Is there good scientific evidence on the effectiveness of rest days for building mass?

Question

I'm wondering if there is any study that compares two groups doing identical exercises, except that one group has fewer rest days than the other.

I have seen a lot of conflicting evidence on the effectiveness of rest days, with some studies arguing that your body needs rest to grow and to decrease the risk of injury, and other studies showing that virtually everyone can substantially increase workout volume and achieve positive mass returns from it.

Essentially, I want to know the best ratio of exercise to rest for mass. Has anyone seen good academic research on this subject?

Answer 1

I find the ACSM Position Stand is a very credible source of precisely what you want to know.

Evidence category A is backed by significant Random Control Trials (RCT) with significant participants

Evidence category B is based on limited RCT studies, or a subgroup analysis, or is a meta-analysis.

Evidence category C is Non-randomized trials or observational studies.

Source: Progression Models in Resistance Training for Healthy Adults
PROGRAM DESIGN RECOMMENDATIONS FOR INCREASING MUSCLE HYPERTROPHY

Evidence category A. It is recommended that a frequency of 2–3 days per week be used for novice training (when training the total body each workout) (34,49,116).

Evidence category B. For intermediate training, the recommendation is similar for total-body workouts or 4 days per week when using an upper/lower body split routine (each major muscle group trained twice per week).

Evidence category C. For advanced training, a frequency of 4–6 days per week is recommended. Muscle group split routines (one to three muscle groups trained per workout) are common enabling higher volume per muscle group.

citations

34   Candow DG, Burke DG. Effect of short-term equal-volume resistance training with different workout frequency on muscle mass and strength in untrained men and women.J Strength Cond Res. 2007;21:204–7.
49   Cureton KJ, Collins MA, Hill DW, McElhannon FM. Muscle hypertrophy in men and women.Med Sci Sports Exerc. 1988; 20:338–44.
116  Hickson RC, Hidaka K, Foster C. Skeletal muscle fiber type, resistance training, and strength-related performance. Med Sci Sports Exerc. 1994;26:593–8.

Answer 2

There is a paper cited in the top answer for Importance of Rest Days but it looks like it's proceeding from fundamentals rather than looking at the result of an experiment.