Primitive reflexes are automatic, involuntary movement patterns that develop in the brainstem before birth and are present in healthy infants. These reflexes help with survival and movement. The Symmetrical Tonic Neck Reflex (STNR) plays a distinct role in early motor development. The presence and subsequent disappearance, or integration, of this reflex are important markers in a child’s neurological maturation. Understanding the STNR’s function and timeline helps trace the progression of a child’s physical skills and posture development.
The Mechanics of the Symmetrical Tonic Neck Reflex
The STNR is sometimes referred to as the “crawling reflex” because it facilitates the transition from lying on the floor to the hands-and-knees position. This reflex is activated by the movement of the infant’s head and neck. It creates a connection between head position and the corresponding movement of the arms and legs, essentially dividing the body into upper and lower halves for movement.
The reflex presents in two distinct patterns based on head movement. When the infant flexes the head forward, the arms bend while the legs simultaneously straighten. Conversely, when the infant extends the head backward, the arms straighten and the legs bend. This mechanism allows the infant to push their body upward into the quadruped position, which is the readiness position for crawling.
This temporary, patterned movement helps strengthen the muscles in the neck and back, which is beneficial for developing future upright posture. The STNR assists in separating the control of the limbs from the control of the head, a necessary step before coordinated, voluntary movement can occur.
The Normal Window for STNR Integration
The Symmetrical Tonic Neck Reflex is not present at birth; it emerges later, around six to nine months of age. This appearance coincides with the infant’s increasing ability to be on their hands and knees. The reflex serves its developmental function for a short period.
The STNR should integrate, meaning it is replaced by voluntary control, between nine and twelve months of age. This integration signals that the higher centers of the brain have taken over movement control. Once integrated, the child can move their head without automatically triggering corresponding limb movements.
Successful integration allows a child to perform independent, coordinated movements, such as looking up while simultaneously crawling forward. If the reflex persists beyond the twelve-month window, it is considered retained and interferes with the acquisition of more advanced motor skills. Integration is a necessary precursor for true hands-and-knees crawling and maintaining an upright sitting posture.
Identifying Signs of a Retained STNR
If the STNR does not integrate on time, the active reflex pattern can create subtle movement difficulties in older children. The persistent link between head and limb movement often manifests as poor posture, resulting in children slumping forward when sitting at a desk. They may frequently wrap their legs around chair legs or sit in a “W” position to compensate for core instability.
Academic tasks can become challenging because looking up at a whiteboard or down at a paper triggers involuntary arm and leg responses. This reflex interference affects visual tracking, making it difficult to shift gaze smoothly from near to far distances. This may lead to difficulties copying work or skipping lines while reading. The child may also exhibit difficulty sitting still or appear fidgety, as they unconsciously attempt to counteract the retained movement pattern.
Poor hand-eye coordination is another common indicator, affecting skills like catching a ball, handwriting, and messy eating habits. In physical activities, a retained STNR can hinder learning to swim, particularly the breaststroke, where lifting the head should not cause the lower body to sink. These difficulties stem from the brain’s continued reliance on the automatic reflex pattern instead of developing independent, voluntary control over the body’s upper and lower segments.