Adenosine Triphosphate (ATP) is the means of energy generation in all cells. It has a limited supply in the muscle and the intensity and duration of the physical activity limit its supply of energy.

In the first few seconds of activity, energy is freed anaerobically from the energy bonds in phosphates stored in muscle:

ATP (+ enzyme ATPase) = ADP + P + Energy (for biological work)

After this, ATP must be regenerated by one of the body's energy systems:

The Phosphagen System (The Alactic Energy System)
Anaerobic Glycolysis (The Lactic Acid System)
The Aerobic System (The Oxygen system)

All energy systems are used to regenerate ATP, but the level of contribution to this process by each energy system is governed by the intensity of the activity. The table below summarises this process:

Energy System	Speed/Duration	Fuel	Needs Oxygen?	Sporting Example	Notes
ATP/PC (Alactic Anaerobic)	Immediate (Flat Out) (Lasts around 10-15 sec)	Chemical ATP/PC	No	Sprinting Athletics Field Events Weightlifting	Small muscular stores of ATP & PC are exhausted quickly leading to a rapid decline in immediate energy.
Anaerobic Glycolysis (Lactic Anaerobic)	Short term (High Intensity) (Lasts around 2 min)	(Glycogen) Glucose	No	400 m Sprint 800m Racket sports	Lactic acid is a by-product of this system and its accumulation causes rapid fatigue due to pH change.
Aerobic System	Long Duration (Low Intensity) (Main energy system used from 2-3 minutes onwards)	Fat/Glucose Mixture	Yes	Long distance running Long distance cycling Recovery system for most sports	This system is limited by the availability of Oxygen. Trained athletes can use a higher percentage of fat as a fuel thus saving glycogen.