The pelvis doesn't walk by itself: Wider pelves reduce the cost of walking over unstable surfaces.

Walking over variable and/or unstable terrain is a key aspect of daily life and was crucial to the evolution of bipedalism. The ability to find gait solutions that maintained stability over uneven terrain for the least increase in metabolic cost was likely a hallmark of locomotor effectiveness in early humans, in particular acting as an adaptive pressure to shape various morphological traits, including those related to mass distribution. Previous research has shown that having a lower center of mass increases the stability of gait and may potentially decrease cost, both over variable terrain, as well as during load carrying. Here we investigate the interactions between these two tasks (walking over variable terrain with loads) in the field. Not only do people with relatively wider pelves (and thus a lower center of mass) have significantly lower costs at all conditions, but carrying a load specifically mediates the walking over unstable surfaces. One possible reason for this is because people with relatively wider pelves also decrease their step-to-step (stride) variability over the unstable surfaces. The increased stability and reduced stride variability may be a reason for humans' ability to blunt the increase in cost of transport for a given load's percent of body mass.