Mechanism of Muscle Growth
During exercise muscles are put under stress which eventually causes micro tears in the muscle fibres.

These micro tears put pressure on the nerve endings which causes the pain. 
The body will then repair the tears during rest and recovery using proteins and sufficient fuel. The muscle will then become slightly bigger and stronger.

Energy systems used to repair these lesions are the following

There are three different types of energy systems:


Phosphocreatine
Lactic Acid
-Aerobic

1.Phosphocreatine Energy System


The ATP-PC system lasts for 8-10 seconds.
During the ATP-PC system a phosphate breaks off of the ATP to form ADP and a lot of energy.
ADP will then bind with another free phosphate to form ATP.
The ATP-PC system is an anaerobic energy system as it dose not use oxygen. 


Lactic Acid Energy System


Glucose is broken down sustain performance for a longer amount of time than the ATP-PC system.However this system does not use oxygen so is not suited for events over 90 seconds.
When the ATP-PC system starts to fade the process of anaerobic glycolysis begins.
Glycogen is stored in the liver and is broken down without the presence of oxygen.
This produces lactic acid which limits energy production.


Aerobic Energy system


The aerobic energy system is constantly being used in your body to supply the muscles with energy.
In sport the aerobic energy system is used for events that last anything over 90 seconds.
The aerobic system can be used constantly but only if there is a constant supply of glycogen and fatty acids.
The aerobic system uses oxygen, glucose and fatty acids to yield large amounts of ATP.
The system takes place within the mitochondria and produces two bi-products, carbon dioxide and water.