Bone answers to how fast you load it, not how much you load it

Skeletal adaptation responds to strain rate and novelty, which is why long steady mileage builds so little of it.

A distance runner covering a hundred kilometres a week is loading their tibia several hundred thousand times. A high jumper does perhaps forty hard takeoffs in a session. Bone density at the hip and shin usually favours the jumper, and by a margin that volume alone cannot explain.

Bone reads the strain signal rather than the repetition count. The cells sitting inside the mineral matrix respond to how quickly the tissue deforms and to strain patterns they have not seen recently, and they habituate fast. After a few dozen cycles of an identical load the signal for building has largely gone flat. Everything after that is accumulated fatigue in the material with very little remodelling reward attached, which is a poor trade.

Two consequences follow, and both are inconvenient for how endurance training is usually written. First, adding mileage is close to the least efficient way to make a skeleton stronger. Second, the loading that does build bone looks alarmingly like the loading people avoid in the name of safety. Hops. Direction changes. Landings. Heavy, fast lifting. Short, sharp, varied and stopped long before it becomes a workout.

The failure mechanism follows the same logic. Bone stress injuries are not usually a single overload; they are the material accumulating microdamage faster than remodelling clears it. That process has a nasty middle phase, because the first step in repair is resorption of the damaged section, so for a period the site is measurably weaker than it was when the problem started. Keep running through that window and a stress reaction can become a fracture during a session that felt no harder than the previous twenty.

I would rather see a young endurance athlete lift and jump twice a week than add another easy hour, and I accept that this costs them a little aerobic development. It buys them a tissue that improves on a timescale of months and cannot be rushed later.

The other thing bone wants is unpredictability, which is why sports with side-to-side loading protect the skeleton better than sports that only push front to back. A tibia loaded from one direction for years becomes strong in one direction.

Then somebody changes surface, or shoe, or terrain, and the load arrives on an axis the bone never trained for.