How Your Energy Systems Overlap : Aerobic & Anaerobic Training


Fuelling exercise to enable you to perform optimally is no game, in fact, it’s a multi-million pound industry that drives scientists and sports teams from around the world! Researchers invest millions of pounds into finding the best ways to fuel training and sport, with aerobic and anaerobic energy systems at the root of most of the research. You see, when we exercise our bodies recruit different energy substrates to fuel us i.e. carbohydrate, fat or amino acids. The substrate used depends on your energy stores and the type of physical activity you are performing, but it isn’t always black and white, and there are times when energy systems overlap.

Aerobic and Anaerobic energy pathways

Aerobic literally means ‘in the presence of oxygen’, whereas anaerobic means ‘without oxygen’, so in relation to exercise this means that the type of exercise being performed is either with or without the free flow of oxygen. As a rule of thumb the aerobic system requires 60-80 seconds to produce energy to resynthesize ATP from ADP + P (the main energy source to the human body). In order for this to happen, the heart rate must increase enough to transport the required amount of oxygen to the muscle in order to break down glycogen in the presence of oxygen. The key thing to note with the aerobic system is that the glycogen breakdown occurs with oxygen, consequently there is no lactic acid production enabling the athlete to train for longer.

The anaerobic system can only provide energy for a period of 8-10 seconds meaning it is the main energy pathway for activity of a short and explosive nature. Since the anaerobic system is without oxygen, the duration of energy release is low and the build- up of lactic acid is high.

Overlap of energy systems

Different types of exercise or sporting activities vary according to their duration and intensity of activity, consequently there is rarely a time when just one energy system is used. As a result the body depletes certain energy sources during exercise based on the type of exercise performed, and therefore in most occasions the body will overlap between aerobic and anaerobic energy pathways. So except for very short duration activities such as a 100m sprint, the body will use both energy pathways. It is possible for a coach to establish what energy pathways an athlete is using based on the level of lactic acid in the blood. There is a threshold of 4 millimoles of lactic acid which indicates that the anaerobic and aerobic systems both contributed to the resynthesis of ATP in equal amounts. Should there be higher levels of lactic acid in the blood, then this indicates that the anaerobic energy pathway predominated. Conversely, lower levels of lactic acid means that the aerobic pathway predominated. Similarly, higher heart rates during activity also indicate that the anaerobic pathway was the main energy source, whereas lower heart rates indicate aerobic energy.


So what is the relevance of knowing what energy systems you are using? Well, the fact that matters is that your body will cleverly adopt the most appropriate energy pathway to suit the activity it is performing. Where this comes in handy is when an athlete needs to prepare for the impending performance/ training session. Diet and supplement planning depends heavily on the type of activity performed e.g. an athlete preparing for a marathon will profoundly benefit from a pre- event carbohydrate load compared to someone readying for a 100m sprint final. In the middle ground is sports such as football, rugby, tennis and basically everything else…these sports have a major overlap of energy pathways, so consequently the loading of carbs the night and morning before is integral, whilst adequate sources of high glycaemic index carbs are also needed for during the session/ event. Knowing the parameters that each energy pathway works in also enables athletes to train specific energy pathways which best suit their sport/ event.


Bompa, T, O. (1999). Periodisation. Theory and Methodology of Training. 4th Ed. Human Kinetics.

About the Author

Job Role Qualified Dietitian and Sports Nutritionist Qualifications BSc (Hons) Sports Science | BSc (Hons) Dietetics Tom has always participated in sport both recreationally and competitively which led to an unquenchable thirst for information on anything health, nutrition and fitness. After leaving school Tom went on to play for a football academy during which time he studied Sport and Exercise Science. From here he went on to study a BSc (Hons) Sport Science at UEA followed by his second BSc (Hons) degree, this time at the University of Hertfordshire studying Dietetics. Tom has worked in the fitness, educational and clinical nutrition industry starting out at David Lloyd Health and Leisure Clubs. He then went on to work as a Dietitian (RD) in the NHS, during which time he conducted clinics for healthy eating, weight loss and weight gain, as well as specialised consultations on Diabetes, IBS and Coeliac disease to name a few. He has vast amounts of experience at devising diet plans and supplement regimens, as well as working in the community with schools and competitive athletes. As Head Nutritionist and Supplement expert at Discount Supplements Tom is here to provide current and evidence based health and nutrition information to help you reach your health and fitness goals!
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