Age takes the acceleration long before it takes the endurance

Motor units serving the fastest fibres are lost first, which is why veterans keep their engine and lose their first two steps.

A thirty-six year old midfielder can still run all afternoon and can no longer beat anyone over five metres. That combination looks like a mystery if you think of ageing as a general decline. It is not general. It is selective, and it works from the fast end down.

Motor neurons are lost with age. When one dies, the fibres it supplied are orphaned, and neighbouring neurons can reinnervate some of them. The neurons most likely to be lost are the large ones serving the fast, high-force fibres, and the ones doing the rescuing are typically slower. So the fibres get taken over and converted, and the muscle gradually becomes a slower version of itself while retaining a good deal of its cross-sectional area. The athlete looks the same in the mirror and produces force at a lower rate.

Tendon changes alongside it, becoming more compliant and less able to store and return energy quickly. Since sprinting and jumping depend on that elastic return, the loss compounds. Rate of force development falls faster than maximal strength, and maximal strength falls faster than aerobic capacity, which is why endurance careers extend past forty and sprint careers do not.

The injury consequence follows from the same physiology. A muscle that produces force more slowly is worse at braking, and braking is the protective function during high speed running. That is a substantial part of why hamstring injury rates climb with age even in athletes whose training has never been better managed, and it is not something a load chart can fix.

Recovery is the other axis. Satellite cell numbers decline, the inflammatory response after damage is less well regulated, and protein synthesis is blunted in response to the same dose of training. The same session leaves a mark for longer.

What I would take from this practically is that the older athlete needs more of the quality they are losing and less of the volume they can still tolerate. The instinct usually runs the other way, because fast work is uncomfortable and easy work feels productive.

You lose fast fibres by not using them, and ageing already has a head start.

Do the sprinting. It is the part that leaves quietly and does not come back on its own.