The ideal rep range for muscle growth is 8-12 reps.
You might have heard mechanical tension, muscle damage, and metabolic stress are the three mechanisms for muscle growth. However, without mechanical tension – tension exerted on the muscles from movement and external loads to reduce, produce, or control force – you don’t create muscle damage or metabolic stress.
So, mechanical tension is really the driving force of muscle growth, and muscle damage and metabolic stress are just the physiological results of it. This is why lifting a lighter weight to (or near) failure produces gains in muscle size similar to that produced by lifting a heavy weight to failure
In other words, mechanical tension can be created either by lifting heavy loads for fewer reps or by lifting medium loads for more reps. So, either can create a
stimulus for muscle growth (16). Not to mention, regularly varying sets and reps is an effective means of improving muscle strength and size (17, 18, 19, 20).
From a practical perspective, the evidence on sets and reps tells us that there’s no specific rep range for maximizing muscle size, and the smartest approach is to use both heavy-load, low-volume work with lighter-load, higher-volume work in an undulating fashion.