Contrary to what was thought many years ago, we now know that longer rests between sets are more conducive to muscle hypertrophy (Schoenfled et al 2016; Fink et al 2017). Decades ago (and even still to this day) short rests were thought to be more conducive to muscle hypertrophy for two reasons:
They increased metabolic stress more.
They increased the expression of hormones related to muscle hypertrophy such as testosterone, IGF-1 or GH (although the latter is not directly linked to muscle hypertrophy).
However, correlation does not imply causation and for example in the study of Fink et al 2017 or that of Ahtiainen et al 2005, it was seen that longer rests between sets were more optimal for muscle hypertrophy independent of the hormonal environment generated. This was yet another example that physiology is redundant, global and complex.
What about rest between training days?
Although we know less regarding this and know that this depends on many factors, some studies conclude that 2-3 days of rest would be optimal for muscle hypertrophy (Coffey et al 2007). Other studies in rats conclude the same (Takegaki et al 2017; Takegaki et al 2019).
In my opinion, the need for so many days of recovery between exercise sessions is very relative, being that the training programming, intensity, volume, muscle failure and above all the profile of the subject will determine the recovery capacity. To give an example, if the studies are done on beginner subjects, recovery will be greater due to the high degree of muscle damage suffered by beginners in strength training. The same is true for age, diet of the subjects, sleep, stress, etc.
Regardless of this, a recent study in rats by the same author shows that shorter recovery periods between workouts of 24 hours or 8 hours (double training session) increased mTOR more than recovery periods of 72 hours.
Independently of this, a recent study in rats by the same author (image 1), shows a curious fact that shorter recovery periods between workouts of 24 hours or 8 hours (double training session) increased mTOR more than recovery periods of 72 hours. However, muscle protein synthesis was higher resting 72 hours between sessions than resting 24 or 8 hours, even though mTOR was more active. This was because shorter rests inhibited to a greater extent the initiation factor eIF4F, which need not be dependent on mTOR.
This is one more example of many that I always give, that the molecular is always complex and does not always correlate with the expected physiological outcome that is hypothesized.