• Alactacid system (ATP/PC)
• Lactic acid system
• Aerobic system

Alactacid System (ATP/PC)

• Uses 90 grams of ATP for the initial 1-2 seconds of the cycle. The following 10-15 seconds use Creatine Phosphate (CP) as the energy source.
• Limited amount of ATP is available regardless of the presence of oxygen. Can generate energy for a brief time.
• The cycle runs roughly for 8 – 12 seconds.
• Exhaustion of ATP and CP can cause fatigue.
• Heat is the only by-product of this system, which results from muscle contraction.
• Recovery takes around 30 seconds to 2 minutes. It requires resynthesis of CP.

Lactic Acid System

• Uses carbohydrates in the form of glucose and glycogen as fuel sources.
• ATP is readily available, but at the cost of high levels of glucose catabolism.
• The system runs for 30 seconds at maximum effort and for 3-4 minutes at 70-80% efficiency.
• Building up of lactic acid in muscle cells causes fatigue.
• System by-product includes pyruvic acid, which is converted to lactic acid.
• Recovery time takes 30 minutes to 1 hour as lactic acid is converted into glycogen.

Aerobic System

• Uses carbohydrates, fats and proteins as fuel.
• More efficient in terms of ATP production.
• System runs from 3 minutes to 24 hours.
• Depletion of carbohydrates and switching to fat as fuel exhausts the system as fats require more oxygen to metabolize, which leads to an increase in body temperature and respiration rate.
• By-products include carbon dioxide and water.
• Recovery window is short if the system runs for a brief period. If not, recovery could take days (because glycogen replenishment requires a prolonged time period).