When it comes to ambulation, toddlers have their own particular style. At first glance, they may just seem a little uncoordinated — as if they haven't quite learned the rules of walking yet. But is there actually more to it than that? Researchers in the U.K. took a close look at the way small children move and came up with some surprising results.
On close inspection, a toddler's stiff-legged waddle is completely different from an adult's more flowing run. The idea that children move the way they do because they are yet to learn efficient coordination is to miss the real story, says Dr. Jim Usherwood of the Royal Veterinary College in London. "Some scientists see a young child appearing to walk and run uneconomically and then attribute it to being underdeveloped," he says. Usherwood questioned this idea, knowing he would need to take a completely new approach if he was to correctly interpret the factors that account for kids' clumsy movements.
Usherwood wondered whether it was actually diminutive stature that was responsible for their awkwardness. Certainly a child's physical dimensions are not those of an adult — morphologically speaking, children are not just small adults. Developing a new theory about the way children run, Usherwood's theory — that children's short legs struggle to find enough time to do the work to push them into the air while running — is published in the Journal of Experimental Biology.
To get a first-hand look at children's movements, Usherwood and his colleague Dr. Tatjana Hubel recruited 18 children — including two of Usherwood's own daughters — who ranged in age from 1.1 to 4.7 years. The scientists used a camera system to track the position of reflective dots placed at specific locations on the children's limbs as the toddlers moved at various speeds along a track. They also measured the forces that were exerted on the floor by the subjects' steps. "We were careful not to tell them what gait to use," Usherwood says — Hubel would ask the children to either match their parent's speed, or to go faster or slower to ensure that their movements were not subject to bias.
Despite their best efforts, however, the measurements did not seem to make sense. When all of the children were compared to the same size scale — to see how well they agreed with the predictions of the models that account for how adults walk and run — one anomaly kept reoccurring: timing. Simultaneously, Usherwood realized that the mechanical models — some of which are based on inverted pendulums — missed out one key physiological factor: muscle. "You activate the muscles and there is a physiological cost to activating it," Usherwood says. In other words, measuring movement cannot rely on a change in physical position alone but must also take into account the amount of effort, or energy, put in. This, he says, was not accounted for in the models that were based solely on the physics of movement.
Of course, children's legs are significantly shorter than those of adults and so they have less time to push up and away from the ground. This means their muscles have less time to contract and generate the power they need to move, which, in turn, leaves their feet on the ground for a longer proportion of each stride than those of adults. "You can see that by watching a three year old running," says Usherwood," they barely get off the ground." A child under a certain age simply does not have the amount of muscle necessary to create the amount of lift seen in adults. By calculating the amount of muscle that is necessary to generate the movements, Usherwood realized that he could deal with the troublesome time factor.
Usherwood and his colleague built a single piston that was the length of the individual's leg as a model to represent moving people. By doing so, they could calculate the amount of muscle required to produce the power necessary to propel the individuals along at the speeds that he and Hubel had measured. Accordingly, the new model successfully reproduced the movements of youngsters and adults. The team concludes that children move the way that they do simply because they are smaller than adults. Their short limbs do not have enough time to produce the high power needed to lift them into the air when running, not because they have not yet learned the fancy footwork skills of adults.