For the legs, it is simple: they push as long as they can. Then they stop.
Da Vinci: The Muscles of the Leg
Sooner or later, one day, their apparent spinning will cease, and they will plant and then lean over until they are horizontal. They will plod, pendulum-like; fold and arc; push follicles long; grow dry and cracked and unheeded; fat and flesh will pave over the noodles of blood pulsing visibly and the skin will once again sag and smooth; the temperature sinks, the limbs stiffen and the intestines drain. These changes in the first few months occur so slowly and take place with such inexorability that there is something almost ritualistic about them, as though fitness capitulates according to specific rules, a kind of gentleman's agreement to which the representatives of frailty also adhere, inasmuch as they always wait until fitness has retreated before they launch their invasion of the new landscape. By which point, however, the invasion is irrevocable. The enormous hordes of fat cells that begin to infiltrate the body's innards cannot be halted. Had they but tried a few hours earlier, they would have met with immediate incineration in the machine of motion; however, everything around them is quiet now, as they delve deeper and deeper into the moist darkness. They advance on the Old Rag, the Chesapeake Bay, Rock Creek, swim out to Roosevelt Island. They proceed to Tyson's Corner, Sugarloaf, and up Old Angler's, the black substance in the Potomac. And they arrive at the legs. As yet, they are intact, but deprived of the activity to which end their whole construction had been designed and honed, there is something strangely desolate about them, like a production plant that workers have been forced to flee in haste, or so it appears, the stationary vehicles shining yellow against the darkness of the forest, the huts deserted, a line of fully loaded cable-buckets stretching up the hillside.
The moment fitness departs the body, it belongs to frailty.