Introduction to energy systems
When we exercise, depending on intensity and duration, fed state and other variables, a mix of energy systems is used, but often one dominates, depending on the nature of the activity. Food is fuel, and understanding the principles of refueling and types of fuels required for particular activities is important in maximizing exercise performance.
ATP and PCr System (anaerobic, alactic)
ATP (adenosine triphosphate) is the primary molecule of energy for human movement. At rest we have a small store of ATP, perhaps a few seconds of intense performance worth. This is replenished by another important molecule, phosphocreatine or creatine phosphate (PCr). Energy from this system is supplied very quickly but lasts less than 10 seconds before it needs to be replenished with rest. This energy source is not ‘nutrition critical’ under normal circumstances, although creatine supplementation may enhance the PCr system. This is sometimes called the ‘alactic’ system.
Additional energy systems are available for more sustainable energy production as exercise duration increases, all of which ultimately produce ATP.
Lactic acid system (anaerobic)
Beyond the energy supply of the PCr system, and with continued high-intensity activity, the lactic acid system takes over and produces another supply of ATP, limited by the rate of oxygen availability, for up to about 90 seconds depending on intensity and lactate tolerance. This system will dominate in a 400 metre runner or 100 metre swimmer for example, although most activities longer than about 10 seconds use a mix of energy systems.
Glucose stored in muscle as glycogen is the main source of energy substrate for this system. Performance can be limited by inadequate storage of muscle glucose, or inhibition of the rate at which it is made available. These two systems, alactic and lactic, supply fast energy.
Oxygen supply maintains the aerobic system more or less perpetually as long as a fuel source is available, energy again being ultimately provided as ATP. The aerobic system uses blood glucose, muscle glycogen, blood fats, muscle triglycerides and even ketones (breakdown of fats) — and proteins in certain circumstances. Needless to say, this is the dominant energy system of longer exercise duration and day-to-day living. Understand, though, that a mix of fuel sources and energy systems, including anaerobic glycolysis (lactic acid system) will occur even in a marathon or long bike race, in, say, acceleration up a hill or a sprint finish.
Carbohydrate most important
Carbohydrate (glucose) is the most important fuel for athletes. Even though fat provides a theoretical almost unlimited supply of energy for most activities, which is valuable in longer-duration sports, it cannot provide “fast” energy like glucose (and PCr), and this is limiting for most sports. Glucose or ‘blood sugar’, which is also stored in liver and muscle, is the result of the digestion of carbohydrates including starches and sugars from foods like breads, grains, cereals, pasta, vegetables, sugars and fruits.
Low-carbohydrate diets are not suitable for athletes – even endurance athletes that rely heavily on the fat-fuelled aerobic system. Failure to maximize glucose and glycogen storage, and its rapid accessibility, will degrade performance and every second or fraction counts. Even amateur and recreational exercisers may notice a difference.
Examples of the main energy systems in various events
- 100 metres – ATP/PC (alactic system)
- 200 metres – ATP/PC and anaerobic glycolysis / lactic acid system
- 800 metres – anaerobic/lactic, aerobic glucose
- 10,000 metres – aerobic glucose and fatty acids, some anaerobic/lactic
- Marathon – aerobic glucose and fatty acids, minor anaerobic/lactic
- Most running team sports over the duration of a game – all of the above!
Note that both lactate from anaerobic glycolysis and glycerol from the breakdown of triglycerides (fats) into fatty acids and glycerol, can be recycled for energy use.