From what I can gather, the heaviest winner of the Boston Marathon in the entire history of the race, tipped the scales at 173 lbs. Today that weight would be unthinkable on the winner’s podium for a marathon. The average winner for a marathon is five feet, seven inches (plus or minus an inch) and 140 lbs, plus or minus a few pounds. There’s a pretty good biological explanation for this.
In endurance athletic events, multiple competing forces are at work. The first one we should discuss is “work required.” It takes more energy to move more mass, after all Force is equal to Mass times Accelerations, F=MA, so the less mass, the less force required for acceleration. Less massive runners have smaller workloads than more massive runners.
The second thing to talk about is heat load. Smaller people actually have better surface to volume. A 5’7″ 140 lbs individual (Body mass index of 21.9) has an expected surface area to volume ratio of 2.92. A 6′ 1′ 167lb (BMI 21.9) individual has a surface to volume ratio of 2.77. Surface to volume ratio calculator used here: https://stats.areppim.com/calc/calc_bsa.php
If you do the division, the shorter runner has a 5% advantage in surface to volume ratio. And a 19% advantage in workload (moving mass).
So the shorter runner has a higher surface to volume ratio, this means that there’s an inherently more efficient cooling system based on how much volume is creating heat, and how much surface there is to spread it out. Even using the exact same BMI numbers, our 5’7″ 140lb runner has a biological advantage over the 6’1′ 167lb runner. And the shorter runner has a 27 lb advantage in less workload.
But what about stride length? Honestly stride length is nearly irrelevant here. The 5’7″ runner has an average stride length of 28″ and the 6’1″ runner 30″. That translates to 7% more distance per stride, or in real terms the shorter runner has to take 7% more strides to maintain the same distance as the taller runner. But we do have an advantage for the taller runner, but that same advantage is almost completely negated by wind resistance. The taller, bigger runner catches more wind.
From https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1331759/ “The energy cost of overcoming air resistance in track running may be 7·5% of the total energy cost at middle distance speed and 13% at sprint speed. Running 1 m behind another runner virtually eliminated air resistance and reduced V̇O2 by 6·5% at middle distance speed.” So not only does the bigger, heavier runner have a weight penalty, but also an air resistance penalty, and the faster you go, the more of a penalty you incur, from the same article: “In running and walking against wind, O2 intakes increased as the square of wind velocity.”
So why 5’7″ and 140 lbs? Well, a 6’1″ person is 6″ taller than the average marathon runner. If we go 6″ shorter to a 5’1″, 116lbs individual (same BMI) the surface to volume ratio increases to 3.09 so the same heat dissipation advantage should apply. The average stride length decreases 3″ though, to 25, so the shorter runner has a 6% advantage in surface to volume ratio, but an 11% disadvantage in stride length. So the advantage of less mass (18% advantage) and advantage of better heat dispersion, what’s going on? Well, lung volume increases with height. Our 5’7″ average marathon winner is going to be more efficient at turning air into oxygenated blood, and expelling carbon dioxide.
But can’t the shorter runner just breath more? Not quite, at the cellular level, O2 and CO2 exchange happens via passive transport, so breathing faster doesn’t speed that process up too much once you’ve reached saturation of CO2 in the blood. Now the 6’1′ runner should also have this same advantage over the 5’7″ runner, but remember he has a less efficient heat radiation system and a higher work load for weight and wind resistance.
Where does that leave us? Well just like every firearm is a deliberate set of compromises between weight, accuracy, and rate of fire, the endurance running event of “marathon” favors people who are 5’7″ and 140 lbs because they are at the intersection of a bunch of biological systems that are all involved in running fast for a long time. Some marathons offer the “Athena” (for women) and “Clydesdale” running categories for heavier runners, so that they have a competitive category against their peers. However in this post I’ve already shown you how three people with the exact same BMI have advantages and disadvantages based on height. So hopefully in the future there’s more weight classes so runners can see how they stack up against people in their peer groups rather than just overall and by age/gender.
As a bigger guy, I’ve finally started losing some weight and dropped below 200 lbs for the first time in decades. This means I’m out of the Clydesdale class, at least for now. Two cheesburgers might put me back in if I’m not careful. But since 2018 I’ve ran a half marathon every year save 2021 (stupid COVID) and this is the first year where I broke the under 2 hour mark (something I hadn’t done since 2014, as a Clydesdale). I’m never going to win, but I can still run. At least for now.