What Causes Your Muscles To Grow?

Muscle growth is triggered by three primary mechanisms:

Mechanical stress - created by applying heavy loads / high forces to your muscles over a full range of motion.

Metabolic stress - caused by the accumulation of lactate and other metabolites in your muscles which cause a burning sensation when you lift moderately heavy loads, lots of times.

Muscle damage - New exercises, eccentric exercises and movements which place load on your muscles in stretched positions are all likely to cause muscle damage which can contribute to muscle growth. Tread carefully with this one!

For optimum muscle building results aim to overload your muscles with a combination of mechanical stress and metabolic stress. This can be  achieved by getting progressively stronger in a variety of rep ranges and performing a proportion of your sets to muscular failure.

Compound exercises like squats, lunges, rows and presses should form the bulk of your training supplemented with some isolation exercises to target specific muscles from different angles. You will also need to eat sufficient volumes of food to fuel the growth.

For a much more detailed look into these mechanisms check out this excellent piece by Brad Schoenfeld.