When you begin to exercise your body must immediately adjust to the change in activity level. Energy production must increase to meet demand. As a result the predominant energy system and fuel source used changes throughout the exercise in order to maintain the required level of performance.
Responses to anaerobic exercise
- In order to immediately meet the sudden higher energy demand, stored ATP is the first energy source. This lasts for approximately 2 seconds.
- When stored ATP is broken down into ADP + P, the rising ADP level stimulates Creatine Kinase to begin the breakdown of Phosphocreatine.
- The ATP-PC system can only last 8-10 seconds before PC stores are depleted.
- The lactic acid system (Anaerobic glycolysis) must then take over as the predominant source of energy production. High intensity (but sub-maximal) exercise can last for between 3 and 5 minutes using this system
- If the exercise continues at a high intensity, and so Oxygen is not available at a fast enough rate to allow aerobic metabolism to take over, the production of lactic acid will reach the point where it interferes with muscular function. This is called the Lactate Threshold.
- Muscles begin to fatigue when ATP resynthesis can no longer match demand.
Responses to Aerobic Exercise
- Due to the necessity of Oxygen being present for aerobic metabolism, the first few minutes of low to moderate-intensity exercise are powered by anaerobic metabolism.
- Continued low to moderate-intensity exercise is then fuelled by carbohydrate and fat stores using aerobic metabolism.
- The intensity and duration of exercise determine which fuel source is used. Fat metabolism is a slow process and so can only be used as fuel for exercise at less than 60% VO2 max.
- Carbohydrate is a much faster fuel source and so can be used for exercise up to 80% (in trained individuals).
- Carbohydrate stores within the muscle and liver can fuel exercise for up to 80 minutes. As carbohydrate stores get lower, the body has to rely more and more on fat stores.
- The intensity of exercise which can be maintained drops as fat cannot supply the required amount of energy.