Aerobic metabolism and the limits of running performance
Proper training for endurance sports is counter-intuitive. To put in your fastest performance, it’s important to train often at a slower pace.
In this post, I’ll discuss the science of why this works, make some recommendations about how to exercise better, and discuss the limits of how fit a person can be.
The physiology here is fascinating, and understanding it can help make you into a better athlete. Understanding how your body processes fat and carbohydrates can also help you understand obesity, diabetes, and maybe even cancer. Understanding how your brain processes sugar can help you understand willpower.
Your cells can burn fuel via two different pathways. They can generate energy using oxygen, inside of the mitochondria, by burning either fat or carbs. This is called the aerobic pathway (the aerobic pathway produces energy via the krebs cycle, if you remember your high school biology). Your cells can also generate energy outside of the mitochondria, without oxygen, by breaking down sugar into lactic acid. This is the glycolytic, or anaerobic, pathway.
The aerobic pathway is limited in how quickly it can produce energy, because oxygen is limited. The anaerobic pathway can produce more energy, faster, because oxygen is not required, but it also produces lactic acid as a byproduct. The acidity prevents your cells from making more energy, so you hit a limit — if you run too fast, your legs start to burn, and you can’t run any faster.
If you stop or slow down, that burning feeling goes away, and you are able to move again. Lactic acid isn’t just a harmful byproduct, it’s also a fuel your aerobic pathway can use. The krebs cycle can take lactic acid as an input, to produce energy, so your aerobic pathway is actually working to clear those byproducts, and help your muscles recover. Cooling down with slow exercise helps you recover from hard, fast exercise.
You can best train your aerobic pathway by exercising somewhat slower than most people do, for longer durations. By training your aerobic pathway, you train your body both to produce less lactic acid, and also to clear it away faster. Since the aerobic pathway can use fat for fuel, training it also lets you exercise for longer without running out of fuel.
There are two important thresholds here. One is the aerobic threshold, where the aerobic energy pathway is starting to be insufficient, the glycolytic one starts to contribute some energy, and lactate slowly builds up. The second is the anaerobic threshold (or lactate threshold), where even the glycolytic pathway starts to get overwhelmed, and lactic acid builds up significantly. Qualitatively, a graph might look like this.
To be as fit as possible, most of your training needs to be done below the aerobic threshold, to maximize endurance ability, with a small amount of high intensity work, to strengthen your heart.
Testing my physiology
I wanted to know how this works in practice. I’m an amateur runner, hiker, and mountain climber. What is fast exercise and slow exercise, for me?
To give some concrete numbers, let’s say I can run a 10k race at a pace of 6:40 minutes per mile. If I really push it for one mile, I can go a bit faster, maybe 5:40. If I want to get as fit as possible, should I be out running some very painful 6 minute miles? Or some moderately strenuous 7 minute miles? Or something much slower?
To understand what’s going on in my body, I went to a sports physiology lab to measure my athletic performance. In the lab, I ran a few miles on a treadmill, at various paces, to measure how my metabolism worked. I wore a gas mask to measure the composition of the air I exhaled, and the test practitioner would prick my finger every five minutes, to measure my blood lactate levels. These are my blood lactate levels, from an initial test:
The anaerobic threshold is usually defined as 4 mmol/L of blood lactate, which for me meant I was running 7 minute miles. The anaerobic threshold is the point at which I’m unsustainably generating lactic acid faster than my body can clear it — anytime I exceed this point, I’m putting a timer on how long I can perform at that level. The more I exceed this point, the faster I will wear out. While the test picked my one hour limit at 7:00 pace, I can run a bit faster for a 40 minute race, or significantly faster for a mile.
The aerobic threshold is a slightly more vague concept — for medium intensity exercise, the body makes a gradual transition from one energy pathway to the other. The aerobic threshold is often defined as the point where you accumulate 2 mmol/L of blood lactate. For me, the aerobic threshold was a relatively slow pace, jogging 9 minute miles.
If I want to train to run faster, you might think I should get out and put in a lot of hard performances. Actually, my body will react best if I run a lot of 9 minute miles.
I did the first test at the start of a training season, and had myself tested again, 3 months later:
In-between these two tests, I was exercising at least 10 hours a week, a mix of hiking up hills and jogging on flat ground.
I got fitter in those 3 months. My heart rate was a bit lower, for a given running pace. The bigger change was that my lactic acid levels stayed low up at a faster running pace than before. For the first test, my aerobic threshold was at a 9:00 running pace, with my heart beating 140 times a minute. For the second test, it was at a 7:30 pace, with my heart rate at 157.
The training I did in those 3 months wasn’t very fast, I intentionally kept my heart rate under 140, often under 130, for almost all of my workouts. Doing a large volume of slow training made me better at fast running, though, because my slow twitch muscles became better at clearing lactic acid.
I tried running a 5k (3.1 miles) as fast as I could, before this training cycle and afterwards. My time went down from 20 minutes (6:27/mile pace) to 19:13 (6:12/mile pace).
The test was also able to measure whether my muscles were burning fat or carbohydrates, by measuring the gas I exhaled. As of the second test, my metabolism worked like this, I could burn up to 235 fat calories per hour, and the rest had to come from carbs:
I made some improvement in my fat burning ability, between tests:
In the 3 months between tests, I grew more mitochondria in my muscle cells. Because I have more mitochondria, I can run faster while producing less lactic acid. My cells still preferentially burn carbs instead of fat, though.
A future goal, to improve my fitness, is to teach my body to better use fat as fuel. To do this, I probably need to do more long workouts where I stay under my aerobic threshold. I need to eat less often, while training, so that my body is forced to rely on its fat reserves. I will do some of my cardio first thing in the morning, without eating first. I might also try going on a higher fat diet, to retrain my metabolism.
For me, this goal has nothing to do with losing weight, the goal is to be able to keep going for longer.
Your body doesn’t store a lot of carbohydrates. Your muscles store some as glycogen, and there’s some more glycogen stored in your liver. That all totals to 2,000 calories, at best. Suppose I tried to go out and run a marathon, at a 7 minute/mile pace, I’d be burning over 1000 calories of carbs per hour. I’d run out of energy in about 2 hours, after travelling only 17 miles. This is why beginners tend to “hit the wall” in that race — they make it maybe 20 miles and then have to slow down dramatically. You can maybe endure for longer, by eating sugar during the race, but there are also limits on how quickly you can absorb food.
Everyone’s body contains a lot of fat as fuel, by contrast. I’m skinny, but I still have around 11% body fat, so I have maybe 18 pounds that I could burn through. That’s 63,000 calories in reserve. Maybe I can burn 250 fat calories per hour, while exercising slowly. So, in theory, I could walk for about 10 days straight before I’d run out of energy (not quite that long, because the body also burns calories at rest — but you get the idea, the body stores much more energy in fat than carbs).
I’m not sure how high my fat utilization could get. One book suggests that I could maybe double it to 500, and some elite athletes have been measured burning as much as 900 calories an hour. If I can train my body to do that, I should be able to go much further without running out of fuel.
As a mountain climber, the events I’m training for take much longer than a marathon, but they’re done at a slower pace. Summit day on a big mountain may involve 12–18 hours of climbing. I try to eat a little, through the day, but it’s actually kind of hard to eat while you’re pushing yourself at high altitude — despite the exertion, your appetite declines, high altitude can cause nausea. Food and water need to be carefully kept warm enough to consume, if you’re not careful a candy bar will get rock hard, an uninsulated water bottle will freeze solid.
To be more successful in this environment, it would help to be able to burn fat more readily, to move quickly, to keep going longer, and to keep my body warm.
Training recommendations, for beginners
For people that aren’t into exercise, this is probably all very abstract. If you don’t get out much, most exercise is good for you, and the key is finding something safe that you enjoy enough to repeat.
That said, thinking about the science here could help you design a better program. Sedentary people tend to have very little ability to burn fat, while exercising. Their mitochondria are atrophied from lack of use, and their body relies mostly on burning sugar. There’s a kind of paradox here, in that an obese person’s body is good at storing fat, but has lost the ability to burn that fat as fuel. This has a lot of number of consequences for health.
For one, if you can only burn carbohydrates, to exercise, it’s going to be hard to lose weight by exercising. Typically, you’ll deplete your glycogen stores, then crave carbs and sugar to refill these, then not lose much weight. Low blood sugar is associated with a decline in willpower. If you’re trying to eat well, at the same time as you work on exercise, this could sabotage your diet goals. If you’re exercising hard, these problem will be exacerbated.
To get out of this trap, consider exercising more slowly. If you’re out of shape, you might benefit more from walking for an hour rather than running for 20 minutes. Eating a lower carb diet might also help reduce this dependency.
The kind of slow cardio that I use for training isn’t painful, it’s a kind of exertion that feels good that I can do for a really long time. I’m not out of breath, I can have a conversation all through a hike or run. I often wonder if exercise classes and trainers screw up this part — they push beginners so hard that their lungs are burning and they’re sore for days. At worst, the clients get injured. On average, they fail to convince people that exercise is enjoyable or worth doing at all.
Finding your training zones
For the enthusiastic athlete, this stuff should be worth paying attention to. If you want to develop your long term potential as an endurance athlete, you don’t want to get out and run fast, every time. You want to figure out a sustainable pace that you can keep up for hours, and then spend a lot of time training at that pace (hiking, jogging, biking, or whatever). You can supplement that with some faster efforts, if you want to do a short race, or something.
You don’t really need an exercise lab to estimate your thresholds, but having a heart rate monitor helps a lot.
To find your aerobic threshold, I would start by guessing at it, with some simple formulas. Phil Maffetone recommends simply subtracting your age from 180, to come up with the threshold number, possibly adjusting that number up or down a bit based on your training history.
Or try these steps to estimate it on a treadmill.
Measuring your anaerobic threshold is a bit easier. Try running a race at the fastest pace you can maintain for 30 minutes. Your heart rate should rise at the beginning and then plateau around some limiting value. Look at your average heart rate during this plateau — looking at just the last 20 minutes of that race should work well.
What are the limits of human performance?
It’s interesting to look at my test results and ask how much fitter I can still get. This is roughly how my lactate curve compares to a pro marathon runner:
An elite athlete like that can run aerobically at 5:00 pace or a bit faster. If you want to know what that feels like, try getting on a treadmill sometime, crank up the speed up to 12 mph, and see how long you can last for. I would fall off the treadmill after a minute or two. Professional marathon runners can keep that up for over two hours.
I think it would be impossible for me to get that fit. But it’s interesting to think about what the limiting factor is.
Running a mile burns a certain amount of energy, and the amount doesn’t change much depending on running pace. The test I did produced a graph of my running economy, across various paces:
This is measured in terms of how much oxygen is needed, to run a mile. It’s normalized by body weight, so that you can better compare different runners to each other.
As far as I can tell, my economy is already good. I wouldn’t have guessed that, but I suppose it makes sense — I have long legs and skinny calves (less weight to swing). Compared to Europeans, Kenyan runners have longer legs for their height and skinnier calves.
It is possible to improve running economy. It’s something that goes up a bit, just from running more frequently. It can also be specifically trained, in some interesting ways — Paula Radcliffe improved hers by improving her jumping strength (vertical leap height) and by reducing the flexibility in her hamstrings and lower back (she became 2'’ less flexible in the sit and reach test). We usually think about flexibility as a good thing, but having less makes your legs springier and gives some kind of elastic recoil.
Let’s assume that my economy never improves. If I need a constant 200 mL of oxygen/kg/km, I can predict how much oxygen I need to run at a given speed. For me, the equation would rearrange to:
Oxygen = 5.33 mL O2/minute/kg * X miles/hour.
To run faster, I need to be able to consume more oxygen, each minute. Everyone has a limit on how fast they can consume oxygen, this is called V02 max. It’s mostly just a measure of how big your heart is, and how much blood it can pump with each beat.
My VO2 max is somewhere around 53 (mL O2/kg/min). I can’t sustainably run 12 miles per hour, since that would take 64 (mL O2/kg/min). Even ignoring lactic acid buildup, my heart simply couldn’t pump enough blood to supply that much oxygen. The best endurance athletes in the world have a VO2 max around 80-90.
I tried to establish my VO2 max with a few different methods, each of which gave similar results. I tried to measure my limit directly in the lab — at the end of my lactate test, I asked the test practitioner if we could crank up the speed, I ran a few minutes at faster speeds, up to 11 miles per hour, and I measured a 53. I’m not sure if I gave it my all, but it felt close to my limit (I felt like I was choking in the mask, and I didn’t want to try 11.5 mph)
Here’s a table which lets you estimate vo2 max from running race times. This table also puts me around 53:
World records at most running distances can be predicted quite well with this table, by choosing a VO2max of about 85 for men and 75 for women.
Note that VO2 max is measured per kilogram, so if I want to improve mine, I need to get lighter or I need to enlarge my heart to pump more blood. If I could lose 10% of my body weight, my V02 max should be 10% higher. I’m currently at 11% bodyfat, though, so this would be a bit of a challenge, I’d have to lose muscle as well as fat, leaving only the muscles that were necessary for running. World class runners are incredibly thin:
So, if I got much lighter, My maximum could get up to 58. Beyond that, I would need a stronger heart. The best way to train your heart, to increase stroke volume, is interval training — exercise with your heart at the highest rate it can go, for a short period of time, rest a little, and repeat.
That kind of training is at odds with the long distance aerobic stuff recommended above— running slowly, for long distances, triggers changes in your muscle cells (you grow more mitochondria, capillaries, and aerobic enzymes). Running quickly can develop the heart, but doesn’t help as much with endurance. Optimal training for an elite athlete is a careful mix of both of those approaches, maybe 80–90% long duration slow exercise and 10–20% high intensity.
The science of how the heart grows, from exercise, is interesting. There are two ways your heart can enlarge, one pathological, and one beneficial. If your blood pressure is always high, the heart wall thickens. The muscle units (sarcomeres) multiply in parallel, making the whole wall thicker and less compliant, eventually you die of heart failure. Weightlifting also causes a similar thickening, but I think it’s not so harmful, because the stress is intermittent and the changes don’t accumulate as badly. If you stress your heart through high heart rate aerobic exercise, the muscle units multiply in series, making the heart chamber larger, and increasing the volume of blood pumped with each beat.
Since VO2 max is mostly a measure of heart stroke volume, it can also be roughly estimated from your resting heart rate:
VO2 max ~= 15.3 *(maximum heart rate/resting heart rate)
For me, that formula gives 55, again in the same range as all my other estimates.
In general, V02 max is a hard thing to improve, and there’s a strong genetic component to it. Most people can only improve it a bit through training. My resting pulse is sometimes as low as 50 right now, suppose that with a lot of interval training I could get it down to 40. If that formula works, then my VO2 max might be 68. That’s really just a guess — I’ve tried some interval training and not had that adaptation, but I could do more, or do it more consistently. It’s hard to really know your limits without pushing them.
It’s safe to say that I’m never going to reach 90. So, the biggest thing that would keep me from becoming a world class runner is that my heart will never be as strong. There’s various other reasons — those guys are shorter, and thus can run faster without overheating. They might be born with a different distribution of muscle fibers. They’re younger, and they’ve been training for longer, so they can run more with less injuries. But the biggest factor is just that cardiac output is higher. Genetics plays a limiting role, when we’re talking about human extremes.
Humans, incidentally, can’t run very fast compared to other animals. Horses have a V02 max of 180, siberian huskies measure at 240, pronghorn antelope at 300. (Someone did, in fact, get two pronghorn running on a treadmill, while wearing gas masks).
Dogs have summitted Denali and Aconcagua. Dogs should have no problem breathing at the top of Everest, but it’s never been tried since they can’t handle the technical climbing required to get there.
Despite our inability to sprint, though, humans have a rare ability to outrun other animals over long distances. While we lack speed, we can run for a long time without overheating and kill prey when we catch up.
A few more tangents
Other than running faster, improving your fat metabolism could benefit your health in a number of ways.
If the body’s reliance on carbohydrates gets too extreme, the end result is the common illness diabetes. Slow exercise that improves your fat metabolism should decrease your risk.
Better fat metabolism might help women with PCOS with fertility.
Cancer cells develop most of their energy through glycolysis. It’s unknown if eating a low carb/high fat diet can prevent or treat any forms of cancer (seems unlikely, with all the possible causes), but there’s a possible mechanism where it could help.
Improving your fat metabolism might even improve your willpower. Psychologist Roy Baumeister has conducted a number of experiments showing that exercising one’s willpower lowers blood sugar and, conversely, people with low blood sugar have worse willpower. Our brains rely on sugar for energy, but improving your fat metabolism can help spare limited glucose so it can be directed for important brain functions.
Is that enough reasons to convince you that slow cardio exercise is good for you? Take a long walk and think it over.
