Why do muscles grow?
Muscle growth has been one of the most studied things in sports science.
In its very essence, muscle growth occurs, when the muscles go through something they have not experienced before.
In fact, muscle growth is simply an adaptation, with which the body prepares for more.
Depending on the type of stimulus, the muscle growth may result in greater maximum strength, or, greater strength endurance.
As a trainee, you must acknowledge that in order to keep sufficiently stimulating your musculature, you must expose it to new constructive stress.
This can mean:
1. Increasing the working weight
2. Increasing the working volume
3. Decreasing the rest times between sets
4. Increasing training frequency
All of the above is a part of what we refer to as "progressive overload", which again, implies that you should gradually increase the demand upon the working musculature.