Is Rest Days good for muscle & strength?

The evidence points that way, though it's not yet conclusive. There is no direct meta-analytic literature under the label "rest days," but the closely related question of optimal resistance-training frequency has been studied extensively: multiple meta-analyses find that training a muscle group roughly twice per week produces greater hypertrophy than once weekly, provided total weekly volume is matched, implying that adequate spacing (i. That's reflected in an evidence score of 58/100 for its role in muscle & strength, drawn from 6 studies, with an estimated impact on longevity rated moderate (35/100).