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."