What do brushing your teeth, shaking someone’s hand, and reading this article have in common? A skill called “crossing the midline”. The midline is an invisible line from head to feet, separating the two sides of the body. We cross that line any time we move a foot, hand or eye into the space of the other foot, hand or eye. We also cross the midline with our tongue when we use it to move food from one side of our mouth to the other.
Why is Crossing the Midline Important?
If you recorded all the times you crossed the midline in a day, you’d fill several pages of a notebook. Even the act of writing them down would involve crossing the midline! Here are a few reasons this skill is necessary for daily life:
It allows us to smoothly perform practical life, self-care, and recreational tasks. Driving a car, sweeping a floor, cooking a meal, taking a shower, putting on socks, playing sports, and similar activities all require crossing the midline.
It’s necessary for visual tracking. Aside from activities like watching movies or scanning the field during a soccer game, we rely on visual tracking for reading because our eyes must continually cross the midline as they move across a page.
It develops a dominant hand. Developing a dominant hand is important for refined fine motor skills like cutting, writing, and throwing a ball. A child must be able to cross the midline to strengthen one hand for mastery of these tasks.
How Does Crossing the Midline Develop?
Around 3 months, babies can cross the midline with their eyes as they visually track an object moved in an arc in front of them. By 6 months they begin reaching across the body with one hand, and around 8 months they cross the midline with both hands by transferring objects from one hand to the other. By age 4, children generally cross the midline with ease.
Development of midline crossing is integrally connected to these sensory and gross motor skills:
In order to cross the midline, your baby must first develop a sense of his body. Every physical interaction with his environment activates receptors in his skin, muscles and joints, which develops his proprioceptive sense. Proprioception includes an understanding of the relative position of our body parts, and it goes hand-in-hand with midline crossing. When you absentmindedly scratch an elbow, for example, you don’t have to look at your body to know where your elbow is and how to reach it with you opposite hand.
Bilateral integration, or the two sides of the brain communicating seamlessly, is not present at birth. Midline crossing and bilateral integration reinforce each other: The more your baby crosses the midline, the stronger the brain communication—and the stronger the brain communication, the better the ability to cross the midline. One of the most important tasks for this mutual reinforcement is crawling. When your baby crawls, he continually crosses the midline with his hands and eyes, sparking constant “conversation” between the right and left hemispheres of the brain.
A strong base and good postural control allow for controlled movement across the midline. You will see this when your baby can sit up on his own (around 6 months) and has both hands free to reach across his body during play.
Many crossing the midline activities involve coordinated twisting of the body.Transitional movements (shifting from lying down to sitting up, for example) help develop your baby’s ability to effectively rotate his trunk.
How You Can Help
Midline crossing won’t become a standout skill until your baby is 3 or 4 years old. But, because its foundation is directly linked to gross motor and sensory development, fostering movement and exploration is one of the best ways you can help your baby learn this skill. Give him plenty of tummy time and freedom to move and play. Because crawling is so important for effective midline crossing (and many other areas of development), try to keep your baby crawling as much and for as long as possible. If you are worried about your baby not crossing the midline, your pediatrician can offer guidance.