But, as quickly as it appeared, STNR disappears when a baby is between nine and 12 months and they’re getting the hang of walking.

In some cases, a baby can continue with the STNR after they’re a year old and pretty decent at walking. If that happens, don’t panic: Like the other parts of a baby’s development, they each have their own timelines. So, it might take them a little longer to outgrow this one. If that’s the case, you may notice your baby doing things like:

  • Walking on their hands and feet (like a bear)
  • Scooting while they’re sitting
  • Going directly to walking without a real crawling stage
  • Preferring to lounge on the floor instead of sitting up

As always, if you’re worried about your baby’s development and how they’re progressing, it’s best to discuss it with your pediatrician instead of letting your mind run wild. Chances are, everything is fine. And if it’s not, your doctor can help you navigate what comes next.