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Predictors of Endurance Training Performance

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In contrast to strength training which is primarily about increasing the ability to produce force, endurance training is aimed more at improving the ability to sustain a given amount of force or power output for extended periods.

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

Now, it’s worth noting that a lot of ‘how long is necessary’ depends on the event and the term ‘endurance sport’ covers a tremendous amount of ground.  Strictly speaking, pretty much any event lasting about 2 minutes or longer has an endurance component and aerobic endurance becomes an increasingly more important contributor to performance as the duration moves past that point.

Here’s a good example, the German track cycling team which set a world record and won the gold in 2000, training for the 4km team pursuit (an event that lasts about 4 minutes) spent the majority of their training time doing easy aerobic work with only a small amount of intensity work (that occurred in the form of stage races done every few months and short periods of interval work right before their main event).  That’s for an event lasting 4 minutes.

Rowers, whose even lasts roughly 6 minutes or so do the same, an enormous amount of aerobic work for the same reason.  Sure there’s an anaerobic component but it’s typically done in fairly small amounts to ‘sharpen’ the athlete right before their event. The predominant training is aerobic.

Tangentially, you might keep that in mind the next time you read an article about how a mixed martial arts guy (who may be doing repeat rounds of 4-5 minutes with a short rest) should be doing nothing but interval work for conditioning.  Because, simply put, the guy with the bigger aerobic engine will outperform the guy running on higher anaerobic capacities.  The aerobic guy will not only recover better between rounds but, since he can generate more energy aerobically during the round, he won’t gas as fast.  Which isn’t to say that fighters of any sort should be doing nothing but or enormous amounts of aerobic work mind you; both extremes are going to result in poor performance.   But I’m getting off topic.

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.

Why?  Because anything longer than about 2 minutes is going to be aerobic in nature.

With that out of the way, let’s look at some of the primary predictors of endurance performance.


VO2 Max

The one I imagine most are at least familiar with (by name anyhow) is VO2 max.   I’ll spare you the equation but VO2 max is basically a representation of how much oxygen the body can use at a maximal effort level.  It’s a combination of factors including the amount of oxygen that the heart can pump as well as how much oxygen the skeletal muscle itself can use.   So it ends up representing both central (heart, blood, lung) and peripheral (skeletal muscle) factors.

VO2 max used to be considered the primary determinant of endurance performance although now we know better.  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 best all you can say is that a high VO2 max is required for optimal endurance performance; that is you’re unlikely to be a great endurance athlete if your VO2 max is low.  But, by itself, a high VO2 max is not sufficient.  Put differently, you need a high VO2 but that’s not all you need. I hope that is clear.

I’d note that VO2 max can be expressed in a couple of different ways and which way is ‘right’ depends on the sport in question.  Some express VO2 in absolute terms, the amount of oxygen the body can use.  So you might see a value for VO2 max of 6 liters of Oxygen per minute (6 l O2/min).

However, it’s usually better to express VO2 in terms of body weight (especially for sports where body weight isn’t supported).  So you see values along the lines of 65 milliliters Oxygen per kilogram body weight per minute or 65 ml O2/kg/min. Basically, dividing by body weight lets you scale the absolute value to the weight of the athlete.

It can get more complicated than that for other sports but that’s not really relevant to this article.

One final comment, 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.  I’ll come back to this in a later article when I look at traditional endurance methods versus the current fascination with intervals for ‘quick results’.


Of Thresholds and Pedantry

When it was realized that VO2 max per se wasn’t a very good predictor of performance, folks 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 larger predictor of performance than VO2 per se.  To explain this, I must do 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 is therefore

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

All other things equal, the second will be expected to perform better than the first.  You can also work the math so that someone with a higher VO2 max but lower ‘effective’ VO2 can be outperformed by someone with a lower VO2 max but a higher ‘effective’ VO2. 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

Despite the lower VO2, the first athlete would be predicted to outperform the first since he can sustain a higher percentage of his maximum.

Now, the next question becomes what this threshold is and at this point we get into a huge degree of pedantic debate.

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.

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 an example, it was originally thought that the anaerobic threshold represented the point below which the athlete was using aerobic pathways and above which anaerobic pathways started to dominate.  Turns out it’s not that simple, you always use a mix of aerobic/anaerobic pathways and it’s not a switch at the supposed anaerobic threshold.  The lactate threshold is equally contentious as it turns out that lactate isn’t causing the problem with fatigue in the first place, hence lactate threshold is an equal misnomer.

I think you get the idea.

But here’s the point that the people debating this have missed: the argument about what to call this threshold is completely irrelevant practically. It’s a bunch of nerd scientists arguing terminology which is what nerd scientists like to do.

The concept is what’s important.

And conceptually what’s important is this: every athlete will have a level of performance (and this can be expressed in terms of speed, power output, or VO2) below which they can sustain that speed for extended periods and above which they fatigue quickly.   Below that level they can go until they get bored, at that level they can sustain it for extended periods but it hurts, above that level they fatigue relatively quickly (with how quickly being determined by how far above threshold they are and how much they are willing to suffer).

That threshold simply represents the maximal level that they can perform at for extended periods without fatiguing where ‘extended’ generally means 20-60 minutes.  What you call it doesn’t matter, it’s the concept that is important.

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.

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).  Ideally, FTP represents the highest power that can be sustained for one hour.  But since hour time trials are mentally and physically grueling, FTP is usually estimated by having an athlete determine their best 20 minute power output and then adjusting that downwards by about 5%.  Training levels are set relative to FTP (a topic I’ll discuss in a later article).

Other coaches take a different approach, Chris Carmichael for example recommends a 3 mile time trial done at a steady state pace (e.g. cover the distance at the same speed the whole way).  The heart rate, speed and/or power that can be sustained over that distance is taken as an indicator of the threshold I’m talking about.  Training levels are set relative to that threshold.

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.


The X-Factor: 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.

Basically efficiency represents how well or poorly the body converts energy (from the breakdown of carbohydrate and fat usually) into usable work (e.g. force or power production).  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).

In the piece I linked to above, I cited a study which suggested that Lance Armstrong improved his efficiency by roughly 7% over a number of years but that data is now being called into question and it looks like it may have been fudged and that no such change occurred.

Basically, it’s unclear whether training can meaningfully impact exercise efficiency or if it’s all mainly just genetic.  Hopefully more research will help solve this little quandry.


Summing Up

So those are the three primary factors determining endurance performance.

Schematically we might say:

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.

And, as I’ll discuss in future articles, each of the above has its own set of determinants and drivers, some of which are trainable and some of which probably are not.

Before finishing, it would be remiss of me not to mention that, obviously you can’t distill sports performance down to an equation like the above.  There is always more to it than just physiological determinants; to think otherwise is absurd.

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, in any case, now you have an overview of the primary physiological determinants of endurance performance. As noted, future articles will examine some of the determinants of each, what affects how those systems adapt and how to train them.

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