Repair pays off only when the skeleton is used -- we can invest in repair because we know the animal is active/foraging and we will get paid back on the investment.
Low loading is a sign that we should cut back on maintenance -- the animal is sick, injured, starving etc and it's better to conserve energy.
The optimal amount of exercise is what tells your body to invest in repair, without acquiring repair debt. Sedentary life is the misleading signal, because it tells the body you're declining when you aren't.