Progressive overload
Muscle adapts to demands that go beyond what it can already handle. So continued growth needs the demand to keep rising over time — more weight, more repetitions, more sets, better leverage, or shorter rest between them. A training stimulus that never changes stops producing change.
Pooled across studies, the number of hard sets per week tracks with muscle growth — more generally means more, up to a point. The relationship flattens, and eventually the recovery cost outruns the benefit.
Protein intake
Building new muscle protein requires amino acids, and the signal that starts the process is especially sensitive to one of them, leucine. Eating protein and loading the muscle act on the same pathway, which is why doing both beats doing either one alone.
The pooled training literature reports the benefit of higher protein intake levelling off around 1.6 g/kg/day. Studies on single meals found that muscle protein synthesis after resistance exercise rose with the size of the protein dose and then plateaued — which is the basis for spreading protein across the day rather than piling it into one meal.
Energy availability
Building tissue costs energy, and an energy surplus makes it easier. A surplus is not strictly required — how trained someone is, how long they have been training, and their starting body composition all affect whether growth is possible at maintenance or even in a deficit — but the evidence that a modest surplus helps the process is consistent.
A large surplus does not speed things up in proportion. Beyond what the body can put to use, the extra is stored as fat — which is why "aggressive gain" is a trade-off between speed and how much fat comes with it, not a shortcut.
Recovery and training frequency
Muscle protein synthesis stays elevated for a while after training, then returns to baseline. Spreading the weekly workload across more sessions keeps that signal raised more often, which is the reason, in principle, to train a muscle more than once a week — and the pooled research on frequency broadly agrees, once total workload is held equal.
Sleep
Sleep is when a large share of recovery happens, and cutting it measurably changes body-composition results under otherwise identical conditions. In a controlled deficit, short sleep shifted the loss away from fat and toward lean tissue.
The same study is cited under fat loss. It belongs in both places, because it is one of the few interventions that changes the make-up of a result without changing the diet at all.
Protein quality and source
Protein sources differ in their amino acid make-up and in how easily they are digested. Dairy protein in particular has been examined — in a systematic review — for its effect on muscle mass, strength and physical performance in middle-aged and older adults. For most purposes, the total protein you eat in a day and how you spread it matter more than the source, as long as the sources are reasonably complete.
