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What Determines Endurance Performance?

In contrast to strength training which is about increasing the ability to produce force or increase muscle growth, endurance training is aimed at improving the ability to sustain a given speed or power power output for extended periods.  Like strength performance, there are number of physiological determinants of endurance performance.

That is, while increasing the total amount of force/power is clearly important (in that it will increase speed), being able to sustain that force/power for long enough to compete well is at least as important.

In this article, I want to look at the three primary determinants of endurance performance and talk a little bit about each. I’m not going to talk about the specific determinants of each or how to train for them; this is just meant to be an overview, introductory article on the topic.

The Importance of the Aerobic Engine

Strictly speaking, any event lasting longer than about 2 minutes can be considered an endurance event.  I mean this in the sense that the size of the aerobic engine tends to be critically important to overall performance.  Even at 2 minutes, aerobic metabolism contributes upwards of 50% of the total energy production.  As the event duration increases, this approaches 99%.

Even runners in the 800m (an event taking just under 2 minutes) do a fair amount of pure aerobic work.  This increases at the mile and by the time someone is running 5-10k or longer, most of the training does in aerobic in nature.

This might help to put it into better perspective.   One of the track cycling events in the Olympics is the 4km team time trial, an event lasting roughly 4 minutes.  The German team, which set a world record and won the gold in 2020 spent the majority of their training time doing pure aerobic work with only a small amount of intensity work done in the last 10 days before the event.  That’s for an event lasting only 4 minutes.

Similarly, rowers whose events last roughly 6 minutes do the same, an absolute metric ton of pure aerobic work.  A fairly small amount of high-quality work is done on top of that to sharpen the athlete right before the event.  But mostly it’s just grinding aerobic miles.

Keep this in mind the next time you read an article about how MMA athletes should be doing nothing but interval work because their matches are only 4-5 minutes with a short rest in-between.  As many have found out, a big aerobic engine prevents them from gassing during the round and they recover more quickly between rounds.  Boxers ran road work for decades for the same reason.

As the events go up in duration, the amount of aerobic work requires goes up significantly.  A road cyclist or marathon runner may be doing 80% of their training as aerobic work with only 20% higher intensity work.   A big aerobic engine, topped off with some quality work, is the key to success.

Of course, as the events increase in duration, the contribution of aerobic metabolism to performance goes up and up.  While a cyclist racing a criterium (a race done on a fairly short course with lots of corners) needs the ability to jump coming out of the corner, the duration of that race (an hour) requires a large aerobic engine.   As the distance goes up, the contribution of aerobic metabolism goes up to and this is reflected in the training done.

And with that out of the way, let me look at the three primary predictors of endurance performance.

VO2 Max

The predictor I imagine most are at least familiar with (by name anyhow) is VO2 max.    I’ll spare you the physiology but this represents how much oxygen the body can use at a maximal effort level.  A number of different factors go into determining VO2 max including how much blood the heart can pump, how much oxygen can be carried in the bloodstream and how much oxygen the muscle can use.

VO2 max includes both central (heart, blood, lung) and peripheral (muscular) factors.

In the early days of sports performance, VO2 max was considered the PRIMARY determinant of endurance performance although that turns out not to be the case.    Two individuals with an identical VO2 max can perform at drastically different levels and it’s not unheard of for someone with a lower VO2 max to outperform someone with a higher VO2 max for reasons you’ll understand in a second.

At most you can say that a high VO2 max is required for optimal endurance performance.  You’re unlikely to reach the top without it.  However, it is not by itself sufficient for success.

Just for completeness, let me note that VO2 max can be expressed in a couple of different ways.  One is to express it in absolute terms.  So you might see a value of 6 liters of oxygen per minute.  It’s usually better to express VO2 max in terms of body weight.   So you might see a value of 65 milliliters of oxygen per kilogram body weight (65 ml O2/kg/min) where the absolute VO2 max is divided by bodyweight.   Which is “right” depends on the sport and doesn’t really matter for the purpose of this article.

Note: I want to point out that VO2 max and “aerobic endurance” are not synonymous and are actually controlled by different mechanisms.  Just because you have a high VO2 doesn’t mean you can actually perform well over extended periods.   It just means that you can hit a high peak uptake level.  Developing endurance is still a different factor, one I’ll address when I write about methods of endurance training.

Of Thresholds and Pedantry

When it was realized that VO2 max per se wasn’t a very good predictor of performance, scientists started looking for a better predictor.   Conceptually it became clear that the percentage of VO2 max that an athlete could sustain for extended periods was a much greater predictor of endurance performance than VO2 max per se.  To show why this is I must show some math.

Consider two athletes with an identical VO2 max of 75 ml/kg/min.  But let’s say that one can only sustain 60% of that level for an hour and the other can sustain 80% of that level for an hour.   Their “effective” VO2 will be:

  • 75 ml/kg/min * 0.60 = 45 ml/kg/min
  • 75 ml/kg/min * 0.80 = 60 ml/kg/min

And all other things equal, the second athlete will be expected to perform better than the first.  Because at the end of the day endurance sports aren’t determined by who has the highest peak output but who can maintain the highest output for longest (yes, there are other factors).

This also explains how someone with a lower absolute VO2 max may outperform someone with a higher VO2 max.  So consider an athlete with a VO2 max of 65 but who can sustain 90% of that and let’s compare him to an athlete who has a VO2 of 75 but can only sustain 70% of that.

  • 65 ml/kg/min * 0.90 = 58.5 ml/kg/min
  • 75 ml/kg/min * 0.70 = 52.5 ml/kg/min

What you see is that the athlete with the lower VO2 max has a higher “effective” VO2 max than the athlete with the higher VO2 max. So already we have a better model to predict endurance performance.  With only VO2 and some sustainable threshold, we can see who can sustain the higher effective output level.

But now we ask what this threshold is or represents.  And in doing so get into a huge pedantic debate.

The Pedantry of the Threshold

Over the years, various thresholds have been named including (but not limited to) the lactate threshold (LT), the ventilatory threshold (VT), the onset of blood lactate accumulation (OBLA), the anaerobic threshold (AT) or individual anaerobic threshold (IAT) and many others.  Recently, cycling has started using lactate threshold 1 and lactate threshold 2 to represent different concepts.  And there are plenty I’m forgetting.

Strictly speaking, all of these represent subtly different measurements although I’d be hard pressed to explain the differences in any meaningful way.  And researchers, who seem to have little better to do than argue, have spent a LOT of time debating what those terms mean, if they are the same or not, and whether they are accurate or not.

And in doing so they miss the point completely.

And researchers, who seem to have little better to do than argue, have spent a lot of time arguing about what those terms mean and whether they are accurate or not.

But in doing so they miss the point entirely.

Here’s one example to make the point.  Early on it was thought that the anaerobic threshold (AT) represented the point above which you switched from aerobic to anaerobic energy pathways.  And it was that switch that caused you to start to fatigue.  Hence the AT was the highest level you could maintain for extended periods.

But it turns out that it’s not that simple.  The body is always using a mixture of aerobic and anaerobic pathways so it’s not as if there is some point where it switches.  The lactate threshold is an equally contentious term since it turns out that it isn’t lactate (but H+ ions) causing fatigue to begin with.  Hence lactate threshold is an equal misnomer.  I think you get the idea.

Missing the Forest for the Trees

But here’s what these researchers have missed in their decades of pedantic bickering: the argument about what to call the threshold or even what it exactly represents is completely irrelevant in a practical sense.  It’s just a bunch of nerd scientists arguing about terminology and minutiae which is what nerd scientists do to stay off the dole.

The terminology is not important. The CONCEPT is.

Because conceptually what’s important is this: every athlete will have some threshold of performance output below which they can continue for extended periods and above which they fatigue quickly.  If they are exercising right at that threshold, it may hurt but they can sustain it.  Well below it and they can keep going until they get bored.  Above that and they may fatigue in anywhere from 30 seconds to 5 minutes depending on how far above it they go (and how much they are willing to suffer).

What you call the threshold doesn’t matter an iota to the above definition.  What matters is that this threshold represents the maximum output that can be performed for extended period without fatiguing.  Here extended usually means from 20-60 minutes and sports often use time trials of that duration to determine the threshold.

Even the mechanistic basis of it isn’t that important unless it helps you find better ways of improving it somehow (another issue I’ll address in a later article).  Whether it’s a shift to anaerobic metabolism or an increase in lactate or whatever is simply irrelevant to either the existence or the practical relevance/importance of the threshold.

Field Testing the Threshold

Many coaches will use some sort of field test to determine this.  For example, in the power meter community, the typical metric is called functional threshold power (FTP).   FTP represents the highest power that can be sustained for one hour.   Since hour time trials are mentally and physically grueling, FTP is usually estimated by having an athlete determine their best 20 minute power and then reducing that by about 5%.  Once FTP is known, real-world training levels are set based on it.

Other sports have similar approaches but it all ends up being the same thing: regardless of the name, what is important is the highest level of effort that can be sustained without rapid fatigue occurring.  What you call it doesn’t matter, what’s important is what it represents.  Everything else is pedantic debate.

The X-Factor: Exercise Efficiency

Finally there is what is essentially the X-factor to endurance performance which is efficiency.  Now I wrote a little bit about Exercise Efficiency when I did the series looking at steady state vs. interval training but I’ll recap some of that here.

The human body is actually exceedingly inefficient.  During most activities, for example, of the total energy produced or used by the body only about 20-25% of it actually goes to producing meaningful work, the rest is lost as heat.   That’s simply about the level that human skeletal muscle works at.

In a conceptual sense, exercise efficiency represents how well or poorly the body converts energy into usable work.   A higher efficiency means that, of the energy used, more goes into producing mechanical work.  Which, from a performance standpoint means that the athlete with the higher efficiency can generate a higher power/force output for a lower energy investment.  Hence they don’t fatigue as fast.

As it stands, researchers still aren’t clear how much efficiency can change, what determines it (Type I fiber number seems to be a key) or if it’s just genetic (e.g. superior endurance athletes start with a high efficiency and that’s part of why they are superior).    It’s possible that exercise efficiency can change over time but it occurs very very slowly.  This might explain why endurance athletes continue to improve their performance long after their VO2 max and thresholds have stopped improving.

Determinants of Endurance Performance

Ok, so we have three primary determinants of endurance performance and we might schematically say that:

Endurance Performance = VO2 max * Functional threshold * Efficiency

And that would give a pretty decent approximation of what someones ultimately performance might be.  Or at least what their predicted performance might be.  Now each of those has its own physiological determinants along with ways to best (or potentially) improve them, a topic I addressed elsewhere.

I should mention before finishing that obviously real world sports performance can’t be distilled down into pure physiology like that.

Equipment, motivation, willingness to hurt, having a good team (in sports where that matters), etc. all play a role in determining who will win a given event and often the most well trained athlete still loses because of some non-physiological factor.  If it didn’t, all you would have to do is take athletes into the lab, test the above, run some math and decide the winner.  And that’s just not how it works.

But none of that changes the primary physiological determinants of endurance performance I’ve just described.  They may not determine who wins and loses but they can’t be ignored either.

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