‘Fibre activation’ is the biggest variable, according to Burkhard Bilger:
A muscle is like a slave galley, with countless rowers pulling separately toward the same goal. Synchronizing that effort requires years of training and the right “neural hookup,” Ivy said. Those who master it can lift far above their weight. Max Sick, a great early-nineteenth-century German strongman, had such complete muscle control that he could make the various groups twitch in time to music. He was only five feet four and a hundred and forty-five pounds, yet he could take a man forty pounds heavier, press him in the air sixteen times with one hand, and hold a mug of beer in the other without spilling it.