Why a muscle gets tired

Muscle fatigue has a more precise definition than you might think: it is the point at which the muscle no longer contracts, even though the nerve keeps sending the order. It is not that you stop wanting to; the order arrives and nothing happens.

The honest thing to say is that we do not fully know why. What we have are factors that have been linked to it. One is ATP, the molecule the muscle uses to pay for every contraction: when the reserves run low, function falls off, and that matters more in a short, very intense effort than in a long, gentle one. Another is lactic acid, which builds up, lowers the pH inside the cell and gets in the way of the enzymes. And another is the imbalance of sodium and potassium, the salts that make the cell membrane work.

The body has an elegant defense against this. In muscles that hold out for hours, like those of the lower back, the fibers do not all work at once: they take turns. It is a team that rotates so no one wears out alone.