First law: Muscles grow only if you apply a load of stress greater than your muscles had previously adapted to.
During workout, you are actually causing tiny tears (known as micro-tears) in the muscle fibres by lifting weights. After you workout, your body repairs or replaces damaged muscle fibres through a cellular process where it fuses muscle fibres together to form new muscle protein strands. This is the process by which muscles grow (scientifically termed hypertrophy).
If you lift light weights (and do more repetitions), your workout causes “only few” micro tears in the fibres. If a workout causes “only few” micro-tears in the fibres, then little muscle growth will occur as a result because the body figures it doesn’t need to grow to deal again with such a minor stimulus.
How to apply a proper load of stress on muscles?
The main way is to lift progressively heavier weights. This additional tension on the muscle helps to cause changes in the chemistry of the muscle which helps grow muscles.
Second law: Pump and fatigue actually don’t help muscles grow.
Grow from overload. Muscle pump is worthless in terms of muscle growth. The pump you feel when training is a result of blood being “trapped” in the muscles. It is a good psychological boost and isn’t a bad thing but it is just not an indicator of muscle growth. High-repetition workouts fail to sufficiently overload muscles to trigger growth, even though they deliver quite a pump.
Many guys think that a burning sensation in their muscles is indicative of an intense, “growth-inducing” workout. The “burn” you feel is simply an infusion of lactic acid in the muscle, which is produced as a muscle burns its energy stores.
What triggers muscle growth, then?
Overload. You can make incredible muscle gains by doing heavy weight sets, mass-building (compound) exercises, steadily increasing weight and reps (overload).
My verdict:
High-rep drop sets, giant sets, super sets, etc. are for the magazine-reading crowd. Such training techniques simply do not stimulate growth like simple, heavy sets do.