A new study examining how much energy males and females spend carrying babies in different positions reported that women expend less energy carrying their babies than men in all carrying positions. Carrying a baby on one’s back was the most energy efficient and people carrying babies this way were able to maintain their unloaded walking speeds. The study was published in Evolutionary Human Sciences.
Carrying babies is one of the few activities that is universal in all populations of humans. However, there are variations between cultures in how this is done. Cultures differ in materials that are used to “sling” babies i.e., in preferred devices used to carry children.
There are differences in the preferred location on the body where the children are carried. There is also reason to believe that these differences translate into differences in metabolic costs — how much energy the body expands to carry a child. There are also differences in how much mobility of the person carrying the baby is impaired.
In scientific circles, there is some debate on whether genders differ in how effectively they carry loads. The author of the new research, Cara M. Wall-Scheffler, has argued that “women are excellent (i.e. both economical and efficient) load carriers (and walkers generally), whereas others have argued that there are no sexually dimorphic characteristics that influence locomotor energetics or kinematics in one way or another.”
She notes that these differences might come about from different ways in which efficiency in carrying loads is calculated. To further study the gender differences in mobility and energy use, she conducted a study aiming to explore the energy costs and mobility outcomes of walking humans carrying their own babies overground in a forest. She set out to particularly investigate effects of different terrain (e.g. carrying on inclines), different carrying positions, and of carrying babies in different developmental stages.
Fifteen people (nine women and six men) were asked to walk on a forest path under different loading conditions. They walked while carrying their own baby on their side, front, and back in a random order. Side carrying involved placing the baby on the hip with one leg around the back and one leg around the front. All participants used a baby sling weighing 640 grams (1 pound).
Mass of the parent, of the baby, stature, strength of lower limbs of the parent, and several elements of the parent’s body were measured. Average age of participants was around 37 years. It was similar for mothers and fathers. Average age of babies was around 16 months with substantial variation between babies. Average mass of babies was around 11 kilograms (24 pounds), with substantial variation in mass between babies. Fathers on average weighed 19 kg (42 pounds) more than mothers.
The study participants walked about 1 km on a forest path that had varying width and terrain, including inclines, declines, and curves. The path had no switchbacks, an average grade of 1.0%, and was balanced with inclines and declines. The ground cover included ferns, salal, nettles, and native rose, and trees could be up to the path edge with roots crossing the path, making the ground somewhat uneven in patches. The participants walked both unloaded and carrying their own baby in different positions in a randomized order.
Wall-Scheffler calculated two types of metabolic costs of movement – cost of transport (cost per unit distance, e.g. how much energy does a kilometer/mile traversed expend) and cost of locomotion (cost per unit time, e.g., how much energy does a minute or an hour spent in movement expend).
Results showed that, when all carrying positions were viewed together, carrying a baby in a sling did not significantly change the overall speed with which the route was traversed compared to walking without carrying a load. However, speed did decrease slightly when the baby was carried in the front and on the side. Of the entire group of participants, carrying the baby significantly increased the energy cost, regardless of the loading position.
Carrying the baby on the back showed the lowest energy cost per distance traversed (cost of transport), but there was no difference in the energy cost for time spent in motion (cost of locomotion). Walking on inclines was shown to significantly shorten stride length, but people did not make shorter strides when carrying a baby, compared to when they were unloaded.
In all conditions, women were significantly more economical (i.e. they used less energy than men to complete the task), both when cost of transport and cost of locomotion measures were considered. They also used less energy per kilogram of mass moved than men. Results showed that carrying a baby on the side becomes more and more inefficient as the size of the baby increases.
On the other hand, medium weight infants could effectively be carried in the front position, but this efficiency decreased dramatically (i.e. costs of transport increased) when heavier infants were carried in the front. Carrying infants on the back was the most efficient regardless of the size of the baby.
“Given these findings, it is clear that people are excellent load carriers! For most typical loading positions (those tested here), the increased cost is similar regardless of whether the mass is symmetrical (i.e. front or back) or asymmetrical (i.e. on the side). In fact, a 7.6% incline is more challenging than a 16% load!” Wall-Scheffler concluded.
The paper makes an important contribution to our knowledge of the mechanics of human motion. However, it also has limitations that need to be taken into account. Notably, the sample size was quite small and all participants carried babies regularly. Additionally, all carrying in the study was done using a baby sling. It is possible that results might not be the same on a different sample and using other methods of carrying.
The paper “Women carry for less: body size, pelvis width, loading position and energetics” was authored by Cara M. Wall-Scheffler.
