What rest days do for muscle.

Muscle is built after the workout

Muscle protein synthesis, the process that builds new muscle protein, rises within hours of a session and stays up. After one heavy session it was more than double at 24 hours and nearly back to normal by 36 (MacDougall, 1995). In people new to training it was still about a third above rest after 48 hours (Phillips, 1997).

In trained people the rise is shorter (Tang, 2008), so there is no fixed window that closes.

How long recovery takes

In the first weeks of a new program, much of that building goes into repairing damage; only from about the third week does it line up with real growth (Damas, 2016), and early size gains are partly swelling (Damas, 2016). Sets taken all the way to failure took longer to recover from, a day or two, than sets stopped a few reps short (Morán-Navarro, 2017).

Soreness is not a measure of growth or of recovery.

How often to train a muscle

Training a muscle twice a week grew it more than once a week in an early review (Schoenfeld, 2016). With the weekly number of sets the same, later reviews found that how often you train a muscle makes little difference to growth or strength (Schoenfeld, 2019; Grgic, 2018), so it can follow your week.

With three or four workouts a week, rest days give each muscle about two sessions a week with a couple of days between them. More rest days do not mean more growth.

Protein on rest days

Protein matters on rest days as much as on training days, not more. Muscle stays more responsive to the protein you eat for about a day after a hard session (Burd, 2011), and the daily total is what counts most. In a review of 49 trials, extra protein stopped adding gains at about 1.6 g per kg of body weight a day (Morton, 2018); a sports-nutrition position stand puts most active people at 1.4 to 2.0 g (Jäger, 2017).

Spreading it over three or four meals is a sensible default (Schoenfeld and Aragon, 2018).

Rest days in Perdura

Your plan sets your training days and your rest days, with the same protein target on both. Rest › Check-in says what kind of day today is, and a walk, your steps or a weigh-in on a rest day count toward your weekly check-ins.