How strong is the evidence for Rest Days?

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 evidence is currently rated moderate (58/100), built on 6 published studies, including 5 meta-analyses -- a reasonable base, but not yet enough for full certainty.