Muscle hypertrophy is primarily driven by the product of mechanical tension and time under tension.

Study: Equalization of Training Protocols by Time Under Tension Determines the Magnitude of Changes in Strength and Muscular Hypertrophy

This study found that whether you lift slowly or quickly, as long as your muscles are under tension for the same total time, you grow the same amount of muscle. So it’s not just how heavy you lift, but how long you keep the muscle working that matters.

Study: Optimizing Strength and Hypertrophy: The Combined Effect of Intensity and Velocity Loss Thresholds in Bench Press Training.

This study found that lifting heavier weights for longer (with more reps before stopping) made muscles grow bigger, which matches the idea that muscle growth happens when you push hard and keep going for a while.

Study: Physiological adaptations and practical efficacy of different blood flow restriction resistance training modes in athletic populations

This study shows that lifting weights slowly with restricted blood flow makes muscles grow better, because keeping the muscles under tension longer helps — which matches the idea that muscle growth needs both heavy lifting and holding it for a while.

Study: Ribosome Biogenesis as a Putative Bottleneck to Skeletal Muscle Hypertrophy: Mechanisms, Human Evidence, and Practical Modulators

Muscles grow when you lift weights over a long time, not just from one workout. This study shows that your muscles need to build more protein-making machines, and that only happens if you keep lifting consistently.