Why belly laughs make us weak at the knees

IF YOU have ever fallen about with laughter, scientists now think they can explain why: humour makes some people, literally, weak at the knees.

Researchers at Leiden University, in the Netherlands, decided to investigate why some people become "weak with laughter", a phrase that is used in many languages. They recruited volunteers who were shown, or told, jokes, and while they laughed their physical responses were monitored.

The team measured a reflex in the subjects' legs by stimulating a nerve behind the knee and measuring the contraction of the calf muscle. This is known as the H-reflex and provides a measure of how ready people's muscles are to move.

The researchers found that when people were told jokes that made them laugh out loud, the reflex almost disappeared, but it was unaffected by jokes that only brought a smile. Gert van Dijk, a member of the research team, said: "When the reflex is strong, you can say the muscles are ready to fire. When it is low, you know the spinal cord has said to the muscles they had better shut up for a while and not move."

The aim of the study, published in The Lancet medical journal, was to look at a phenomenon called cataplexy, a rare side-effect of the sleep disorder narcolepsy. When sufferers from the condition laugh, they drop to the floor and are unable to move for several minutes while remaining fully conscious. Scientists believe there is a switch in the brain, which normally operates only during sleep to prevent people acting out their dreams, but which may also be triggered by laughter in susceptible individuals. While this is extreme in narcoleptics, one ordinary person in six has laughter-induced weakness.

"We hoped we would see people with narcolepsy have a bigger drop in the reflex," Dr van Dijk said. "But that is not what we found. They did have a drop, but normal people had exactly the same reaction. You do get weak at the knees if you laugh out loud."

But he was unable to spread the jokes that made the study's subjects fall about. "They were told by students," he said. "I don't know what jokes they told and I don't think I want to know. From outside the room it was clear that the volunteers certainly found them funny and that was the point."

Suggested Topics
Start your day with The Independent, sign up for daily news emails
Latest stories from i100
Have you tried new the Independent Digital Edition apps?
Independent Dating
and  

By clicking 'Search' you
are agreeing to our
Terms of Use.

iJobs Job Widget
iJobs General

Recruitment Genius: Search Marketing Specialist - PPC / SEO

Negotiable: Recruitment Genius: This is an opportunity to join the UK's leadin...

Recruitment Genius: Sales Administrator

Negotiable: Recruitment Genius: This caravan dealership are currently recruiti...

Recruitment Genius: Invoicing Clerk

£14500 - £17500 per annum: Recruitment Genius: This company are contractors to...

Recruitment Genius: Executive Administrator / Marketing Assistant

£20000 - £25000 per annum: Recruitment Genius: This leading supplier of packag...

Day In a Page

Greece debt crisis: What happened to democracy when it’s a case of 'Vote Yes or else'?

'The economic collapse has happened. What is at risk now is democracy...'

If it doesn’t work in Europe, how is it supposed to work in India or the Middle East, asks Robert Fisk
The science of swearing: What lies behind the use of four-letter words?

The science of swearing

What lies behind the use of four-letter words?
The Real Stories of Migrant Britain: Clive fled from Zimbabwe - now it won't have him back

The Real Stories of Migrant Britain

Clive fled from Zimbabwe - now it won’t have him back
Africa on the menu: Three foodie friends want to popularise dishes from the continent

Africa on the menu

Three foodie friends want to popularise dishes from the hot new continent
Donna Karan is stepping down after 30 years - so who will fill the DKNY creator's boots?

Who will fill Donna Karan's boots?

The designer is stepping down as Chief Designer of DKNY after 30 years. Alexander Fury looks back at the career of 'America's Chanel'
10 best statement lightbulbs

10 best statement lightbulbs

Dare to bare with some out-of-the-ordinary illumination
Wimbledon 2015: Heather Watson - 'I had Serena's poster on my wall – now I'm playing her'

Heather Watson: 'I had Serena's poster on my wall – now I'm playing her'

Briton pumped up for dream meeting with world No 1
Wimbledon 2015: Nick Bollettieri - It's time for big John Isner to produce the goods to go with his thumping serve

Nick Bollettieri's Wimbledon Files

It's time for big John Isner to produce the goods to go with his thumping serve
Dustin Brown: Who is the tennis player who knocked Rafael Nadal out of Wimbeldon 2015?

Dustin Brown

Who is the German player that knocked Nadal out of Wimbeldon 2015?
Ashes 2015: Damien Martyn - 'England are fired up again, just like in 2005...'

Damien Martyn: 'England are fired up again, just like in 2005...'

Australian veteran of that Ashes series, believes the hosts' may become unstoppable if they win the first Test
Tour de France 2015: Twins Simon and Adam Yates have a mountain to climb during Tour of duty

Twins have a mountain to climb during Tour of duty

Yates brothers will target the steepest sections in bid to win a stage in France
John Palmer: 'Goldfinger' of British crime was murdered, say police

Murder of the Brink’s-MAT mastermind

'Goldfinger' of British crime's life ended in a blaze of bullets, say police
Forget little green men - aliens will look like humans, says Cambridge University evolution expert

Forget little green men

Leading evolutionary biologist says aliens will look like humans
The Real Stories of Migrant Britain: An Algerian scientist adjusts to life working in a kebab shop

The Real Stories of Migrant Britain

An Algerian scientist struggles to adjust to her new life working in a Scottish kebab shop
Bodyworlds museum: Dr Gunther von Hagens has battled legal threats, Parkinson's disease, and the threat of bankruptcy

Dying dream of Doctor Death

Dr Gunther von Hagens has battled legal threats, Parkinson's disease, and the threat of bankruptcy