Muscle growth, or hypertrophy, is the result of a cascade of physiological events triggered by mechanical stress on muscle tissue. When you lift weights, you create micro-tears in muscle fibres. Your body responds to this damage by fusing muscle fibres together and adding new protein strands, making each fibre thicker and stronger. This process - called muscle protein synthesis - is what drives long-term size gains.
Three primary mechanisms drive hypertrophy: mechanical tension (the force generated by lifting heavy weights), metabolic stress (the burn, pump, and accumulation of metabolic byproducts from higher rep work), and muscle damage (the soreness-inducing micro-tears from intense training). All three contribute to growth, which is why neither exclusively heavy lifting nor exclusively light pump work is optimal - a combination yields the best results.
Muscle protein synthesis is elevated for 24-48 hours after training, which is why training frequency matters. Hitting each muscle group twice per week keeps protein synthesis elevated more consistently than once-weekly training. Sleep is when growth actually occurs - growth hormone peaks during deep sleep, and it's during this window that damaged muscle tissue is repaired and strengthened. Protein intake provides the amino acid building blocks for new muscle tissue. Without adequate protein (1.6-2.2g per kilogram of body weight), even perfect training produces minimal growth. Understand these mechanisms and you can evaluate any training method rationally rather than relying on gym myths.