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Thursday, June 24, 2010

Taking a Little Walk: What Factors Make a Difference to Toddlers?

The “Question” feature in the New York Times Science Times section today focused on an issue about babies’ early attempts at walking alone. A reader inquired whether what she had been told was true-- that babies could actually walk much earlier, except that their vestibular systems were not developed enough to control their balance until about a year of age. The “Question” editor, C. Claiborne Ray, reported on the inquiries he had made and noted the need for development of the nervous system and its control of the leg muscles.

There’s actually a lot more to walking than that answer implies. Perhaps the first point to examine involves the facts about development of the vestibular system. This system, embedded in the skull near the inner ear, and sending messages along the 8th cranial nerve as the auditory system does, is actually one of the first sensory systems to come “on line” in the course of development. It’s a primitive system which we share with some fairly simple living creatures, and essentially it signals information about the pull of gravity and about the movement of the head. Lean forward or tip your head sideways, and the changed direction of the pull of gravity on parts of the vestibular system signals parts of the brain about the new position. Turn your head, or undergo rotation as you do when going around a corner in a car, and that information is also detected and sent. There is little or no conscious awareness of vestibular activity, but messages from the system enable the body to stay upright by changing muscle tension to counteract accidental movement. They also enable us to know when an object is really moving in front of our eyes, and when an image is moving across the eyes because of head or body rotation. A great deal of vestibular function is already present at birth, and you can see it at work if you hold a young baby in a horizontal position and suddenly move her downward a few inches. The Moro reflex, a movement pattern governed by vestibular activity, causes the baby to fling her arms outward in a grabbing movement.

So , we see that the newborn has almost-mature vestibular function, but she can’t walk (though she can display a stepping reflex when held in the right position). What’s the problem? Why can’t the baby bear her own weight on her legs? Part of the difficulty is the factor mentioned in the Times “Question”: the immaturity of the myelin coating of nerves in the lower part of the body makes conduction of messages poor, and signals from the brain do not effectively control muscle contraction and relaxation. Like other aspects of development, this one follows a cephalocaudal pattern, with the head end maturing more rapidly than the “tail” end. It’s usually about a year before the baby has voluntary control over the legs (and by the way, that developmental pattern also means that toilet-training is not really possible in the first year of life, because voluntary control over the lower part of the body is needed for that purpose).

We haven’t finished yet. There are more factors that enter into the walking equation. One important one would be obvious to us if we were talking about adults, but we tend to forget that it works for babies too: muscle strength. Young babies don’t have strong leg muscles. For them, as for adults, muscle strength increases with exercise. Chances to push with the feet or pull to stand help to strengthen the leg muscles until they have the power to support the weight of the body. In fact, the late and much-respected developmental scientist Esther Thelen showed that a “baby treadmill” that exercised the legs could improve the age at which Down syndrome babies walked-- an event that is ordinarily delayed for these children.

Even when independent walking is achieved, we can see from watching toddlers that they do not yet have a mature gait. Their large heads and relatively short legs make it hard for them to keep their balance when walking, and they compensate by bending their knees a bit and placing their feet far apart to lower the center of gravity and stabilize their position. As their ankles are not yet very strong, toddlers often keep the foot extended and hurtle, from place to place on tip-toe, a gait that has its problems but helps compensate for ankle weakness. (Ideally, they get to use and strengthen those muscles by walking barefoot rather than in hard, unbending “baby shoes”.)

Their first walking experiences have enormous fascination for small children-- so much so that they may not even notice painful results. I have a vivid memory of seeing my older son at 11 months, prancing barefoot over painfully prickly holly leaves, then a month later, when the excitement had worn off, yelping when he stepped on one. That joy of early mastery must be included as one of the factors that helps walking emerge as a new skill.

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