The short version
Muscles grow when they are challenged with enough tension, you recover from it, and you eat enough protein and energy to build new tissue. Training is the signal; food and sleep are the building materials.
Tension is the main driver
Heavy-enough weights taken close to failure create high mechanical tension on muscle fibres. That tension is the main trigger for growth. The 'pump' and soreness are side effects, not goals.
Effort matters more than the exact rep number
Sets anywhere from about 5 to 30 reps can build similar muscle if they are taken close to failure — roughly 0–3 reps left in the tank. Moderate ranges (6–15) are usually the most time-efficient and joint-friendly.
Recovery and food
- Protein: about 0.7–1 g per lb of bodyweight (1.6–2.2 g/kg) per day
- Calories: a small surplus speeds growth, but beginners can grow at maintenance
- Sleep: 7–9 hours; poor sleep blunts recovery and appetite control
How fast is realistic?
A beginner might gain 1–2 lb of muscle per month in the first year, and much less each year after. Scale weight goes up faster than that during a bulk because of water, glycogen and some fat.