General explanation: Muscle growth (hypertrophy) is essentially the body building more muscle protein over time than it breaks down. When muscles are loaded with heavy weights, physical stress is created. This stress acts as a signal to the body’s cells that they must grow and become stronger to cope with similar stresses in the future. This process requires an interplay between the mechanical force from the lift itself, the activation of the muscle’s own stem cells to repair damage, and the chemical environment (lactic acid and metabolic byproducts) that forms in the muscle when it works hard.