Exercise Intensity & Energy Source
Energy is primarily supplied from two sources:
During exercise, we use a combination of these energy sources. At a high-intensity the main source of energy is carbohydrate and at a low-intensity fat is the predominant source. As there is a limit to the amount of carbohydrate that can be stored in the muscles, high-intensity work can only be sustained for short periods. We have large stores of fat so lowintensity work can be maintained for long periods.
Intensity and Energy Source
The following table, adapted from O'Neil (2001), shows the relationship between exercise intensity (% of your Maximum Heart Rate) and the energy source (carbohydrate and fat).
Carbohydrates, fat and protein all play a part in energy metabolism and for a certain volume of oxygen the energy released will depend upon the energy source. It is possible to estimate which particular fuel (carbohydrate, fat or protein) is being oxidised by calculating the Respiratory Exchange Ratio (RER). RER is the ratio of carbon dioxide (CO 2 ) produced to oxygen (O 2 ) consumed and is known as the Respiratory Quotient (RQ).
The RER for protein is approx. 0.8 but as it plays a very small part in energy metabolism. A value between 0.7 and 1.0 indicates a mixture of fat and carbohydrate as the energy source. A value greater than 1.0 indicates anaerobic respiration due to more CO 2 being produced than O 2 consumed.
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