Calculating the Limits of Endurance - Intensity versus Time

Thurber et al in Herman Pontzer's lab at Duke used the Race Across USA (what dummies!) to look at metabolic rate relative to basal metabolism. Over an arbitrarily long period, humans can only sustain effort at 2.5x resting metabolism. In a marathon it's almost 16x (for less than 1 day), in the Tour de France it's almost 5x (for 23 days) and for someone crossing Antarctia it's 3.5x for 95 days. Lay article here, paper here.

Additional Evidence for the Central Governor Hypothesis: No Claudication-Like Symptoms With Sustained Maximal Effort

Two theories have emerged to explain a maximal exertion limit in exercise.

1. Catastrophic failure. This is the "common sense" idea that limitations of local physiology set the maximum exertion - e.g., oxygen supplied to muscle, muscle fiber recruitment, and metabolite accumulation; and
2. The Central Governor model (CGM), where some central regulator sets the limit. This regulator is often assumed to be in the central nervous system, and has been explained either as active limitation of exertion, and also as depletion of neurotransmitters during exercise.

For a more detailed explanation of these models, I refer you to Ament and Verkerke (2009.) They primarily defend the catastrophic failure model, and while they do give a fair shake to CGM, they also have some interesting arguments against it, as well as an excellent reference section (with references to Tim Noakes, probably the best-known proponent of CGM.)

It should be said that nothing in biology is ever simple, so I would agree with Ament and Verkerke that at the very least it's worthwhile distinguishing between slow- and fast-twitch muscle fibers. In pointing that out, they imply but don't really defend the point that BOTH of these models may be correct, i.e., CGM in Type I slow-twitch muscle fiber, and catastrophic failure in Type II fast-twitch. It also seems that CGM proponents like Noakes are mostly interested in slow-twitch (he's an endurance runner) and I'm interested in this model for the same reason.

A bit of circumstantial evidence to bolster the CGM argument, at least for slow-twitch fibers is the phenomenon of claudication. Long story short, claudication is what happens when you have a blockage in a peripheral artery in your leg. Some blood may still be getting through, but not enough - and the problem is revealed with activity, so that when the person tries to run or even walk, they get extreme tiredness, pain and cramping, often in a matter of blocks, and at a very consistent distance. It's basically the peripheral analog to chest pain on exertion (stable angina) though in that case it's because of a blockage in a coronary artery in your heart. In some cases, especially in diabetic people whose circulation is damaged by the disease, skeletal muscle can actually be infarcted - that is, severe damage to muscle cells occurs causing tissue necrosis much like in a myocardial infarction.

If the catastrophic failure model is correct, then athletes pushing themselves and only being limited by peripheral metabolism in their muscles would sometimes infarct their muscles like this, and yet it doesn't happen. If CGM is correct, you would expect that the Governor keeps us out of trouble by keeping us away from the limit - except in people whose vascular systems have undergone rapid pathological deterioration (not just deconditioning), at a rate which perhaps the feedback system to the Governor can't adjust fast enough to take into account. Not only that, the experience of maximal exertion during exercise is not at all like claudication or muscle infarction. (Admittedly I don't have a satisfying mechanism for the peripheral vasculature's feedback to the Governor, but it is very interesting that anatomically nerves and vessels do tend to travel together in bundles.)

The most tempting possible objection to using the lack of claudication or infarction events as support for CGM is to say that catastrophic failure is correct, but the cardiopulmonary system is the constraint that sets maximal exertion, not the peripheral muscle. That is, in a runner, the quads are nowhere near their "theoretical limit" but the heart-lung system is at its maximum (whether it's cardiac output or diffusion-limitation, as occurs in a very few elite athletes.) But this just moves the site of the potential infarction, since now we would be infarcting our hearts instead of our limbs if that's where the limitation is, with no outside governor, and again, this never (or almost never) happens. (A very few elite athletes are diffusion limited, where cardiac output is so great that blood is moving so fast through the lung's capillaries that there's not enough time for oxygen to get into the blood, but this is certainly not the case for the vast majority of humans.)

A second possible objection is to say that catastrophic failure is true because at sustained maximal exertion there IS infarction, e.g., after a marathon, runners sometimes show CK values on the order of 10,000 to 100,000. Yes, but a) CK levels in infarcts from diabetes are actually much lower, in the low thousands; b) chronic mechanical damage to muscles would produce the same effect; c) these runners don't describe acutely feeling part of their quad suddenly dying like an infarct - it's a gradual process, and d) such elevations would happen in acute exercise as well. But they do not.

I suspect that the final correct answer will be that there is a CGM for slow-twitch muscle and not for fast-twitch, and this is why things like neurological changes e.g. after surgery or trauma, placebos, motivation, and anecdotally SSRI use have an effect on endurance sports but not as much in acute strength-and-speed events.

The Longer the Race, the Weirder the Racers' Physiology, and the Less You Can Generalize From Them

A study by Saugy et al on PLoS ONE makes (or implies) the claim that super-long ultras (as in, 200 miles) don't damage muscle as much as just "regular" ultras of 100 miles.[1] The data for this paper comes from 25 runners (9 completing) the 2011 Tor des Géants (TdG) in the Italian Alps. (Video of that race here). To be sure, it's a cool paper, and I admire them not only for conceiving and executing these studies, but also for getting anyone to put up with their experiment during the race! They do a number of measures on the nerves and muscles of runners before and after the race, and compare against a control group of people who are merely sleep-deprived; this way they can get an idea to what extent the wear-and-tear of super-long ultras is related to sleep deprivation. When races get to be this stupid-long (in space and time), part of the endurance is the lost sleep.

Here's the part that the press picked up: in addition to comparing racers to sleep-deprived controls, the paper also compares the results from one race to another - specifically, to the shorter Ultra Trail du Mont Blanc of 2009,[2] which is "only" 100 miles, with 40% of the elevation change of the TdG. Counter-intuitively, runners in the shorter race showed more fatigue and inflammation than in the longer race. Interesting! The authors go on to discuss reasons why this might be, mainly that the pace in a longer race is slower, which is less punishing. Specifically, the pace in the longer race here was 15% slower, and some of the wear-and-tear parameters were 50% or more lower!

That does make sense, but it raises another interesting question they don't address about inter-individual exercise response differences - which I think is more likely to be relevant here than in most experiments. We can't be sure the difference is because of any difference in the race experiences themselves, because the racERS are different too. The longer the event, the more systematically different the racers are from racers in shorter events. Not only will people train differently; more importantly, in these groups of racers at the margins of human endurance, you are certainly enriching for genetic variants that allow this kind of performance and which are likely to affect the response to this kind of stress. You're out at the very end of a long tail, many standard deviations away from the mean. The harder the race, the more you're enriching. Basically, they're a bunch of mutants! Consequently, the clearest support for this theory would require that they sample the same runners. This would be hard to pull off, but it might make a difference.

Above: the route during the Tor des Geants, i.e. what's wrong with you people. From Jiloutside.
Below: a few of the Tor des Geants runners. Get it? From screenrant.com.


To my knowledge this has not been studied, but here's an experiment that would test it. Take a group of people who ran a 26-mile road marathon, and another group who have run a mountainous 100-mile ultra. Train them equally, and control for pace so you expect them all to finish in around the same time. My prediction is that even with the same pace and distance and training, the 100 milers would have less wear-and-tear than the marathoners. Because the 100-milers are further out on the tail of the genetic distribution. On the other hand, if the wear-and-tear is the same, then Saugy et al were right not to worry about it. (Part of the improvement in athletic records may be because of new mutations and/or a more efficient sorting mechanism to find these athletes, discussed here in the prediction of 2038 for the first sub-2-hour marathon.)

Granted, the difference between people who run 200 miles versus 300 miles is probably not as great as the difference between people who road marathons and 100-milers. And, it was probably hard enough getting this data for any runners, let alone the same runners in multiple events - and try getting a budget to train people and an ethics board to approve your experimental endurance race. But interesting study and look forward to further results from this crew.


References:

1. Saugy J, Place N, Millet GY, Degache F, Schena F, Millet GP. (2013) Alterations of Neuromuscular Function after the World's Most Challenging Mountain Ultra-Marathon. PLoS ONE 8(6): e65596. doi:10.1371/journal.pone.0065596

2. Millet GY, Tomazin K, Verges S, Vincent C, Bonnefoy R, Boisson R, Gergelé L, Féasson L, Martin V. (2011) Neuromuscular consequences of an extreme mountain ultra-marathon. PLoS ONE 6: e17059. doi: 10.1371/journal.pone.0017059

The First Human to Walk Around the World

...was Dave Kunst of Minnesota. (Or at least the first verified, per the article). His brother was killed by bandits in the mountains of Afghanistan, then later his mule died in the Australian outback. (When your mule dies, you know you're somewhere harsh.)

People have run around the world too of course. My biggest regret of 2004 was that I had a chance to join the pack and run with world-runner Jesper Olsen on his aptly named World Run as he came up the Cali coast and somehow I thought I had more important things to do that day...but at least there's a World Run 2!


Good on Dave Kunst, but after 30 minutes in the smaller and mildler San Diego outback I'm pretty much bored. Wow...image credit Stefan_G. on panoramio.

The Paleo Theory Takes a Hit

[Added later: turns out caloric restriction doesn't extend life either, at least in primates; it's just unpleasant. And if you're reading this, you are one. Don't bother telling me about rodent studies.]

As nice as it is to see people trying to unify theory with action, it's even better to compare actual data against your theory, and when the theory doesn't fit the data, you fix the theory.  (Not the other way around.)  Or at the very least, you decrease your certainty about it.

Recently, Pontzer et al published a paper in PLoSONE (and Pontzer summarized it in the NYT here) about metabolic rates in a group in Africa.  Meet the Hadza:

The Hadza live in simple grass huts in the middle of a dry East African savanna. They have no guns, vehicles, crops or livestock. Each day the women comb miles of hilly terrain, foraging for tubers, berries and other wild plant foods, often while carrying infants, firewood and water. Men set out alone most days to collect honey or hunt for game using handmade bows and poison-tipped arrows, often covering 15 to 20 miles.

Paleos par excellence! A paleo wet dream! It doesn't get better than this without a time machine. So what did the researchers find, carefully measuring metabolism using two separate isotopes?

"...average daily energy expenditure of traditional Hadza foragers was no different than that of Westerners after controlling for body size." That is to say, "We found that despite all this physical activity, the number of calories that the Hadza burned per day was indistinguishable from that of typical adults in Europe and the United States. We ran a number of statistical tests, accounting for body mass, lean body mass, age, sex and fat mass, and still found no difference in daily energy expenditure between the Hadza and their Western counterparts."

There are two aspects to controlling one's physique: exercise and diet. The paleo evangelists of the world love to tell us endurance athletes that we're bad, bad people for doing long-term cardio instead of being miserable for short bursts in gyms. It turns out that side of the equation is less important than we all thought, judging by this research (which doesn't exist in isolation - look at the references). What it does mean is that diet becomes even more important.

Evolutionary Economics takes issue with the conclusion and also refers to Razib Khan's point that metabolism has undergone differential adaptation in humans who settled different parts of the world, and ate different things. (No kidding - for one extreme example, look at the fat distribution on the now very multi-ethnic sumo wrestler corps in Japan and this cannot be disputed.) That being the case, I would also argue that different populations of humans might respond differently to varying diets and exercise regimens. There has clearly been selection in muscle proteins, that your partly-sequenced blogger has found himself and his ancestors subjected to.

But most importantly, I hope the paleo exercise people start to settle down in the preaching.

The Chemistry of Runners High

Most interesting, there's an anandamide spike - to simplify, a pleasure chemical that is the endogenous compound for the receptor that THC stimulates. Chocolate has anandamide in it, which has been advanced as one reason for people's strange behavior around it. Include me on that list.

I originally found this at io9, which questions the evolutionary value of runner's high (or if it's just noise in an overtaxed system); one possible explanation is obvious in io9's link to a great Youtube video of an African persistence hunter. One of the reasons I started writing this blog was to puncture the Just-So stories of paleo enthusiasts who often claim that running is "unnatural" and bad for you. Yet here we have our species' remaining hunter-gatherers doing exactly that! (Apaches are reported to have done the same thing.)

Paleo Workouts: Running and Lifting Outside

"No no, let's move him this way. I've been neglecting my delts this week."

Move Naturally (MovNat) seems to be kind of a hybrid of middle-distance running and high-impact gym-type workouts. More importantly, it looks fun. I might try this. The idea that
is probably a useful vehicle for thinking about fitness, but it has holes, and people aren't consistent iin the way they think about it. Case in point, the subject of this very article talks about eating paleo but then gets inspired by the Tarahumara of Mexico. News flash: the Tarahumara are about as non-paleo-diet as you can get! They get HUGE portions of their calories from corn.

Still, this is the first time I've been intrigued by a trendy work-out philosophy. Because you can do it outside. And it's fun. And it would also seem to correct the deficiencies decried in lifting-oriented exercise (by endurance types) as well as in endurance exercise (by lifting types).

Previous entries on the paleo/lifting vs running debate:

- Paleo Running in Kenya and Mexico Highlands

- Paleolithic humans did eat grains - #1, #2, #3, #4

- The endurance vs. lifting jihad, in paleo terms: #1 and #2

I am an Alpha-Actinin 3 Knock-Out Homozygote

I just found this out today when I reviewed my DNA test results. To learn why that's significant for distance runners, go here.

Where Are the Tibetan Marathoners?

I ask because a flurry of papers in the last month or two has shown that they've racked up a bunch of oxygen-efficiency mutations, unsurprisingly. What's interesting is it shows that where humans live can have rapid evolutionary effects on us, even in just a few thousand years (Tibetan and Han Chinese populations split less than three millennia ago). Most interesting is that one of the mutations much more common in Tibetans was already associated with improved athletic performance. (Science paper wasn't up yet so here's an article from the lay press; here are two more.)

So why don't we see more of these guys and gals out on the course? My prediction is that before long we will, although I'm presuming (since I haven't read the paper either) that these are genes which improve aerobic metabolism rather than muscle mass and will therefore enhance performance in endurance sports.