After just a couple of days on bed rest, the material of your body begins to feel different: softer, heavier, a burden to the bone beneath. The thud of the heart in the chest feels deeper: each beat shifts your frame a little. Even though you haven’t used your back for anything, it aches — and when you twist into a new position the ache swivels along with the muscles, can’t be left behind. You fall asleep throughout the day but can’t sleep through the night, and when you bend a limb at the joint, it’s not the transparent sensation…