Exercise Intensity & Energy Source

Energy is primarily supplied from two sources:

  • Carbohydrates - in the form of glycogen stored in the muscles
  • Fat - stored around the body

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 predominate 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 low intensity work can be maintained for long periods.

Intensity and Energy Source

The relationship between exercise intensity (% of your Maximum Heart Rate) and the energy source (carbohydrate and fat) is as follows:

Intensity % MHR % Carbohydrate % Fat
65 to 70 40 60
70 to 75 50 50
75 to 80 65 35
80 to 85 80 20
85 to 90 90 10
90 to 95 95 5
100 100 0

Respiratory Exchange Ratio (RER)

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 (CO2) produced to oxygen (O2) consumed and is known as the Respiratory Quotient (RQ).

If carbohydrate is completely oxidised to CO2 and water (H2O) then the relationships is as follows:

  • 6O2 + C6H12O6 » 6CO2 + 6H2O + 38ATP (Adenosine Triphosphate)
  • RER = 6CO2 ÷ 6O2 = 1

If fat (e.g. palmitic acid) is completely oxidised to CO2 and H2O then the relationships is as follows:

  • C16h32 + 23O2 » 16CO2 + 16H2O + 129ATP
  • RER = 16CO2 ÷ 23O2 = 0.7

The RER for protein is approx. 0.8 but as it plays a very small part in energy metabolism, it is not important here. 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 CO2 being produced than O2 consumed.

Associated Pages

The following Sports Coach pages should be read in conjunction with this page:

Associated Books

The following books provide more information related to this topic:

  • Principles of Anatomy and Physiology, G.J. Tortora et al., ISBN 0 06 046704 5
  • Strength Training Anatomy, F. Delavier, ISBN 0 7360 4185 0
  • Atlas of Skeletal Muscles, R.J. Stone et al., ISBN 0 697 13790 2
  • The Muscle Book, P. Blakey, ISBN 1 873017 00 6
  • Advanced Studies in Physical Education and Sport, P Beashel et al., ISBN 0 17 4482345
  • Physical Education and the Study of Sport, B. Davis et al., ISBN 0 7234 31752
  • Essentials of Exercise Physiology, W.D. McArdle et al., ISBN 0 683 30507 7
  • Physical Education and Sport Studies, D. Roscoe et al., ISBN 1 901424 20 0
  • The World of Sport Examined, P. Beashel et al., ISBN 0 17 438719 9
  • Advanced PE for Edexcel, F. Galligan et al., ISBN 0 435 50643 9
  • Examining Physical Education, K. Bizley, ISBN 0 435 50660 9
  • Sport and PE, K Wesson et al., ISBN 0 340 683821
  • PE for you, J. Honeybourne, ISBN 0 7487 3277 2