In young adult men, lifting heavy weights with few repetitions and lifting lighter weights with many repetitions both lead to similar increases in muscle size when done consistently over four weeks.
Mechanism
Synthesis from 1 study
Whether you lift heavy weights or do lots of reps with light weights, your muscles get stressed in two ways: they’re pulled hard, and chemicals build up inside them. Both of these signals tell your muscles to make more protein, which makes them bigger over time. That’s why both methods can build...
Most probable mechanism
When muscles are worked hard, whether with heavy weights or many repetitions, the stress causes tiny tears in muscle fibers and builds up waste chemicals inside the muscle. These two signals tell the muscle to make more proteins, which repair and thicken the fibers, making the muscle bigger over time.
Mechanical tension from muscle contraction stretches and stresses muscle fibers, causing microtears in the contractile structures and surrounding membranes
Metabolic byproducts such as lactate, hydrogen ions, and inorganic phosphate accumulate during prolonged contractions, creating a biochemical signal within the muscle cell
Both mechanical stress and metabolic byproducts activate the Akt/mTOR signaling pathway, which increases the cell's capacity to produce new proteins
Increased protein synthesis leads to the addition of new myofibrils and sarcomeres, resulting in thicker muscle fibers and increased muscle mass
Less supported by current evidence, but not ruled out
Lifting very heavy weights forces the body to recruit the strongest muscle fibers, which have the greatest potential to grow. These fibers respond by adding more contractile material, making the muscle larger.
High mechanical loads activate high-threshold motor units that control fast-twitch muscle fibers
Fast-twitch fibers experience greater force production and structural strain during contraction
Repeated activation of these fibers triggers adaptive growth responses, increasing their size and contributing to overall muscle mass
Evidence from Studies
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