Ask Men’s Fitness: Is Running Bad for Bulking Up?

Not exactly, says powerlifter Mike Israetel, professor of exercise science at the University of Central Missouri. It works like this: Muscles grow best when you give them lots of sets of heavy weights, then let them recover between workouts. “Anything that interferes with the ability to lift hard by fatiguing the muscles, or with the ability to recover, is going to reduce muscle growth somewhat,” says Israetel, who’s also head science consultant for Renaissance Periodization and author of The Renaissance Diet.
So if you’re lifting for your legs, yeah, running on “legs day” might not be a good idea. But a few laps won’t undo all your work.