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Walking or running efficiently, your locomotor muscles might not agree

The views expressed are those of the author and are not necessarily those of Scientific American.

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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

Scicurious About the Author: Scicurious is a PhD in Physiology, and is currently a postdoc in biomedical research. She loves the brain. And so should you. Follow on Twitter @Scicurious.

The views expressed are those of the author and are not necessarily those of Scientific American.

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  1. 1. PrettyOld 10:16 am 01/2/2012

    Humans did not evolve to walk upright. I have a creator.

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  2. 2. Percival 11:34 am 01/2/2012

    Well, this helps explain why humans are able to run down horses; I’ve always wondered about that.

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  3. 3. HowardB 4:56 pm 01/2/2012

    Humans are the only animal that can run long distances without stopping, without over heating .. because we can lose weight by breathing and sweating … all other animals have to stop and pant. Hence we can run-down, catch and kill many other animals that are superficially faster than us.

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  4. 4. podboq 8:15 pm 01/2/2012

    Humans did not evolve to walk upright, they evolved to bury their heads in the sand.

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  5. 5. LarryW 9:55 pm 01/2/2012

    Is this study valid? My sense is walking or running on a treadmill is very different than doing the same on a real track or sidewalk, for example. Has there been any study comparing the two?

    Having done both, I detected a difference on how my muscles are used. Using both a treadmill and walking on a normal surface to recover form injury, the experience was quite different.

    The treadmill moves one’s forward leg back, which does not require one’s body to be propelled forward with toe, foot, ankle, and leg muscles. The treadmill allows considerable passivity. Also, the energy and effort I experienced walking on a normal surface at say 3 mph noticeably exceeded that same effort on a treadmill.

    Also, anecdotally, I have read cautionary tails involving the strain to hips using a treadmill, which doesn’t seem to occur walking on a normal surface.

    For these reasons, the study to me seems inaccurate.

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  6. 6. scicurious 10:36 pm 01/2/2012

    @LarryW, I agree with you that the treadmill is a major limitation here. I think in this case it is probably warranted, as the leads required to do electromyography while people run around a track would probably have to be awfully long. I know that you can avoid many of the problems associated with treadmill running (the propelling problems especially) by using a slight incline. They did not record using an incline in this study, but using one might ameliorate the issues you raise.

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  7. 7. kluddy99 7:45 am 01/3/2012

    Based on your description of the study, (I will need to look it up) there appear to be several problems with this study: the assumption that EMG activity correlates to efficiency, the assumption that EMG activity alone correlates to work done by the body, neglecting to account for the technique of walking and running, specifically the coordination of the firing of muscles, the contribution of passive tissue, and the transfer of momentum.
    Although attractively simple, the methods as described don’t hold up.
    PS, I really don’t think that your picture is a human leg, looks more like a shoulder!

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  8. 8. scicurious 8:39 am 01/3/2012

    kluddy99: ARGH. I have been trying and trying to find out of the leg, and every time I replace it it puts the arm one in instead. One more time.

    The author did recognize that EMG does not directly correlate to efficiency of an individual muscle. The idea was to correlate the activity via EMG as the best measure. As for the work done by the body, that’s not what they’re after here. They wanted to see whether, at our overall most efficient pace, our individual muscles were also overall most efficient. I feel like they succeeded in that goal, but you are right that EMG is not the best measure. Unfortunately, the best measure (using pellets in the muscle itself) can’t really be done in humans.

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  9. 9. LarryW 11:16 am 01/3/2012

    @scicurious. I don’t want to seem too snarky but the suggestion of using a treadmill instead of a normal walking surface because otherwise the leads would be too long, struck me as funny. I’ve heard there is this newly discovered phenomenon called radio waves that could be used to transmit information, in place of attaching long copper wires. Just an idea.

    As for inclined treadmills, my experience says inclined treadmills suffer from the same problems I previously described.

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  10. 10. scicurious 2:18 pm 01/3/2012

    @LarryW, I was joking about the long leads. :) Probably they used the treadmill a) because of being able to keep a REALLY constant pace, which most untrained humans can’t do and b) because almost all other studies on human pacing have been done using a treadmill. So yes, the treadmill is not the best thing to use here, but until it becomes the standard to use track running or some other non-treadmill method, the treadmill is what people will expect to see here as a gold standard, whether or not that’s the best measure.

    And don’t worry about being snarky, I was being plenty snarky myself. :)

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  11. 11. mspf777 5:09 pm 01/4/2012

    Hmmm. Seems to me if we used walking/running as a way to lose weight (i.e., burn calories), constantly speeding up and slowing down would be best (most inefficient use of the muscles?)–makes one wonder if those of us with inefficient muscles actually burn the most calories and thus (leap of faith here), are the thinnest???

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