What really limits how long and hard we can run: our lungs, our heart or our brain?

What limits endurance is still argued over, and the argument is about where the ceiling sits: in the plumbing or in the brain. The classic answer is VO2 max, the highest rate at which your body can take in and use oxygen. Push harder and harder on a treadmill, and at some point oxygen uptake stops rising even though the work keeps increasing. Untrained adults top out around 27 to 40 millilitres per kilogram per minute; elite runners reach 77 to 85, and the highest value ever tested, 101.1, belongs to the triathlete Kristian Blummenfelt.

The ceiling has two halves. Supply is the chain that carries oxygen from the lungs, through the heart and blood, to the muscles; demand is how fast the muscles' mitochondria can burn it. For trained athletes, supply is probably the bottleneck, which is why the banned drug EPO, which boosts VO2 max, became a doping product in endurance sports. Yet VO2 max predicts race results surprisingly poorly, because runners differ in economy and in how well they resist fatigue.

So what is fatigue? A rival idea, the central governor, says your brain slows you down before your muscles fail, cutting how many muscle fibres it recruits to keep a safety margin, and you feel that cut as tiredness. The physiologist Archibald Hill floated a similar idea in 1924; Tim Noakes revived it in 1997. Critics answer with athletes who push past the limit anyway, like Dorando Pietri, who collapsed five times in the last stretch of the 1908 Olympic marathon.