The tonic neck reflex is an automatic movement pattern in newborns where turning the head causes the arms and legs to extend or flex in a predictable way. It’s one of several primitive reflexes that babies are born with, and it typically lasts about six to seven months before the brain matures enough to override it. The most recognized version is often called the “fencer pose” because the baby looks like they’re holding a sword.
What the Reflex Looks Like
The most familiar form is the asymmetrical tonic neck reflex, or ATNR. When a baby turns their head to one side, the arm and leg on that side straighten out while the opposite arm and leg bend. So if your baby’s head turns to the right, their right arm and right leg extend, and their left arm and left leg curl inward. It’s subtle in some babies and quite obvious in others, but the pattern is consistent.
There’s also a less commonly discussed version called the symmetrical tonic neck reflex (STNR), which works along a different axis. Instead of responding to the head turning left or right, the STNR activates when the head tilts forward or backward. When a baby’s chin drops toward their chest, the arms bend and the legs straighten. When the head tilts back, the arms extend and the legs flex. The STNR appears later, usually around six to nine months, and plays a role in helping babies learn to crawl.
How It Works in the Nervous System
This reflex is driven by sensors in the neck joints called proprioceptors, which detect the position of the head relative to the body. Early animal research showed that the reflex depends on processing in the upper portion of the spinal cord, near where it connects to the brainstem. The neck proprioceptors sense a change in head position and trigger the automatic limb response without any involvement from higher brain areas. Input from the inner ear’s balance system helps moderate the reflex, preventing it from being too strong or too easily triggered.
Because primitive reflexes like this one are controlled at such a basic level of the nervous system, they don’t require conscious thought or learning. They’re essentially hardwired. As the brain’s cortex develops over the first months of life, it gradually takes over voluntary control of movement and suppresses these automatic patterns.
Why Babies Have It
The tonic neck reflex serves as early scaffolding for more complex skills. By linking head position to arm and leg movement, it gives babies some of their first experiences connecting what they see with what their limbs do. When a baby turns their head and the arm on that side extends, the hand enters their line of sight. This is thought to be an early building block for hand-eye coordination, laying groundwork for reaching and grasping that comes later.
The reflex also helps develop muscle tone on both sides of the body, giving newborns repetitive practice with extension and flexion before they have the brain development to do it on purpose. Think of it as the nervous system’s way of keeping the muscles active and building neural connections while voluntary motor control is still months away.
When It Appears and Disappears
The ATNR is present at birth (and can even be observed in utero) and typically integrates, meaning the brain suppresses it, by about six to seven months of age. You can often see it most clearly between one and four months. After integration, the baby can turn their head freely without their limbs automatically responding, which is essential for learning to roll over, sit up, crawl, and eventually walk.
The STNR follows a slightly later timeline, usually emerging around six to nine months and integrating by about eleven months to a year. Its window of activity coincides with the period when most babies are figuring out how to get on all fours and crawl.
How the Reflex Is Tested
Checking for the tonic neck reflex is straightforward. With the baby lying on their back, a pediatrician slowly turns the baby’s head to one side, holding it briefly with the chin over the shoulder. A normal response is seeing the arm and leg on the face side extend while the limbs on the opposite side flex. The test is repeated on both sides. The response should be present but not overly rigid. If it’s so strong that the baby seems locked into the fencer position and can’t break out of it, or if it’s completely absent, that may warrant further evaluation.
What a Persistent Reflex Can Mean
When the tonic neck reflex doesn’t integrate on schedule, it can signal that the nervous system isn’t maturing as expected. Children with cerebral palsy, for instance, frequently show persistent primitive reflexes including the ATNR, along with delayed development of balance and protective responses like the parachute reflex (the instinct to throw your hands out when falling). A reflex that persists well past six or seven months doesn’t automatically mean cerebral palsy, but it is one of the signs pediatricians watch for when assessing motor development.
Other neurological conditions and developmental delays can also be associated with retained primitive reflexes. The key concern is that a reflex still active past its expected window can physically interfere with the voluntary movements a child is trying to learn. A toddler whose arm still extends every time they turn their head will have a harder time with coordinated, intentional movement.
Effects on Learning and Coordination
In older children, a retained ATNR that was never fully integrated can show up in surprisingly specific ways. Because the reflex links head turning to arm movement, it can disrupt hand-eye coordination, balance, and the ability to use both sides of the body together. Children may struggle with handwriting because turning their head to look at the paper triggers subtle changes in arm tension. Visual tracking, the ability to smoothly follow a line of text across a page, can also be affected, making reading slower and more effortful.
Research from Baylor University has highlighted a connection between retained ATNR and difficulties with reading fluency and comprehension, noting that the reflex disrupts the motor coordination that underlies smooth visual tracking. Children with these issues aren’t lacking intelligence or effort. Their nervous system is still running a background program that interferes with the fine motor control that reading and writing demand. Occupational therapists sometimes work with these children on specific movement exercises designed to help the brain finally integrate the reflex, though results vary by individual.