What scientists know about muscle growth can be summarized in two words: not much. It’s long been theorized that delayed-onset muscle soreness—the aching you sometimes feel a day or two after lifting—is an indication of muscle damage done by hard training. Muscle damage is thought to be necessary, to some degree, for muscle growth. But people are most often sore when they change up their workouts or they’re brand-new to lifting, and some find they rarely get sore at all.
So does soreness matter? If you’re never sore, you’re probably not training with enough variety to cause muscle breakdown. But, according to a 2013 paper on the subject published by the National Strength and Conditioning Association, you shouldn’t chase soreness, either: “High levels of soreness...is a sign that the lifter has exceeded the capacity for the muscle to repair itself...The applicability of DOMS in assessing workout quality is limited.”
A better way to judge your workout? The results.