For a long time the stationary bicycle was emblematic of the grinding futility of keeping fit. You can't even call a bicycle that doesn't go anywhere an invention; you might as well patent an oven that doesn't get hot.
Then came the treadmill: they were introduced into prisons as a punitive measure in the early 19th century, but at least those early models ground a bit of grain as well. Whenever I climb aboard a treadmill I am reminded of Sisyphus pushing his rock up the hill over and over for eternity and I think, at least he didn't have to watch MTV.
So when I heard about the Endless Pool, sometimes referred to as "the swimmer's treadmill", I was doubtful. Do people really need a machine which allows them to battle against a strong current without making any progress? Isn't that what rip tides are for?
Peter and Angela Mynors, of Ealing, kindly allowed me to test their Endless Pool. They have had it in their back garden for about nine years and consider it to be a vital component of their retirement wellbeing. They haven't messed about, either: at 8ftx15ft, it's one up from the smallest size and they have built a log cabin over it to keep it nice and warm. Peter shows me the complex hydraulics that drive the propeller, but the pool itself is simple: you climb in (it's about waist deep), turn it on and water shoots out of a grate at you.
Getting into the groove involves adjusting either your stroke or the current until you find the right pace. Unfortunately, I had my 10-year-old son at the controls and his chief aim was to drive me spluttering against the back wall of the pool. I like to describe myself as a strong swimmer - especially to strangers - but I could not keep pace with the pool's fastest setting for more than a few seconds. It's much more knackering than a few laps at the leisure centre. "When you get to the end of an ordinary pool you stop to turn around and get your breath back," says Peter. The Endless Pool is more than a "swimmer's treadmill". It's a Jacuzzi with a sense of purpose.
· Next week: Lucy Mangan moves a muscle