At the rate that most kids rush around, even watching them is exhausting. But kids don't burn energy just for the sake of it; it takes much more effort for a child to walk and run than an adult. Patrick Willems is fascinated by human locomotion and wondered why children use so much more energy when walking than their parents. Were children's strides using energy less efficiently than their elders, or were they simply using more energy because they were doing more mechanical work while they walked? Measuring the mechanical work done by 3 to 12 year old's as they sauntered along, Willems and his team discovered that children are every bit as efficient as their elders; it's their smaller statures that cost them dear (p. 587).

Despite the old acting adage, Willems remembers that his child subjects were extremely cooperative little walkers, except when he asked them to speed up. Then their competitive sides came out. Some of the youngsters tried `beating the record', instead of walking slightly faster; which wasn't exactly what he wanted. But once he'd overcome the child psychology, Willems and his team were able to track both children's, and adult's, movements with LEDs tapped to their skin as they strode across a force platform. After recording more then 1000 short walks at various speeds, the team converted the LED's movements into moving `stick people' before Bénédicte Schepens and Guillaume Bastien began investigating individual strides.

First the team analysed each stride's energetic components. Calculating the amount of energy used by both young and old to swing their legs and move their weight forward, Bastien, Schepens and Norman Heglund also added another component to the work done during a stride; the mechanical work done while both feet were planted on the ground, pushing against each other. Willems remembers that he was pleased when he realised that less than 10% of each stride's energy was wasted while the feet opposed each other on the ground. Willems explains that although this energetic component had been measured before, he'd been anxious that it would take a larger fraction of the young walker's efforts and make a child's stride more costly than an adult's. But adult and children's strides used energy in the same way. So what was causing the children's costly gait?

The team decided to see whether the children were using more energy, simply because they were smaller, by `scaling' them up to the size of adults. Surprisingly the children's scaled up bodies used the same amount of mechanical energy as the adults! It was simply the children's smaller stature that made them use more energy. Willems explains that children's limbs are like short pendulums, which use more energy to swing than longer pendulums. So the children must do more work swinging their short legs, than adults use with their longer limbs.

But size didn't explain all of the differences for the really young children. The scaled up three year olds were using significantly more energy than adults of the same size, `[which] is probably due to an immature muscular pattern of walking' explains Willems.