The Pacing Puzzle – the most interesting mystery

The running world pays a lot of attention to aerobic and anaerobic physiology, using terms like VO2max, aerobic threshold, lactate threshold, aerobic base, and max heart rate.  All of this is done in an effort to explain endurance performance, to explain what sets and limits the speed at which you can run, cycle, or row.

As interesting as those terms may be, the reality is that scientific research offers little in the way of explaining performance.  For example, the theory that lactate levels set an upper level of endurance performance has been disproven in recent years.  Similarly, VO2max has also been shown to be a poor predictor of performance.

In short, all those physiology terms may seem really neat and make the person saying them sound smart but they really don’t provide much in the way of useful information for explaining endurance performance. Unfortunately, exercise physiology still has a long way to go before it will be able to accurately explain what makes one person a world champion and another person an average runner.

That being said, there is something that actually does a great job of predicting performance and you are likely already familiar with it – the running calculator or race time predictor.  This device enables you to predict your performance at one distance based on your performance at other distances.  For example, using the calculator Greg McMillan has posted on his website I was able to calculate that if your best performance in a 5K race was 20 minutes then you can expect to race a 10K in 41:31.  And Greg’s calculator is not unique – other people publish similar calculators.

The reason we have race time predictors is that there is a known relationship between performance at varying distances. There is a relationship between how fast you can run the 1/2 marathon and the full marathon.  Or between your 10K performance and your 10 mile performance.  The various running calculators use that relationship in order to predict performance at any distance from a know performance at one distance.

Which brings us to the puzzle of pacing.  Think about it for a moment – when you run any distance other than the shortest of sprints (less than 100 meters) you do not run at the fastest pace you are capable of running.  For example, you don’t race a 5K at your all-out sprint pace do you? No. You race the 5K at a slower pace than your all-out sprint pace.  Similarly, your 10K pace is slower than your 5K pace.  And your 1/2 marathon pace is slower than your 10K pace.  In fact, any time you increase the distance you are running, your average pace per mile slows.  And the variance between your 5K pace and your 10K pace, or between all distances, is quite stable and predictable.


Experience teaches us that we have to slow our pace in order to run a further distance.  There is a maximum speed you can run for any distance and that maximum speed varies in a predictable manner based on the distance to be run.  Therefore, in order to maximize performance at any distance runners naturally learn to pace themselves.  New runners quickly learn their 5K pace, their 10K pace, indeed, their pace at any distance.

What happens if you try to run 10K at your 5K pace?  Most any runner can tell you – you won’t make it.  At a short distance beyond 5K your fatigue levels will rise dramatically and your pace will slow precipitously.

This is the pacing puzzle.  Prof. Tim Noakes says, “pacing is the really interesting athletic phenomenon” and he’s right. If you think about it for a few moments you will realize that pacing is a fascinating mystery. What, physiologically speaking, forces you to run slower at longer distances?  Or said another way, why can’t you run 10K or 10 miles at your 5K pace?  Something within your body obviously prevents us from doing so, but what?

Whoever solves the pacing puzzle will likely hold the key to improved performance.  Unfortunately, traditional endurance physiology has mostly ignored pacing because it’s not easy to test in the laboratory. Sure we can easily observe that pacing occurs, but how do you go about testing the cause or causes on a treadmill in the laboratory?  Instead, exercise physiologists have traditionally conducted tests in the lab that have no resemblance to how athletes actually perform during competition.  For example, a VO2max test – which for many years was the gold standard of endurance physiological tests – is performed on a continually inclining treadmill at a continually increasing pace.

The runner begins the test on a treadmill at a slow pace with little incline. The researcher then regularly increases both the speed and the incline until the runner drops from exhaustion (usually reached in 7 – 12 minutes or so). But no one races like that in real life.  Instead, we all naturally learn to pace ourselves in order to ensure our best performance.  Yet, despite the obvious difference between how a VO2max test is performed and the way runners actually run races, VO2max was touted as the primary factor explaining endurance performance.  But the fact of the matter is that you don’t race at VO2max pace. How could VO2max limit performance if you aren’t running at a pace that elicits VO2max?

Perhaps you are in the camp that claims that lactate threshold, not VO2max, exerts the most influence on performance.  Okay, how is lactate threshold measured?  The same was as VO2max.  The runner begins running on a treadmill in the lab at an easy pace.  At regular intervals the researcher increases the speed or resistance (incline) while taking blood samples from the athlete.  As you can see, there is no relationship between how lactate threshold is measured and how runners actually race. (All this talk of lactate threshold is actually a mute point since we now know there is NO lactate threshold and lactate does not cause fatigue.

Some people still cling to this outdated belief which is why I bring it up here.)  And since you don’t race most distances at lactate threshold how can lactate threshold limit performance?  Wouldn’t you have to run at lactate threshold for lactate threshold to limit performance?  What about races, like the marathon, that are run at less than lactate threshold – how could lactate threshold be limiting performance at those distances?

The bottom line is that traditional endurance physiology does not account for how runners actually race in order to maximize performance.  It doesn’t account for the phenomenon of pacing.

Since traditional endurance physiology has no explanation for pacing, where does this leave us?  Lucky for us, there are some in the physiology world who have been thinking about and studying pacing. I will talk about their work in a future post.