A tendon loses its stiffness faster than it built it

Weeks of layoff can undo months of tendon adaptation, and the athlete returns with a spring that no longer stores what it used to.

Three weeks off with a hamstring strain and the Achilles was never the problem. Then the athlete comes back, does two sessions of jumping, and the tendon that has been quiet for four years starts complaining. That sequence is common enough that I have stopped treating it as coincidence.

Tendon is not a passive rope. It adapts, and the property that adapts most usefully is stiffness, meaning how much force it takes to stretch it by a given amount. A stiff tendon stores and returns elastic energy efficiently and transmits muscular force to bone with less delay. Building that takes sustained, fairly heavy, fairly slow loading over months, because the change involves collagen organisation and cross-linking rather than a quick swelling of tissue.

Unloading dismantles it much faster than loading built it. Reduce the mechanical signal and collagen turnover shifts, the matrix softens, and within a few weeks the tendon deforms more for the same force. The muscle attached to it, meanwhile, loses strength more slowly than the tendon loses stiffness. That mismatch is the interesting part. You return with a motor that is close to what it was and a transmission that has gone slack, so the same effort now stretches the tendon further and loads it in a range it has not seen recently.

Add the second-order effect. A more compliant tendon changes the muscle's operating length during the stretch-shortening cycle, so the muscle fibres shorten more and at higher velocity to do the same job, and they produce less force while doing it. The athlete feels flat rather than injured. Jumps are lower. Ground contact is longer. They read this as being unfit and train harder, into a structure that is currently the weakest link in the chain.

This is why I think return from any layoff should be paced by the slowest tissue rather than by how the running feels, and running is exactly the thing that feels fine first.

The classic presentation follows a week or two later. Pain at the start of a session that warms up and vanishes, then returns worse the next morning. That warm-up effect fools people constantly.

It is not the tendon recovering. It is the tissue temporarily behaving well while you keep adding to a bill that arrives at breakfast.