The Asymmetrical Tonic Neck Reflex (ATNR) is an involuntary, automatic movement pattern observed in infants that is foundational for early neurological and motor development. These primitive reflexes are present from the fetal stage and help the newborn survive and build complex motor skills. The ATNR must emerge and then disappear on a set timeline to allow for voluntary movement to take over.
Understanding the “Fencing Posture”
The ATNR is often called the “fencing reflex” because of the distinct pose the baby assumes when the reflex is triggered. When an infant is lying on their back and their head is gently turned to one side, the reflex causes an asymmetrical response throughout the body, mediated by the brainstem.
The arm and leg on the side the baby is facing (the face side) will straighten or extend. Simultaneously, the arm and leg on the opposite side (the back of the head side) will bend or flex. This pattern of extension on one side and flexion on the other is what gives the reflex its familiar nickname.
The resulting posture mimics the “en garde” stance of a fencer, with the extended arm appearing to hold a sword. Pediatricians often test the ATNR by gently rotating the baby’s head while they are supine to observe this characteristic response. The reflex is typically most noticeable between one and four months of age, as the baby begins to actively move their head.
Developmental Role and Integration
The ATNR begins to develop early, with evidence of its presence as early as 18 weeks of gestation, where it may contribute to the sensation of fetal movement or “kicking”. This reflex is thought to play a preparatory role in the birth process, assisting the infant in navigating the birth canal during a vaginal delivery. After birth, the reflex continues to serve several functional purposes in the newborn.
One function is believed to be preventing the infant from rolling over too early, before the necessary strength and control are developed. The reflex also provides an early mechanism for developing muscle tone and body awareness on both sides of the body. The reflex is foundational for the development of hand-eye coordination.
When the baby turns their head, the extended arm enters their line of sight, linking visual focus with arm movement. This repeated action helps the infant establish an understanding of their limbs in relation to their visual field. The reflex is present at birth and should gradually integrate as the infant’s central nervous system matures.
Integration occurs when higher brain centers inhibit the primitive movement pattern, allowing voluntary, purposeful movements to replace it. The ATNR typically integrates between four and six months of age. It should be fully inhibited by seven to nine months at the latest to allow for more complex motor development.
Signs of a Retained Reflex
When the ATNR does not integrate on time, persisting beyond six or seven months, it is considered a retained reflex that can interfere with subsequent motor skill development. The continued presence of the ATNR forces movement to remain asymmetrical and can make complex, coordinated tasks difficult for a developing child. This non-integration can indicate an underlying developmental or neurological concern.
Observable signs of a retained ATNR in older children include:
- Difficulty crossing the midline of the body (e.g., bringing both hands together).
- Challenges with smooth visual tracking.
- Poor fine motor skills, such as messy handwriting.
- General motor clumsiness or poor balance.
- Struggling to use both sides of the body together efficiently.
- Lack of clear hand dominance.
Other indicators include difficulties learning to ride a bicycle or problems with reading, such as frequently losing their place on the page. This persistence can also affect posture and stability, manifesting as slumping or leaning when sitting. Parents who observe these signs, or a movement pattern that seems to lag behind typical developmental milestones, should consult a pediatrician or a pediatric therapist for a professional assessment.