"Strength needs a gym"

So far I've mostly talked about the kind of exercise that gets you breathing, such as walking, stairs and cycling. The guidelines have a second part that people with the threshold habit are even more likely to skip. The NHS says adults should do strengthening activities that work all the major muscle groups on at least two days a week. The WHO guidelines recommend regular muscle strengthening for all age groups.

Here the threshold belief takes a slightly different form. Strength training, in most people's heads, means a gym with racks of weights and machines. It means a programme, written down, with sets and repetitions. It means an hour. Without all of that it's not really strength training, so without a gym membership and a free hour it doesn't happen at all.

I can't offer you the same depth of evidence here as in chapters 3 and 4, and I'd rather say so than pretend. I haven't checked studies on very short strength sessions for this book. What I can offer is what the NHS guidance says, which sets the bar a good deal lower than the gym picture suggests.

The NHS lists the following among activities that strengthen muscles. It includes carrying heavy shopping bags, heavy gardening such as digging and shovelling, and lifting and carrying children. It includes exercises that use your own body weight, such as push-ups and sit-ups, and working with resistance bands. It says these can be done at home or in a gym. For how hard, it gives one rule. To get health benefits from strength exercises, you should do them to the point where you need a short rest before repeating the activity.

You'll see that there's no mention of how many minutes, or of equipment. What matters is that the muscles work hard enough to need a rest. You can get there in a small space and a short time.

Here's what a ten-minute version might look like, as an example and not a prescription. Stand up from a chair and sit down again, slowly, without using your hands, until your legs want a rest. Do press-ups against the kitchen counter, or against the wall if the counter's too hard, until your arms want a rest. Step up on to the bottom stair and down again, leading with one leg and then the other. Carry something heavy in each hand up and down the hall, such as two full shopping bags or two large bottles of water. Rest between each, and go round twice. That works your legs, arms, chest, back and grip, and it's done in the time the potatoes take to boil.

If you have any doubts about your joints, your back or your blood pressure, check with your GP or a physiotherapist before you start, as the preface said. If something hurts in a sharp or worrying way, as opposed to the ordinary effort of a working muscle, stop.

Strength is also the area where a small regular amount is most clearly better than an ambitious plan you've abandoned. Muscles respond to being asked to work, and they decline when they aren't asked. Someone who does ten minutes at home twice a week is asking their muscles twice a week. Someone who's waiting until they can get to the gym properly isn't asking at all.