Photo-Illustration by TIME (Source Image: Victor Freitas—Pexels via Canva)Strength training is having a moment—and, whether on #GymTok or the real gym, the emphasis is largely on lifting heavy. “That's one key attribute for not just bigger muscles, but stronger muscles and stronger bones,” says Abbie Smith-Ryan, a professor of exercise physiology and nutrition at the University of North Carolina at Chapel Hill.Yet “heavy” is a relative term. The…