Carrying Loads on Heads Found to Be Efficient

African women carry substantial loads with their heads without burning any additional calories because the weight makes their gait better at conserving energy, a study found.

Previous work had shown that African women of certain tribes burned no extra calories from carrying up to 20% of their body weight on top of their heads or with a strap looped across their foreheads.

In addition, scientists had found that when the women carried loads of up to 70% of their body weight, they burned fewer extra calories than did people carrying backpacks.

In the May 4 issue of the journal Nature, researchers report that carrying a load makes the women’s gait more efficient in converting one form of energy to another. That leaves less work for muscles to do, said researchers Norman Heglund of Pharos Systems Inc., a consulting company in South Chelmsford, Mass., and colleagues in Belgium and Italy.

The study focused on potential energy, which an object gets from being raised to a higher position, and kinetic energy, which an object gets from moving. When a person walks, energy is converted back and forth between these two forms.

Researchers tested five women who carried loads with their heads as they walked across a platform that measured forces they exerted on the ground.

Heglund said the process is a mystery. “You can’t see any difference when you look at them,” he said. “Whatever they’re doing is very subtle,” and the women themselves are not aware of it.