The ultimate limit of human endurance has been calculated by scientists who found that pregnant women are close to the maximum.

Researchers at Duke University analysed elite sporting events like the Tour de France and a 3,000 mile ultra-marathon to show what the human body can physically cope with.

They found that the upper limit of energy expenditure was two and a half times the body's resting metabolic rate, or for an average person, burning 4,000 calories a day.

When pregnant, a woman's energy use peaks at 2.2 times their resting metabolic rate.

The findings redefine thinking around what is possible for all humans, no matter what the activity is because everyone hits the same metabolic limit.

According to the research team, anything above the limit that is not sustainable in the long term, for example, marathon runners used 15.6 times their resting metabolic rate over one race.