Endurance leaves before strength does

Detraining dismantles the body in a predictable order, and the qualities that vanish first are not the ones athletes worry about.

Two weeks in a plaster cast, or a fortnight of illness, and the athlete comes back convinced they have lost everything. They have lost specific things, in a specific sequence, and knowing the sequence is more useful than the panic.

Plasma volume goes almost immediately. Within days of stopping, blood volume falls, which reduces stroke volume, which raises heart rate at any given pace and makes easy running feel wrong. That is the change responsible for most of the alarm and it is also the fastest to reverse, often within a week of returning. Mitochondrial enzymes decline over a few weeks and take longer to rebuild than they took to lose, which is the genuinely annoying part of any layoff. Capillary density holds up better.

Strength is more durable than people expect. Maximal force can be largely maintained for several weeks of inactivity, because a good part of it is neural and neural patterns are stubborn. Muscle size declines slowly at first, faster after a couple of weeks of true immobilisation, and returns quickly on retraining thanks to the nuclei already present in the fibres.

The two qualities that fall off first and matter most for injury are rate of force development and tendon stiffness. Explosive capacity relies on fast, well-timed neural drive, and that degrades early. Tendon, as discussed elsewhere, softens with unloading. So the returning athlete has a reasonable aerobic base within ten days, close to their old strength, a nervous system that has become slower to switch on, and a spring that no longer returns what it used to.

They then judge their readiness by the thing that came back first. Running feels fine again, so full training resumes, into a body whose fast qualities and connective tissue are still weeks behind.

I would build a return around jumping and sprinting exposure before I built it around volume, and I would accept feeling unfit for a fortnight to do that.

There is a wrinkle worth noting for older athletes, where the losses come faster and the rebuilding is slower, mostly in the fast fibres.

Nothing about a layoff is symmetrical. Things leave in one order and return in another, and the interval where those two orders disagree is exactly where people get hurt.