There are 3 primary drivers of muscular hypertrophy. To optimize muscle gains, all these factors should be smartly incorporated in your training cycles (micro / meso / macrocycle). Neglecting any of them can leave a lot of growth on the table :
1) Mechanical Tension : It may be the most important factor in training I differ muscle hypertrophy. Sensors on muscle cells (mechanosensors) are sensitive to both the magnitude and duration of loading, and these stimuli can directly mediate intracellular signaling to bring hypertrophy adaptations. The greater the load , greater the muscular force and greater the mechanical tension. Hence progressive overloading is the primary driver for hypertrophy.
2) Metabolic Stress : There is a compelling evidence that metabolic stress achieved during resistance training promotes muscle hypertrophy. Metabolic stress equates to increased buildup metabolites, particularly lactate, inorganic phosphate and H+ ions. The more the reliance on anaerobic glycolysis (15 to 120 seconds sets), the more would be the metabolic stress. Hence high rep sets and drop sets do have their own place in hypertrophy. Several researches debate that metabolic stress has even greater impact on muscle hypertrophy than high force development.
Metabolic stress may increase hypertrophic adaptations due to :->Increased fiber recruitment (training close to failure with lighter loads will lead to more recruitment of type 2 muscle fibers which have greater potential for hypertrophy)-> Elevated hormonal response (like GH, IGF-1) supporting growth-> Altered myokine production-> Cell swelling (or cell volumnization)
3) Muscle Damage : Exercise Induced Muscle Damage (EIMD) can enhance muscular adaptations, although excessive damage has a negative effect on muscle development. The severity of EIMD depends on factors like type , intensity and duration of training. EIMD is highly influenced by type of muscular action (For example , eccentric actions cause more myodamage then concentric or isometric actions.)
EIMD also reduces if a person performs the same exercise consistently (commonly known as repeated bout effect). Higher EIMD means greater inflammatory response and greater protein turnover !!!