In some cases, they suspect, it can be so extreme as to cause a person to suffer a type of narcolepsy, the disease that causes people to fall asleep when they get anxious or experience an extreme emotion.

But while laughter can cause you to fall about, a smile leaves the muscles unaffected and you standing.

The researchers, from the Leiden University Medical Centre in the Netherlands, published their findings in The Lancet medical journal.

Jokes and comedy slides

Dr G. Lammers and colleagues had seen that about 15.5% of the population reported a sensation of weakness in their muscles when they experienced emotion and decided to investigate the truth of the saying "weak with laughter".

They looked at the excitability of the nerves that control leg muscle movement - the more excitable, the easier movement is and vice versa.

This is measured by monitoring what is known as the H-reflex, which is bigger when the nerves are excitable and smaller if they are not.

The researchers measured it in volunteers watching various slides - including comic ones - with content designed to provoke an emotional response. They also told jokes to the volunteers.

When people laughed, the reflex disappeared - indicating they would find it more difficult to use their muscles - while there was no change in people who just smiled.

Sleep reaction

Dr John Rothwell, of the Medical Research Council human movement and balance unit in London, explained why laughing might cause muscle weakness.

The effect was closely related to what happens when the body goes into a dream state, he said.

Under such conditions, the brain uses a mechanism to shut down the spine and prevent signals getting down to muscles to tell them to move.

Dr Rothwell said it was probable that the brain shuts down these spinal systems to stop people acting out their dreams and hurting themselves.

"You can see it come into effect when people fall asleep on a train," he said.

Stranger on a train

"Often their head starts to droop to the side, and that's a sign that the spinal motor system is losing all it's excitability - and that's a normal part of sleep."

But when the mechanism was triggered inappropriately by laughter it could shut down the body altogether, triggering a form of narcolepsy whereby people remain conscious but lose the use of their muscles, the researchers said.