No time to exercise: the minimum that works.

On Monday you open your diary and the gym is the only entry without a time. Tuesday's session loses out to a call, Thursday's to a client dinner. By Friday you've promised yourself a proper start on Monday: three sessions, an hour each.

That three-hour plan is exactly why you never start. I've seen it for more than twenty-five years: people who wait until they have time don't begin. Your body needs far less, provided that little goes on strength.

How to tell you're losing strength

The mirror is late with the news. Everyday movements tell you first.

• The stairs. Two flights and your thighs burn.
• The suitcase. Getting it into the overhead locker has become a struggle.
• The sofa. You push off your knees to get up.
• Your back. It complains after a day in a chair.

Why strength comes first after 40

Muscle peaks between twenty and forty. Then it declines: about half a per cent a year in your forties, one to two per cent after that.

Muscle responds to load. Ask your body to hold a weight and it keeps the muscle. Stop asking and it takes it apart. Astronauts live weightless and lose muscle and bone fast, which is why they do strength training on board every day. A life between chair, car and sofa is the slow version of the same experiment.

Walking isn't enough. It's good for your heart, but only work against resistance preserves strength: a weight, a machine, your body.

The minimum that works

Two sessions a week, half an hour each. Once a week isn't enough for your body to adapt. With two, you get most of the benefit for muscle, strength and metabolism. The benefit for how long you live keeps rising up to about an hour of strength work a week, then levels off.

Five movements, whole body. Sit down and stand up holding a weight. Push: press-ups, even with your hands on a table. Pull a weight towards you. Lift it off the floor with a straight back. Press it overhead. They work many muscles at once, so they do more than exercises that isolate one. If you've spent years at a desk, start on the resistance machines.

Two or three sets of eight to twelve. Pick a weight where the last rep is hard but you could manage two more. Rest one to two minutes between sets. Breathe out as you push. And ask yourself where you felt it: you want the weight that works the right muscle, even if you could shift more.

A little more each time. One rep, one kilo, one set. That gradual increase builds strength more than your choice of exercises does. Keep the same routine for four to eight weeks and log the numbers.

Forty-eight hours in between. Monday and Thursday, or Tuesday and Friday. Muscle grows while you recover. Short on sleep and living on planes, you have less to spare: extra sessions just wear you down.

In your diary, like a meeting. After each session your attention and reactions stay sharper for at least two hours, so a lunchtime workout pays off in the afternoon.

Look at the week ahead and find two half-hours. Put them in your diary now, and in a few weeks the stairs will tell you it's working.