Exercise terminology is a funny thing. Just as you don't need to be a tennis player to get tennis elbow, neither, so it seems, do you need to be a runner to experience a "runner's high". In fact, research suggests that many runners have never experienced the profound sense of wellbeing - supposedly due to the brain being flooded with endorphins - that sometimes occurs during, or just after, exercise. Yet other exercisers, who wouldn't be seen dead in a pair of trainers, have blissed out on activities as gentle as yoga and walking. So what gives?

While early research concluded that, to get an exercise high, you had to work at 76% of your maximum heart rate - and may need to keep that going for two or more hours - more recent findings suggest that nothing like that intensity is required. "Recent studies have found that, as we get closer to our limit in physical terms, far from reaching a euphoric state, we actually feel negative emotions," says Professor Stuart Biddle, head of the school of sport and exercise sciences at Loughborough University. "We're coming round to thinking that you don't have to be on the edge of your threshold to feel pleasure from exercise."

Professor Nanette Mutrie, a sport psychologist at Glasgow University's department of physical activity and health science, offers an explanation: "I think the exercise high is a lot to do with what sport psychologists call flow - a sense of oneness and total absorption in what you are doing," she says. To experience flow, your ability needs to match the task. "If a novice nine-minute-mile runner attempts to run a marathon in three hours," she says, "the challenge is most likely beyond their capability, so they are unlikely to reach a state of supreme enjoyment, and will probably simply feel anxious and uncomfortable. If, however, their goal time is within reach and they achieve it, they get a sense of having mastered something challenging and feel awash with joy."

This theory has currently found favour with sports psychologists. But whatever happened to those pleasure-giving endorphins? It is now known that the body has two endorphin systems: the "central" system in the brain and the "peripheral" system in the bloodstream. And since the two are divided by the "blood brain barrier", elevated endorphin in the blood does not necessarily reflect anything happening in the brain.

"Endorphins might play a part in the exercise high, along with a whole host of other brain chemicals that are hyperactive when we exercise," says Mutrie. "While endorphins are only part of the brain chemical picture, brain chemicals are, in turn, probably only part of the overall exercise high."

One problem lies in the fact that it's difficult to know what any individual means when they talk about a rush. For some, it may just be feeling good that they have notched up another workout, while for others it's almost spiritual in its intensity. One study asked marathon runners to describe their experience. The most frequently picked phrase was "general happiness", rather than "euphoria", suggesting that for most dedicated exercisers, it's not an out-of-body experience.

Put that finding together with the research about intensity, and it would seem that exercise nirvana comes not from pushing yourself to the limit but from setting realistic goals and then basking in the glory of achieving them. Now, that's a result ...