Body & HealthHow it works
Muscles don't grow during training, they grow afterwards.
When you lift heavy, mechanical tension and small tears occur in the muscle fibres. This activates signalling pathways such as mTOR, which ramp up protein synthesis in the cell for 24 to 48 hours. The fibres get thicker, not more numerous. Training is only the stimulus; the building happens during recovery, with dietary protein as the raw material.
Useful: Two hard sessions for the same muscle on one day achieve less than one session with rest in between.
Source: en.wikipedia.org