There is a window after heavy work when the tendon is weaker than before

Loading breaks collagen down before it builds any, so the day after a hard session the tissue is temporarily below where it started.

The tissue does not improve during the session. It improves afterwards, and to get there it goes down first, which is the part nobody schedules around.

Heavy tendon loading disrupts existing collagen. Degradation rises immediately and stays high for a period, while new synthesis takes longer to switch on and longer still to produce anything mechanically useful. For a stretch of roughly a day and a half after a genuinely demanding session, the balance sits on the wrong side of the line. Net collagen content dips. Only afterwards does synthesis overtake degradation and the tissue end up ahead of where it began, and that overshoot is the entire adaptation.

Put two hard tendon sessions inside that window and you have loaded a structure during its weakest phase. Do it repeatedly and you get a tendon that never spends time above its starting point, which is a decent structural description of how tendinopathy develops in people who are training hard and doing nothing obviously wrong. They are not overloading in any single session. They are re-entering the dip.

This is the mechanism behind advice that otherwise sounds like superstition. Why heavy jumping is better spaced two or three days apart than done daily. Why in-season athletes who play twice a week frequently cannot also do the plyometric work that built them in the summer. Why a tendon problem often begins in the week the schedule got congested rather than the week the load was highest.

Muscle does not behave this way to nearly the same extent, and that difference is the trap. Muscle recovers faster, feels ready sooner, and gives the athlete a vote in the matter. The tendon has no vote and only speaks the following morning.

I would go further and say the training week should be built around the connective tissue's rhythm and the muscle should be fitted in around it, which is close to the opposite of how most programmes are written.

There is also a nutritional angle here that I am unwilling to oversell, since the human evidence for feeding collagen synthesis is thin and mostly hopeful.

The reliable intervention is boring. It is the gap between the sessions, and you cannot buy it in a tub.