I'm Drew - a high school biology teacher and endurance athlete who decided to become a scientist. I am currently a PhD candidate in biological anthropology at the University of Massachusetts where I study human evolutionary physiology.


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An evolutionary perspective on cross training for runners

Updated: Nov 10, 2018

(This is a repost of content which first appeared on the old Running Primate blog in July 2017, with a few small updates.)


Why doesn't running give you all the adaptations you need to be optimal at running? (If you believe this to be true, which I do.)  And a related question: Why can running sometimes have side effects that are counterproductive to running (and survival), like overuse injuries, heart scarring (seen in long-time marathoners), decreased sex hormones, and altered immune response?  As is typical for questions that I explore on this blog, the answers lie in an understanding of how evolution works.

(In many ways this is an extension of an earlier post, so I'll just go ahead and link it here and in several places below. The idea there originally came from this paper. )

Some backstory: Athletes and coaches in all sorts of sports disagree about many aspects of training, including cross training (doing exercise other than your sport- so for runners, cross training includes lifting weights, plyometrics, biking, swimming, cross country skiing, etc).  In running, there's a camp who believes running alone produces all the physical changes (biological adaptations) you'll need to be good at running.  The other camp insists that additional fitness gains can be attained through strength training and cross training (biking, etc), gains that make you a faster runner and can't be earned through running alone.  (The fact that our bodies respond to exercise in myriad specific and helpful ways, so as to get better at that specific exercise, is a fascinating biological phenomenon, and beyond the scope of this post. In humans, our physical fitness is extremely "plastic", meaning that we "use it or lose it".  This suggests that evolution has shaped us to be fit for specific physical activities only when we need them, for reasons I discussed here.)  Early in my running life, I lifted weights (for reasons of vanity) and mountain biked (for reasons of fun), but as I became more serious about running in college I largely abandonded these things, believing they took time and energy away from running.  In my 30's I am back at them, for reasons of fun and because I firmly believe they make me faster and healthier (and research supports at least that first assertion).

So let's break down the question.  Remember, we're asking why running alone likely won't make you an optimal runner, and why, if running is healthy and presumably had a function in our early ancestors (as is the bent of this blog), it can have negative health consequences.

1. Increased fitness through training is an evolved, biological trait.  We need to accept this premise for the rest of my argument to make sense.  Every biological trait with a genetic component is subject to evolutionary forces.  The changes that occur in your body in response to exercise most certainly are biological and genetic, and evolution has endowed us (and other animals to varying degrees*) with the ability to respond with exquisite sensitivity to repeated, routine physical activity, thus preparing us to perform better at that specific activity.  (Coaches calls this the "specificity of training principle", which at first glance contradicts my argument in this post.)

*to what degree? The reseach on this question is scant, and understanding this would give interesting insights into the evolution of human endurance. 

2.  Evolution generally does not produce perfect biological traits.  Even ignoring the fact that all organisms die- which deals a massive blow to any idea that organisms are perfectly suited to their environments- just about no biological trait is perfect.  The adaptations that make humans good runners (and long-distance walkers) are subject to wear and tear and injury.  Just about any trait of any organism you think of could, in theory, be better.  Evolution, then, doesn't produce perfection.  Why? Several reasons:

a) Evolution is constrained by genes, environment, and timing. Biologists call this "phylogenetic constraint".  Put more simply, evolution is limited by its starting materials.  The right genetic mutation has to pop up, in the right individual organism at the right time in an environment that allows that gene to produce a favorable result for that organism, one that helps it survive and reproduce.  And so on.  So it's no surprise that the human foot, for example, is just an ape foot with more robust arches and toes aligned in one direction.  Is that ideal for walking and running as a biped? Likely not, but when you start with an ape foot, you're gonna end up with a slightly redesigned ape foot; evolution doesn't start from scratch.

b) Evolution is constrained by competing biological necessities. A human runner a million years ago didn't just need to run.  She needed to find food, avoid getting eaten, produce offspring, dig up tubers, carry stuff long distances, and navigate a complex social and physical environment (hence the huge brain that consumes tons of energy).  Even ignoring the cognitive/brain stuff, at the purely physical level, running is not all that a human (or any organism) needs to do in order to survive.  Natural selection operates on many biological traits simultaneously and the optimal result is a compromise between these competing demands.

c) Energy is a scarce commodity, and your body wants to conserve it.  Perhaps the biggest constraint is imposed by energy.  Getting and using energy is one of the fundamental problems for any biological organism, and there's only so much to go around (we call this an organism's "energy budget").  It's true humans are strange primates in that our evolution was characterized by an expanded energy budget- we eat more and can do more metabolic work than other primates, probably to fuel our huge brains and also an unusually physically active lifestyle.  But, we are also energetically thrifty, and when we expend alot of energy on something (like running), our bodies compensate by reducing energy expenditure elsewhere: basal metabolic rate slows, sex hormone production drops, and (if you're chronically in energy deficit) your immune function decreases too, all to save energy and prioritize immediate survival.  This energetic problem explains why you have to exercise to be physically fit; your body would rather not spend the energy building a strong cardiovascular system, producing extra mitochondria, adding muscle mass, etc, unless it's clearly necessary.  (This is explored in greater depth in that post I keep linking to.)

To illustrate the idea that evolution has to produce traits for competing functions, consider how "evolutionarily fit" an elite athlete really is.  We consider them pinnacles of physicality, but in an evolutionary sense they are more like a pure bred dog: they have a specific set of unusual traits that also makes them generally crappy at being a dog.  To set a marathon world record, you need the upper body strength of a 9 year old and a metabolic engine/body fat percentage that would cause you to starve to death after 2 days without food.  To play most positions in professional football, you likely need the aggressive temperament and social acumen of a rabid wolverine, which would get you exiled from any early human tribe (and indeed, given the incarceration or at least arrest rate of pro football players, gets you exiled from the modern human tribe).  You get my point: being amazing at one thing, any one thing, probably means you're going to suck at other things that would have been necessary for survival and reproductive success a million years ago.

3. Natural selection is concerned with you suriving long enough to pass on your genes.  If you die at 30 after having kids, so be it.  (It's not quite this simple, as being healthy into old age means you can care for your grandchildren, thus helping your genes to continue- but let's keep it simple. For more, google "The grandmothering hypothesis".)  Even though running is generally good for you, it can and does cause wear and tear on your body.  And doing too much can cause overuse injuries.  So long as it helped early humans find food and cover distance, endurance running and the body's responses to it were favored by natural selection, but the side-effects (many of which may appear later in life) came along for the ride because, well, evolution doesn't care.

So.... Let's put this together.  We shouldn't be surprised that running can have negative side effects, given that traits are not perfect, and evolution is blind to deleterious consequences that happen after you've passed on your genes.

This description of evolution as imperfect and constrained also explains why you might need to lift some weights or ride a bike to realize your full potential as a competitive runner.  A bit more fast twitch muscle mass in some key muscle groups (or other adaptations realized through strength training) does, it seems, make you objectively faster, even over long distances.  So why doesn't running alone produce these particular muscular adaptations, the ones you get from strength training? Maybe because they are energetically costly.  Let's say these adaptations increase your performance by 1%, letting you run 1% farther or faster.  This performance increase would matter little on the African savannah, and the cost/benefit math of the energy required to produce these particular adaptations vs. the survival benefit you get doesn't work out well.  In a food scarce environment where early humans lived on the margins of energy balance, energetic thriftiness was far more important than minute improvements in running speed and stamina.  So, natural selection did not favor increased muscle mass and force generating capability as part of the endurance training response.  But because you can elicit these responses through strength training, this suggests that at some point in our evolutionary past, getting stronger in response to short/hard muscular work was an evolutionary advantage, and it was selected for.  This is the main way that fast twitch muscle fibers respond: they get bigger and more powerful.  And as primates have lots of fast twitch muscle which they use to navigate trees, it's easy to imagine that our ability to gain fast-twitch strength through strength training is a holdover from earlier primates. (In fact, when our ancestors split from the ancestors of modern chimpanzees, our muscles changed to favor endurance over brute strength, and this difference is evident in comparisons of human vs. chimpanzee muscle.)  And luckily for us, we can exploit that to gain a bit of fitness that makes us better at something totally different: running.

The science is more mixed regarding the benefits of endurance cross training (biking, swimming, etc).  Of course they confer cardiovasular benefits, but as the muscular action is very different from running, muscular benefits are minimal.  I'd suggest, as others have, that they key benefit of endurance cross training is that it provides a cardiovascular stimulus while resting running-specific muscle groups.  And as you get older like me, and old nagging injuries and weaknesses accumulate and magnify, the benefits from cross training begin to outweigh the fact that you're missing some running-specific muscle training.

Thanks for indulding my ramblings, as always.