When I read or hear about unbelievable physical feats of endurance I am both awestruck and curious. This article distills some current scientific study. It is quite short, so well worth a read.
"The ultimate limit of human endurance has been worked out by scientists analysing a 3,000 mile run, the Tour de France and other elite events.
They showed the cap was 2.5 times the body's resting metabolic rate, or 4,000 calories a day for an average person.
Anything higher than that was not sustainable in the long term."
Duke University conducted this study.
"You can do really intense stuff for a couple of days, but if you want to last longer then you have to dial it back," Dr Herman Pontzer, from Duke University, told BBC News.
He added: "Every data point, for every event, is all mapped onto this beautifully crisp barrier of human endurance.
"Nobody we know of has ever pushed through it."
The study suggests that it is the human digestive system, not the heart, lungs, or muscles that is really critical. Apparently our bodies cannot sustain a higher level of energy use because we can't digest, absorb and process enough calories and nutrients.
Here are the detals of the study: