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2:00:25: Eliud Kipchoge’s lesson in endurance physiology (or biomechanics, or psychology, or nutrition, or…)

May 6, 2017

Earlier on this morning, we witnessed a unique and surprisingly exciting time trial over 26.2 miles in Nike’s Breaking2 project, now being rebranded as #VeryNearlyBreaking2 (allegedly). I thought that, since almost everyone else will, I’d give my thoughts on the event. First, two confessions/declarations: first, my old PhD supervisor and close collaborator Prof Andy Jones was a consultant on this project, as was Dr Phil Skiba who I have got to know very well since he started working with Andy’s Exeter group. Second, I only watched the last 25 km of the event because it was a Saturday and wasn’t going to get up at 4:30 am to watch the first half of a marathon in which almost nothing happens.

I had no inside information about the even, mostly because I didn’t pry, other than the odd “how’s that thing going?” to which the reply would usually be “fine” or “interesting”. Most of what I could glean either was or now is in the public domain, largely thanks to the twitter feeds of Phil and Andy and the work of Alex Hutchinson. But the event itself was, for me, a lesson in the physiology of endurance running and the acute effects of prolonged high-intensity effort. As the post’s title implies, there are plenty of other lessons to learn from this, but this one is mine.

Kipchoge completed the 26.2 miles in a blistering 2:00:25, by ~2.5 minutes the fastest a human has ever run the distance.  At 25 km, he was bang on schedule, but in the last 10 km he lost time, and lost about 15-20 s in the last 5 km alone. Why could he not hold that pace in spite of all of the assistance from the arrowhead of pacers (or the car)? To understand that, I think we need to look at what it takes, physiologically, to run a marathon. We hear a lot about maximal oxygen uptake, lacatate threshold (LT) and running economy, as these largely place limits on what is possible. The maximal oxygen uptake is the size of the engine, the LT is the rev limiter (sort of) and the running economy is the miles per gallon (or, in this case, per gram of glycogen) that the engine can offer. What is less well appreciated is that there is not one “rev limiter” in physiology, but two. Running faster than your LT has implications for endurance because you develop a “slow component” of oxygen uptake which increases the oxygen cost of exercise (it reduces economy, in other words). But the slow component can be stabilised provided you run at a speed slower than what is called your “critical speed”. This is the second rev limiter, and I’m going to argue that it is crucial to Kipchoge’s efforts today.

The critical speed, simply defined (!) is the speed asymptote of the speed-duration relationship (analogous to the critical power in the power-duration relationship). Andy Jones and I have written about these concepts in the scientific literature here and here if you want more detail on the definition.  The key point for this post is that a 2 hour marathon effort requires a running speed that is necessarily below the critical speed, because above this point, the slow component of oxygen uptake cannot be stabilised, leading rapidly to the attainment of maximal oxygen uptake and inevitably to task failure.  But the sustainable pace for a marathon run this fast must have been very close to the critical speed (see Jones & Vanhatalo (2017) for more on this). Thus, Kipchoge’s task was to run as close to his critical speed as possible, and stay there. For the most part, he suceeded.

Kipchoge looked in full control and on pace for much of the Breaking2 effort. But if Kipchoge ran just below his critical speed, why did he slow down in the last 5 km? Well, a normal runner running above LT but below the critical speed is operating in their “heavy domain”, where we strongly suspect glycogen depletion plays a key role in determining exercise capacity, one way or another. The presence of a slow component of oxygen uptake increases the demand on fuel reserves, which probably explains why most studies show marathon efforts being completed just above (but not too far above) LT. We mere mortals cannot get close to critical speed during a marathon lasting 2:30-4:00 hours, but there is good reason to suppose elite athletes can. One reason for that is an apparent “domain compression” in which the LT and critical speed both occur at a very high fraction of maximal oxygen uptake. Another is that elite runners have phenomenal fatigue resistance, in part due to the high percentage of type I (slow twitch) fibres in their muscles.

The above considerations provide a reason for Kipchoge’s basic speed, but not for his slowing down. Obviously, the distance itself places a severe strain on fuel stores, but he was still running exceptionally quickly towards the end, albeit grimacing. He didn’t look like he blew catastrophically. He lost some of the advantaged of drafting as the “arrow” collapsed and the car gapped all of the runners, but again this would probably have a minor influence since this only happened in the last few laps. The effect of heat and dehydration were also probably minimal as it wasn’t hot and he was regularly drinking. That leaves few possibilities for his slowing down. Undoubtedly there would have been some muscle damage at this stage, and we know that progressive, slowly-developing fatigue occurs in the heavy domain. However, the slowing in running pace was not progressive, which leads me to conclude that his critical speed itself may have decreased.

The above idea seems at odds with what we have seen experimentally: fatiguing exercise and glycogen depletion does not seem to alter the critical power in cycling, but these experiments are nothing like prolonged exercise performance in elite athletes.  I have heard anecdotal reports of a diminished critical power at the end of cycle stage races or long-duration time trials. If true, it would explain the fall-off in pace without catastrophic failure.  What makes the 2 hour marathon such a challenge is that it is close to the limit of what we think humans can sustain, and the effort itself likely reduces that sustainable pace in the last 10-15 km.

Kipchoge is the first to treat the marathon like a 2 hour time trial and hold it together for most of the distance. If he can find a way of holding it together for the full duration, an athlete of Kipchoge’s talent really could break 2.

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One Comment leave one →
  1. May 8, 2017 9:26 pm

    Good rationale Prof. Burnley!

    Best regards

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