Jim was an early, confident walker. Greta likes to say that he didn’t learn to walk, he went straight to running. By the time he was about 16 months old, he could already outrun his already-pregnant mother.

Nora, on the other hand, was a late, tentative walker. She took her first steps at around 12 months, and still wasn’t very confident as a walker at 18 months. In this photo, at 17 months, she still clings to their toy kitchen set for balance.

But I’ve just finished reading a fascinating study suggesting that at 14 months, when both of them were walking — Jim with confidence, and Nora struggling — they actually took a similar approach to balance while walking.

A team led by Jessie Garciaguirre might be the first to investigate how infants who’ve only recently learned to walk adapt to carrying heavy loads. Adults generally carry no more than 35 percent of their own body weight (though in some African tribes, women balance immense loads — up to 70 percent of their body weight — on their heads). School-age kids might port 20-30 percent of body weight in backpacks on their way to school. Adults and kids make significant adjustments to posture and gait when bearing loads in excess of 15 percent of body weight. They take shorter steps, and they lean away from the load to compensate (generally this means leaning forward to accommodate a backpack).

So why not put backpacks on toddlers and see how they manage? Garciaguirre and Karen Adolph had previously found that 14-month-olds fall down an average of 15 times per hour while playing. What could possibly go wrong when heavy weights are strapped to their backs?

Actually the infants were carefully monitored and “spotted” by an experimenter during the study to ensure that no one got hurt, and this research can offer tremendous insight into how children learn to walk. The team recruited 27 14-month-olds and fitted them with a vest that could accommodate weights in four positions:

The toddlers then walked along a raised, 3.66-meter walkway that sensed their footsteps while they were videotaped from two different angles. Each child made the walk several times: with no weights, with weights in front, in back, evenly distributed, or on the right or left sides. The weights were customized for each child to ensure a consistent 15 percent of body weight.

Then an observer rated each video to indicate whether the child was leaning forward, backward, or to the side. Do toddlers lean in the same way as adults, to counterbalance the weights they carry? Here are the results:

When the weights are placed on toddler’s backs, instead of leaning forward to compensate, the vast majority actually leaned back! When the weights were in front, they leaned forward. Needless to say, this made walking quite difficult. When the weights were in back, there were an average of 2.69 disruptions in walking gait over the short 3.66 meter walkway. In fact, only 10 of the 54 trials with weights on the back were completed without disruption. With no weights, 47 of the 54 trials were disruption-free.

Infants actually had a harder time with weights on their back than any other condition, including the front or left or right side (this isn’t to say those weight positions were disruption-free: half or more of those trials were disrupted as well). So why is walking with weights on the back so difficult for toddlers? Garciaguirre’s team says it’s probably because it’s more difficult for anyone to walk while leaning back than leaning forward. Since our feet extend forward, we can lean farther forward than backward.

These toddlers, some with over four months of walking experience, simply haven’t yet learned to lean away from a weight to balance it. The amount of walking experience each child had bore no relationship to their walking behavior; the same trends were seen in all the children.

That said, all the toddlers did change their gaits in response to the weights. They took shorter steps, spent more time with both feet on the ground, and they slowed down overall, just as adults do when carrying heavy weights. The failure to lean to balance the weight appeared to account for all the differences between weight-bearing for toddlers and adults.

Related: How we learn to walk.

Garciaguirre, J.S., Adolph, K.E., Shrout, P.E. (2007). Baby Carriage: Infants Walking With Loads. Child Development, 78(2), 664-680. DOI: 10.1111/j.1467-8624.2007.01020.x