Cost of transport at preferred walking speeds are minimized while walking on moderately steep incline surfaces

Research has shown that preferred walking speed results in a minimization of the cost of transport on flat surfaces. However, it has also been shown that over non-smooth surfaces other variables, such as stability, are necessary for task completion increasing the cost of transport. The purpose of this research was to investigate the effect of incline walking on the cost of transport, assessing the effect of raising the center of mass as a potential variable affecting preferred walking speed, such that the cost of transport is no longer minimized. 12 healthy, college-aged male participants completed walking trials on a treadmill at inclines of 0%, 5%, 10%, 15%, and 20% at three different continuous speeds (1mph, 2mph and 3mph) and a preferred walking speed for 4–5 min. Cost of transport was calculated using the oxygen consumption collected during the last minute of each stage. Up to 20% incline, the cost of transport was lowest on each incline for the preferred walking speed trials. On inclines greater than 20%, many participants were unable to complete the task with respiratory exchange ratios less than 1.0. We conclude that inclines up to 20% do not induce an alternative challenge affecting the established relationship that humans prefer to walk at speeds that minimize the cost of transport despite the increased need to raise the center of mass. •For inclines up to 20%, cost of transport was lowest at the preferred walking speed.•While incline walking introduces the costly need to raise the center of mass, this task priority does not affect the established minimization of cost of transport at the preferred walking speed.•During the preferred walking speed trials, a belt speed was chosen that increased vertical speed up to 20%, but then plateaued, implying that there is a compromise above 20% that should be investigated with respect to cost of transport.•During the preferred walking speed trials, efficiency rose more quickly with incline to the plateau than at other speeds.•At inclines above 20%, changes to preferred walking speed resulted in plateaus in oxygen consumption, heart rate, respiratory exchange ratio, and vertical speed.