Walking or running efficiently, your locomotor muscles might not agree
So we’re good walkers, or at least economic ones. But the question is, what MAKES for this efficiency? How exactly are we burning fewer calories at a specific walking pace? Ideally, this means that our muscles, like our bodies overall, are at their most efficient at a moderate walking pace. The calories burned over time are the result of the total metabolic rate of all the muscles that produce locomotion. So since your metabolism is minimized at a moderate walking pace, creating the most efficiency, it would make theoretical sense that your individual MUSCLES are also minimizing their metabolism at that moderate walking pace, and the cumulative effect is one of energy efficiency.
The later part is quite interesting, in which they found that the actual biology disagrees, trading off maximum efficiency with higher flexibility. Thus, biological optimization often takes a multitude of factors into account. In the long run it boils down to fitness, at least in terms of optimization.