Humans walk well. More to the point, we walk EFFICIENTLY. As we evolved to walk upright, we also evolved to do so with great economy, expending fewer calories at an optimal walking pace, but then expending more calories when we either speed up or slow down. We also may be economically efficient runners as well as walkers, we're average for mammals, but our long legs and ability for those legs to take repeated strain suggests we may be on the efficient end of primates (and we're some of the BEST long distance runners on the planet, so we can preen a bit over that one). The jury is still out on running, but as far as walking goes we are the most efficient at a moderate speed (roughly 5 km/hr, or 3.1 miles/hour for men, a relatively brisk walk of 20 min a mile).

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.

Theoretically, it makes sense. At a certain pace, you're overall burning fewer calories and not working as hard, so your individual muscles must also not be working as hard. Right?

Well...possibly wrong.

Carrier et al. "The musculoskeletal system of humans is not tuned to maximize the economy of locomotion". PNAS, 2011.

This paper asked the question of whether the individual muscles in your legs and back are at their most efficient at our most economical walking and running speeds. The authors took 17 men (they excluded women because the paces and efficiencies of movement in women are very different, but I rather wish they'd done both, it would be very interesting to see how some of them differed), and set them up on treadmills (they note that all of the guys were given "habituation" to the treadmill). They attached leads for electromyography, a method for measuring the electrical activity of skeletal muscle. While this cannot record the energy efficiency of each muscle directly, it can observe differences in electrical activity, and you can extrapolate those differences as a measure of how much energy a muscle is using. Lower electrical activity, lower activity of that individual muscle, and this may correlate with the individual metabolism of that muscle.

So they set these guys up on a treadmill and attached leads. They had them alternate walking at their most comfortable pace, near the most efficient pace of 5 km/hour (3 miles/hour), and running at various paces, starting at a VERY leisurely job of 6 km/hour (3.7 miles/hour) and ramping it up to a full run at 20 km/hour (12 miles/hour, or 5 minute miles, a full out sprint for most people). At each pace, they looked at the cumulative muscle activity per distance traveled (CMAPD), to get an idea of how efficient each muscle was at a given pace.

And here's what they got.

(Clicky to embiggen)

This is a summary graph of the 13 muscles they looked at, with walking in black and running in grey, and with a point for each muscle of the speed at which it showed the most efficiency. You can see there's some pretty wide variation, especially in running. Not only that, though most of the muscles show most efficiency at our comfy brisk walk, it's not ALL of them. So while many of your muscles ARE most efficient at a brisk walking pace (and probably contribute to our economy of energy at that pace), not all of them are. This contradicts the hypothesis that ALL the muscles would be most efficient. We're efficient walkers, but not AS efficient as we could be.

Now, you might be thinking "our muscles aren't all tuned in to maximal brisk walking/running efficiency?! TOTAL evolution fail". But actually, that's probably the OPPOSITE of true. In the end, it may be better to have a wide range of musculoskeletal efficiency. After all, you don't JUST walk at a brisk pace. You run, you sprint, you accelerate and jump. You kick. You saunter. You might even dance once in a while. All of these things involve muscles working in different ways. And in the case of things like acceleration in particular (specifically a movement involving your glutes), efficiency when it really counts for acceleration may make more of an evolutionary difference than the efficiency of that particular muscle at a brisk walk. As an example, the authors compare us with horses. Horses are AMAZINGLY efficient movers at their optimal paces of a fast walk and a medium trot. But outside of those gaits, they are very inefficient indeed. They are much more specialized for particular paces than we are. While we are not as efficient, our slight lack of efficiency at our favorite paces may allow us to be more efficient in others, giving us more flexibility, and broadening our range of sustainable speeds. We can jog for a long time, we can walk briskly or slowly, and overall we are more efficient at all of those speeds than a horse might be. So while your muscles may not all agree on a single pace, in the long run (heh) that might be a good thing.

Carrier, D., Anders, C., & Schilling, N. (2011). The musculoskeletal system of humans is not tuned to maximize the economy of locomotion Proceedings of the National Academy of Sciences, 108 (46), 18631-18636 DOI: 10.1073/pnas.1105277108