The black, rubbery mat bubbles unpredictably beneath my back as I hold one sixty-pound dumbbell in each hand, palms facing each other above my chest—knees bent, feet firm—and move my arms out, then lower them, slow and controlled. I feel my chest stretch. I fight gravity. I raise the dumbbells up toward each other again, consciously contracting my chest throughout the movement, and squeezing as hard as I can at the top; I imagine my muscle fibers microscopically tearing; I imagine cells shooting through my body toward my chest like electrical currents, or little guppies rushing in a river;…