Thursday, July 13, 2017

An evolutionary perspective on cross training for runners

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.)

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.  (See the Pontzer papers linked halfway down this page.)  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.

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 were 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.  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 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.

Friday, June 30, 2017

"Chimpanzee super strength and human skeletal muscle evolution"

We know that chimps are super strong-- commonly they're thought of as being six times stronger, pound for pound, than a human.

Fantastic new research (paper here) puts this number at more like 1.5x.  Still, there are clearly differences in human and chimpanzee muscle, and the main contribution of this paper is that they've figured out what this difference is: muscle fiber type.  Chimpanzees, probably like most other primates, have a preponderance of type II (fast twitch) muscle fibers, which generate more force (but fatigue easily) than type I (slow twitch) fibers, which are good for endurance.  (These fiber types differ in protein content, mitochondrial number, stored glycogen and fats, enzymes, capillarization and all sorts of other things that aid in either anaerobic force production of aerobic, oxidative energy production).  Through computer modeling, the researchers rule out other candidate causes for chimpanzees' relative super strength, leaving only fiber type composition and fiber length.  A simple explanation that really makes sense...why would evolution bend over backwards to produce increased strength when something as simple as fiber type ratios will suffice?

But this isn't a muscle physiology paper, nor a primatology paper-- it's really about human evolution.  As our closest living relative, chimps are a great comparison: differences between us reflect the different evolutionary pressures each lineage (the one leading to chimps and another leading to humans) faced over the last 7 million years.  The authors interpret their results as further evidence that hominins (early human ancestors) shifted from the ancestral condition common to all apes-- lots of fast twitch muscle fiber, helpful when you haul yourself around in trees-- to a higher ratio of slow twitch fibers, to complement the multitude of other adaptations that arose for 2-legged locomotion.  Brief mention is made to expanded daily travel distance, and (predictably) I'd again say that running should be included here.  The derived human condition of mostly-slow-twitch muscle, along with all of our other endurance-type adaptations, scream long-distance locomotion.  All in all, this is further evidence telling the story of evolved human endurance.

More on the evolution of human vs. ape muscle here:

Tuesday, June 20, 2017

Pontzer: Economy and Endurance in Human Evolution

Herman Pontzer, whose interests for years have orbited around biological, evolutionary and ecological issues related to human endurance, has finally jumped headfirst into the question of whether/how/why/when human endurance running evolved.  The paper (here) in Current Biology -titled "Economy and Endurance in Human Evolution"- is a review and doesn't present original research.  There is, however, some novel (or at least more explicit) synthesis here.  A few things jump out:

1) He begins by distinguishing the terms economy, endurance, and efficiency.  (I've been saying that greater speed and endurance, which together would permit larger travel distances, need not have been super economical in terms of energy cost if they permitted access to novel food resources.  This isn't a point he makes here but he's hinted at it before and the distinction of terms reminds us that these are different things.)  In short: economy is calories spent to move a given distance, per unit of body mass. Endurance: how far you can go at a given speed.  Efficiency: how well the energy-consuming parts of the locomotor chain are translated into forward motion.

2) The paper outlines the biological determinants of endurance and describes which of these features are observable in the fossil record and which can be inferred through other means.  Leg length and joint surface areas tell us alot, and they fossilize.  Other things don't fossilize but we can make inferences:  Daily ranging distance probably increased in early Homo because there's good evidence for increased hunting and scavenging, and carnivores are known to have larger home ranges than herbivores.  Another intersting point: he chalks human endurance gains up to increased leg muscle and mitochondria, which permit greater VO2 max, which he says is the most important advancement.  I wonder if muscle and mitochondria are really more important than other parts of the cardiorespiratory chain-- hematocrit, capillarization, heart size/stroke volume, pulmonary diffusing capacity, etc.

3) As early hominins adopted part-time bipedalism, they would have lost some locomotor speed and endurance, as their still-long forelimbs were taken out of the locmotor equation.  No longer able to use their considerable upper-body muscle mass for walking, their VO2 max would drop.  A chimp suffers a 22% VO2 max detriment when walking bipedally.  This reinforces the idea that the move to bipedalism was not driven by economy, but rather it evolved despite its costs because it had other benefits (and there are lots of ideas about what those could be).

4) We're reminded that traits enabling modern humans' running economy and endurance didn't show up all at once, but followed mosaic and piecemeal patterns of evolution.  Long legs seem to be present before H. erectus, with Australopithecus (and thus the former didn't immediately represent a big change in walking/running capabilities).  As for the emergence of increased running economy (energy cost to run a given distance), he pegs that at around 1 million years ago, a million years into the era of H. erectus.  (The modern human foot shows up then.)  H. erectus specimens found within the last 20 years demonstrate that this species was highly variable, with some populations retaining primitive traits while others looked more modern in their endurance anatomy (think Dmanisi erectus vs. the lanky Turkana Boy).  Truly modern running economy shows up perhaps as recently as 200,00 years ago, only with our own species, as hips didn't narrow fully until then.  But, even early erectus was enough of a runner that this behavior was almost certainly important.

It's this last bit- discussing when and where the most recent advances in economy and endurance occured- that I find most interesting.  It's also the hardest to pin down.  We are still a long way from understanding when running was used, how important it was to survival, and therefore how strongly natural selection favored running economy and endurance.  Every week it seems like a new find adds complexity to the story of this time period-- evolution tinkered with unique combinations of traits; some populations became isolated while others mixed with their neighbors.  I wonder how close we'll ever actually get to understanding the origins of modern endurance.