Walking down the stairs wrecks you more than walking up

You walk down four flights on Saturday and by Monday your quadriceps hurt more than if you had climbed all four. It isn't bad luck. On the way down the muscle brakes your weight while it is lengthening, and that way of working, which physiologists call eccentric contraction, is the one that tears fibres.

What you feel the next day has a technical name, delayed onset muscle soreness, and three giveaway traits. It appears between twelve and twenty-four hours after the effort, not during it. It bites hardest at around forty-eight. And it goes away by itself in three or four days without you doing anything.

Lactic acid has nothing to do with it, and there are no little sugar crystals in the muscle. The lactate you produce while straining clears from the blood in under an hour, long before anything starts to hurt. What is actually in there are microtears in the fibres and the inflammation that follows to repair them. Theodore Hough described it that way back in 1902, in a Massachusetts laboratory, using an ergograph, a crank device for measuring fatigue, and a few volunteers with a sore finger.

Which gives you the remedy that does work, and it is counterintuitive: do it again. Repeat that same exercise a week later and it will hurt considerably less, and by the third time almost not at all, because the muscle reinforces what was torn. Physiologists call it the repeated bout effect, and the protection lasts for weeks.