The body adapts at two speeds and only one of them is visible

Lungs and legs improve on a timescale of weeks; tendon and bone are working to a much slower clock.

Six weeks of preseason and the running numbers look wonderful. Everyone covers more ground, recovers between efforts faster, finishes sessions that flattened them in July. That improvement is real and it is also the trap, because the systems that got better fastest are not the systems that break.

Aerobic capacity responds quickly. Blood volume rises within days, mitochondrial content climbs over a few weeks, and the heart begins moving more blood per beat almost immediately. Muscle follows at its own pace. Tendon and bone are slower still, and they are slower for structural reasons rather than stubbornness. Collagen turns over across months. Bone remodelling runs a cycle measured in weeks just to resorb old tissue before new tissue is laid down, and during part of that cycle the site is temporarily weaker than it was before you started.

So you get an athlete whose engine has outrun their chassis. They can now produce and sustain forces their connective tissue has not yet been rebuilt to absorb. The fitness is the accelerator. The tissue is the brake, and it is fitted later.

I think this is the single most useful idea in load management, and it survives without any of the tracking arithmetic bolted onto it. You do not need a ratio to know that a player returning from six weeks off should not be running full-speed decelerations in week one, and no number will tell you that the tendon is ready when the heart rate says it is.

Watch where the injuries land in a preseason and the pattern argues for itself. Early on you get calf and hamstring strains in players whose sprinting exceeded their recent sprinting history. Later, once the running has been tolerated for a while, you start seeing bone stress in the shins and feet, because that tissue has been accumulating a debt quietly the whole time and only declares it at the end.

The awkward part is that the slow tissues give almost no feedback. Muscle tells you it worked yesterday. Tendon tells you the following morning, in stiffness that eases with movement, which is easy to dismiss. Bone often says nothing at all until it says something enormous.

Fitness arrives on schedule. Durability does not, and it does not care that the schedule exists.