This story is an adaptation of What are the limits of human endurance?, an episode of CrowdScience presented by Marnie Chesterton and produced by Cathy Edwards. To listen to more episodes of CrowdScience from the BBC World Service, please click here.

In the early hours of a night in late September, somewhere between Athens and Sparta in Greece, a trickle of runners trudge wearily by. It’s dark, it’s raining heavily and these ultra-distance athletes are in the midst of a staggering 246km road race.

These runners are attempting the Spartathlon. Starting at dawn in the shadow of the Acropolis in Athens and finishing in Sparta, the annual event recreates the journey of the ancient Greek messenger Pheidippides, who made the journey in a day and a half to summon Spartan troops. (This ancient ultra-runner is probably better known for his legendary 26-mile (42km) run from the Bay of Marathon to Athens – the origin of the term ‘marathon’.)

The annual race was first held in 1983 and the fastest time of 20 hours and 25 minutes was achieved a year later. No runner has been able to beat that record since. But there’s no lack of people trying.

The popularity of ultramarathons has shot up in recent years – and so too has interest in the science and mechanics of how the body works in these extreme situations.