He’d always been curious about muscles and muscle growth, but the hiatus gave him time to think—pro rugby players, he was well aware, have notoriously short careers. In 2018, his research group was the first in the world to show that human skeletal muscle possesses an epigenetic memory of muscle growth after exercise. Those changes persist; if you start lifting weights again, you’ll add muscle mass more quickly than before. (Cellular muscle memory, on the other hand, works a little differently than epigenetic muscle memory. Exercise stimulates muscle stem cells to contribute their nuclei to muscle growth and repair, and cellular muscle memory refers to when those nuclei stick around for a while in the muscle fibers—even after periods of inactivity—and help accelerate the return to growth once you start training again.)