When Eliud Kipchoge crossed the finish line of the INEOS 1:59 Challenge in Vienna, he not only ran a marathon in under two hours, but he also broke a boundary that people thought was the ultimate limit of human performance. How was such a performance possible? We have just considered the possible physiology behind the 1:59 Challenge and the results are stunning…
To cover a distance of 42.195 km in 1:59:40, equals a speed of 21.2 km/h. Or, in more precise scientific terms: 5.88 meters per second (m/s). For most people, only half that speed is already a great goal, as it would allow one to finish a marathon under the 4-hour barrier.
THE RUNNING ECONOMY
An average elite athlete needs approximately 12.5 - 13.0 ml/min/kg of oxygen per every 1 m/s of running speed. This amount of oxygen is needed to produce the energy required to run at a certain speed. A portion of that energy, however, will be covered by the anaerobic metabolism, hence allowing the actual oxygen uptake to be lower than the total demand.
At a speed of 5.88 m/s, the average oxygen demand of an elite runner is 75.4 ml/min/kg — which equals to approximately 4.5 kJ per Kilogram body weight to cover 1000m.
Most non professional athletes have a maximum oxygen uptake (VO2max) below 75 ml/min/kg. Therefore, the running speed of Eliud Kipchoge – which he sustained for almost two hours — was ran above what most people could tolerate for only a few seconds (or a couple of minutes maximum).
But Eliud Kipchoge isn’t an average elite athlete and the INEOS 1:59 Challenge wasn't the average race. And that is why we assume that the energy demand (or oxygen demand) to run 5.88 m/s is lower than in normal situations. Because of the assumed better running economy of Kipchoge, we use a 0.2 ml/min/kg lower oxygen demand per m/s.
At such a high-end speed a significant part of the energy is needed to overcome aerodynamic drag (visible in the curvilinear shape of the oxygen uptake curve in figure 1). But because Kipchoge was paced throughout the length of the course, it seems fair to assume a reduced aerodynamic drag and — therefore — a reduced energy or oxygen demand for his effort. We can therefore assume that the curvilinear factor of this particular running economy is at least 20% lower, resulting in a 6% lower oxygen demand at 5.88 m/s.
That is why we can assume that the total oxygen demand of Eliud Kipchoge was not 75.4 ml/min/kg but only 70.0 ml/min/kg (see Figure 1).
Fig.1: Oxygen demand of an average elite runner (navy blue dashed line) and assumed oxygen demand for Eliud Kipchoge in the INEOS 1:59 challenge (navy blue solid line). It was assumed that because of the drafting effect, the increase of energy to overcome air resistance as a function of speed is reduced by 20%.
THE BODY
In elite runners, a negative correlation between running economy and VO2max has been reported by scientific studies. Elite runners often do not register a VO2max above 80 ml/min/kg, with values in the range of 75 ml/min/kg seem more common in marathon runners.
In our case here, though, we assume a VO2max of at 78.0 ml/min/kg. Eliud Kipchoge is an outstanding athlete and it seems fair enough to consider his VO2max rather at the high end compared to his peers. A VO2max significantly lower than this seems unlikely, unless the running economy is significantly better than we have considered above.
Highly trained endurance athletes have a reduced ability to produce energy anaerobically through the glycolytic pathway. A high anaerobic — or, more precisely, a high glycolytic — energy supply is simply not needed in endurance events such as a marathon.
The glycolytic sibling of VO2max – the common marker to gauge glycolytic energy production rate – VLamax of such athletes ranges between 0.2 to 0.4 mmol/l/s of maximum lactate production.
In order to be able to run 5.88 m/s for two hours, the VLamax should not be significantly higher than 0.25 mmol/l/s. A significant higher VLamax would basically mean that the glycolytic system is better developed and the athlete would therefore produce more lactate. The slightly higher lactate production by itself may not be a significant performance hampering factor.
But; lactate is a C3 molecule and the result of the glucose breakdown. So, on the other hand, an increase in lactate production also means an increase in glucose demand, which is a performance hampering factor.Therefore a VLamax significantly higher than 0.25 mmol/l/s would make it necessary to either have a significantly higher VO2max and/or a significantly lower total energy demand (further improved running economy).
THE FUEL
We assume that Eliud Kipchoge’s body weight is around 52 kg on race day. And because all metrics used here are expressed as per kg of body weight, the influence of a slightly higher or lower body weight is comparably small.
Furthermore, we also assumed that: he has approximately 43% of muscle mass in his body, of which he uses 75% while running. This results in an active muscle mass of approximately 16.8 kg that contributes to his locomotion. In highly trained athletes, one kilogram of muscle mass stores approximately 25g of glycogen. That means Kipchoge starts the race with a store of 420g of glycogen in his body.
As it can be seen in the race footage, Kipchoge was constantly given drinks during the race. That suggests he managed a carbohydrate intake of at least approximately 20-40g per hour. Hence, the total carbohydrate availability (stored glycogen plus exogenous supply) sums up to approximately 480g. For a two hour effort, this means that his combustion rate should not exceed 240g of carbohydrates per hour. THE RESULTS
To summarize, in this case study we assume:
A body weight of 52kg, 16.8 kg of this being active muscle mass
A total glucose availability of 480g (stored glycogen plus intake during the run)
A reduced energy demand for running, because of an above average running economy and because of the effect of being shielded by other runners
A VO2max of 78 ml/min/kgA VLamax of 0.25 mmol/l/s
In the scenario described here, the average speed of 5.88 m/s would be equal to a stunning 97 % of the anaerobic threshold (6.01 m/s). Or perhaps even more dramatic: a utilization of 92% of the VO2max of our virtual Eliud Kipchoge.The key to this high utilization is the low capacity of his glycolytic system (VLamax). A relatively weak glycolytic systems means that the organism can rely primarily on fatty acids as a fuel at a moderate speed. Significant carbohydrate combustion rates are shifted to higher speed, closer to VO2max. In our case here, at a speed of 5.88 m/s the carbohydrate combustion rate is 221 g/h. As described above, the maximum possible carbohydrate combustion rate was 240g/h without “bonking”.
Fig. 2: Fat and Carbohydrate combustion rates. At a speed of 5.88 m/s the carbohydrate combustion rate is only 221 g/h. Given the assumed total carbohydrate availability of 480g, there is still some wiggle room. WHY NOT ?!
You may say this is all theoretical and too much guess work. In fact, we didn’t use any directly measured and validated physiological facts of Kipchoge to confirm the assumption made here.
BUT:
If any of the metrics of the real Eliud Kipchoge would be significantly different to what we have used here for his virtual Avatar, the other metrics would need to compensate for that change and may easily drift into a highly unlikely range.
Here are some examples:
1. Let’s assume that Kipchoge’s pacemakers would not have reduced his aerodynamic drag efficiently. In this case, his oxygen demand would increase to 74.2 ml/min/kg . At a speed of 5.88 m/s his carbohydrate combustion rate would have jumped from 221g/h to 302 g/h. As described above this is clearly above his limit which is likely in a range of 240g/h.
Without the drafting effect the speed at the same assumed carbohydrate combustion rate of 221g/h would have dropped from 5.88 m/s to 5.64 m/s, resulting in a finish time of 2:04:41 instead of 1:59:40.It turns out that the reduction of the air resistance Eliud experienced is maybe the key element to breaking 2 hours. A 10% reduction in aerodynamic drag results in a reduction of oxygen cost of approximately 2.2 ml/min/kg. The drafting may have provided an advantage of approximately 5 minutes.
2. Let’s assume his VO2max being only 75 ml/min/kg instead of the assumed 78 ml/min/kg. In this case, the carbohydrate combustion at 5.88 m/s would have jumped to 270 g/h, again making it unlikely that he would be able to sustain this speed because the combustion rate would be well above the maximum glucose availability.
Only if his energy demand is decreased further (better running economy) a VO2max value significantly below 78 ml/min/kg becomes possible. The assumed reduction in air resistance by only 20% is a rather conservative estimate. If the V-formation of his pacemakers provided a bigger aerodynamic advantage than assumed here, it actually would open up opportunity to an even faster running time for the marathon, because VO2max values significantly higher than 75 should be within reach. Hence, if combined with a similar great running economy resulting in faster times.
Figure 3: Carbohydrate combustion rate with a VO2max of 78 ml/min/kg (solid line) vs. carbohydrate combustion with a VO2max lowered to 75 ml/min/kg (dashed line). At the average race speed the combustion rate increase from a most likely sustainable rate of 221g/h to 270 g/h.
Higher VLamax
The assumed glycolytic lactate production of maximum 0.25 mmol/l/s is on the low end of what has been measured in highly trained endurance athletes. A higher VLamax of 0.3 or even 0.35 mmol/l/s seems unlikely: Such a high VLamax would have increased the carbohydrate combustion from 221 g/h to 272 g/h. If Kipchoge would be able to take in 90 g/h of carbohydrate, such a high combustion rate of glucose this might be possible, but such high carbohydrate rate seem unlikely in running.
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