Why it's a bad idea to be upside down is all about evolution.
Humans have evolved under the influence of a lifestyle that sees them one way up for the vast majority of every day.
As a result it makes sense that the way the blood is pumped around the body naturally relies on gravity to help.
Putting yourself the wrong way up means it is harder to get blood to the lower limbs and that blood is not easily returned from the head and upper body, with potentially disastrous effects.
Dr Paul Ford, senior lecturer in exercise physiology at the University of East London, cannot think of any type of sportsman who spends prolonged periods of time upside down.
Trapeze artists, rock climbers and bungee jumpers find themselves upside down, but it is rarely for more than a few minutes.
"The main thing is going to be the gravitational effect on blood flow. Normally the heart is supported by the effect of gravity," says Dr Ford.