Landmark studies in the 1960s showed people having sex had an increase in their respiratory rate, heart rate, and blood pressure.
These are all signs the body is working at an elevated rate, similar to that experienced during exercise.
For days he lay on a respirator, treated with painkillers and antibiotics, saying little.
In Wythenshawe Hospital, where he spent more than a week, John asked to see a psychiatrist, but this was Britain, home of the dilatory National Health Service.
More recently, these findings have been replicated by a number of researchers using less obtrusive, miniaturized and wireless equipment, enabling more realistic results.
Again, they found a significant increase in markers of physiological stress, such as heart rate and blood pressure.
An all-points bulletin was issued by Detective Chief Inspector Julian Ross, 43, a terse, authoritative figure with an iron jaw and a manner to match.
Then Mark pulled out the knife and called an ambulance.
Some madman had attacked his friend, the boy told the police.
"This was a seemingly unprovoked attack," he told the media, which were directed to inform the public of the knife-wielding monster, "and we have no idea why this happened." To this day, a British media Web site carries a picture of John, his engaging, shy smile shadowed by a tender hint of mustache. All this took place on June 29, 2003, a very hot Sunday, in a small alleyway in an area that goes by the charming name of Goose Green.
At the time of the stabbing the alley led nowhere; it was cut off by a 40-foot drop.
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It was from this injury that the teenager almost died on the operating table—twice, police tell me.