Last night's viewing - Horizon: The Truth About Exercise, BBC2; Timothy Spall: All at Sea, BBC4

"I have two doctors," the historian G M Trevelyan once said, "My left leg and my right." They seem to have served him well because he lived to 86, not bad for someone who was born when Disraeli was prime minister. And if Horizon: The Truth About Exercise was to be believed, more of us should be signing up with the Trevelyan practice. Amid the blizzard of acronyms that featured in this account of recent research into exercise was NEAT, which is short for Non-Exercise Activity Thermogenesis, pretty much everything you do that isn't sleeping or sitting in a chair. Eulogised by an excitable scientist who looked to be making a pitch for a show of his own, NEAT can make a change to your overall fitness without you really noticing.

In the interests of science and his own health, Michael Mosley pulled on pair of hi-tech underpants (wired to record the minute-by-minute activity of their wearer) and found that he could burn an extra 500 calories a day just by fidgeting and taking the stairs. I'm guessing that he wasn't nearly as comfortable sitting down as he usually would be, with five or six accelerometers stuffed down his trousers, but the results were fairly striking even so. "You burnt an extra 500 calories and you didn't drop an extra drop of sweat," said the excitable man, before continuing to denounce the office chair as a perpetrator of silent genocide: "It's literally killing millions!" he said.

Mosley is a TOFI incidentally (Thin Outside, Fat Inside), which gave him a particular incentive to explore the best way to get fitter. In other words, he's got a lot of visceral fat, which doesn't show but does much more damage. Unfortunately, like many of us, his will is weak and his days are crowded. The good news here was HIT, or High Intensity Training, which promises the same benefits as two and half hours of exercise a week from just three very short sessions. The exercise you do has to be frenzied and extreme (and, in Mosley's case, accompanied by a lot of exhausted whooping) but, not counting the warm up, amounts to just three minutes of flat-out effort a week. The bad news, for Mosley in particular, is that if you're the wrong genetic type no amount of panting and screeching is going to make much of a difference.

After testing the regime for a month, he was pleasantly surprised to find a 23 per cent improvement in his insulin sensitivity (one measure of fitness), but dismayed to discover that in respect of aerobic fitness, he's one of the 20 percent of the population who count as Non-Responders. "How funny," he said as this sank in. "How tragic as well. How incredibly annoying." There have been occasions when that last phrase has been a perfect description of Horizon too, but not this time. This was solid popular science programming, both thought- and muscle-provoking.

The Princess Matilda is now on the homeward leg of her circumnavigation of Britain, a journey that Timothy Spall last night calculated has been conducted at the breakneck pace of a mile a day. Appropriately enough, they were moored up in the town of Amble at the beginning of last night's Timothy Spall: All at Sea and, as in all previous episodes, also paid a visit to Meander, Bickering and Mucking-About-on-Sea. They wandered around Newcastle with Tim doing his Geordie accent and then headed south to Hartlepool, a town they didn't seem very keen to linger in, but then were obliged to anyway, because the waves were a bit scary when they set off for Whitby. After mooring up again, they visited the local maritime museum, briskly: "Shane and I can do a museum in 25 minutes," said Timothy proudly, "We went to Versailles once. It took us 15 minutes." I think that counts as HIT, High Intensity Tourism.