Published February 26, 2014 

Let me start by saying that I don’t actually know much about the topic I’m writing about today and, in a tiny, personal celebration of blissful ignorance, I decided not to try to learn much about the topic I’m writing about before setting pen to paper. I sort of think of this post as a cry for help, so if anyone wants to tell me gently (to spare my feelings), offline (to spare my reputation) what I should  have read before trying to write about this, please drop me a line.

Ok, here’s the thing. Human muscles seem to atrophy with disuse, as anyone will tell you who has had to take a six week hiatus from the gym because of a bunch of gorram injuries. Muscles are a bit like foreign languages and health flexible spending accounts: use it or lose it.

My sense is that many people think that this is a general property of muscles. Or, at least, my sense is that people think that in order for muscles to get big and stay that way, they need to be used.

Don’t get me wrong. I think that’s true of (some, most, all?) human muscle tissue. But just because it’s true for humans, I don’t think that it’s true for other critters, and I don’t think that it had to be true for human muscles, whatever that “had to” might mean.

Part of the reason I was thinking about this is because of the self-control literature I’ve been subjecting myself to, in which people say that self-control is “like a muscle.” In that word “like,” self control researchers seem to have in mind, first, that self control is “like” a muscle in that muscles get tired with use over the short term and, second, that muscles get stronger with use over the long term.

Now, the first part probably has to do with the way that muscles work. Muscles turn chemical energy into mechanical energy. This is accomplished through mysterious processes going on in the cells, and the little power plants that do the work run out of fuel, produce metabolic byproducts, and heat up, some or all of which reduce their ability to produce mechanical energy, which is why each rep is harder than the last one. There are, then, physiological constraints that tissues face over time. This reduction in output is, in short, a necessary side effect.

What about the second bit, the idea that muscles get stronger with use over longer time scales? Or, in the same vein, why do human muscles seem to get weaker over the long term if you don’t use them.

Who says Gorillas don't know squat?

Who says Gorillas don't know squat?

Which brings me to gorillas. I don’t know about you, but I’ve spent a fair bit of time watching gorillas in zoos, not to mention Gorillas in the Mist, and they strike me as, in a word, lazy. Juvenile gorillas do seem to frolic a bit in the enclosure, but adult males seem to punctuate their bouts of sitting around munching on foliage with short intervals of sitting around not munching on foliage. Ok, sure, in the wild, right, every now and then two male gorillas will get into a fight, and I’m sure that provides a robust work out. And of course in zoos, no such battles occur because no zoo would put potential rivals in the same enclosure, and risk the loss of a precious gorilla.

Lazy zoo gorillas, then, don’t seem to get much of a workout, but those dudes are huge.

So, from my completely informal observations that gorillas in zoos are A) lazy and B) buff, I infer that there’s nothing intrinsic about (gorilla) muscles that requires that they atrophy with disuse.

If human muscles atrophy if unused, but gorilla muscles don’t, then it seems that some sort of explanation is required.

My guess is that the relationship between how much you use a muscle and how big it gets is a design feature, not a byproduct, and this relationship varies across species.

For gorillas, my guess is that no matter what sort of life you’re leading, you might need your muscles at any given moment. In the case of males, this is either to defend the harem against rivals, or, on the other side, fight to take over a harem. If such attacks might occur at any time, then muscles must be kept robust more or less continuously, independent of whether you’re working out regularly. As with everything else, there’s a tradeoff, and the metabolic cost of maintaining the muscles is a good investment insofar as the defenses must always be manned.

Maybe something different is going on with humans. People are unusual in many ways. In How the Mind Works, Steve Pinker quotes Ambrose Bierce’s Devil’s Dictionary, which defines humans thusly:

Man, n. An animal so lost in rapturous contemplation of what he thinks he is as to overlook what he indubitably ought to be. His chief occupation is extermination of other animals and his own species, which, however, multiplies with such insistent rapidity as to infest the whole habitable earth and Canada.

That last part is relevant to the present point. People live and probably have lived in tremendously different ecologies. The ecology probably influences which muscles are worth paying the metabolic cost to maintain. Are you doing a lot of persistence hunting? Probably you want to invest in big leg muscles. Paddling a kayak? Best to have big upper body muscles.

And of course the best way for the system to know which muscles are worth investing in are the one that are being used. So, a good design feature is to build up muscles for tomorrow that you’ve been using a lot today. Other muscles, by virtue of the fact that you’re not using them, aren’t worth the cost. So, unused muscles are hung out to dry.