Energy for Exercise: Energy Systems and ATP Resynthesis

Energy for Exercise

The body requires energy to perform all activity, from basic functions such as breathing and digestion to intense exercise.
The body’s energy currency is known as adenosine triphosphate (ATP). All energy-requiring functions utilise ATP to drive their processes.
ATP is synthesised through the breaking down of macronutrients (carbohydrates, fats and proteins) in our diet, which releases energy for ATP production.
The formation of ATP from adenosine diphosphate (ADP) and an inorganic phosphate group (Pi) is called “ATP resynthesis”.

The body employs three energy systems to resynthesise ATP: the ATP-Pcr system, the lactic acid system, and the aerobic system.

The ATP-Pcr system (Phosphocreatine system) provides energy for short-duration, high-intensity activities. This system has a rapid rate of ATP resynthesis but has a low ATP yield.
It uses creatine phosphate (Cr) stored in the muscle to instantly regenerate ATP. No oxygen is needed, hence it’s also called the “immediate” or “anaerobic” system.

Also known as anaerobic glycolysis or the anaerobic system, the lactic acid system is engaged during high-intensity activities that last for about 30 seconds to 2-3 minutes.
It resynthesises ATP through the breakdown of glucose or glycogen without the need for oxygen, but it results in the production of lactic acid.
It has a moderate rate of ATP resynthesis and can produce a greater ATP yield than the ATP-PCr system, but less compared to the Aerobic system.

The aerobic system is the primary system responsible for ATP resynthesis during prolonged, low to moderate intensity activities.
This system uses oxygen to breakdown carbohydrates, fats, and to some extent proteins, to resynthesise ATP.
It has a slow rate of ATP resynthesis, but provides the highest ATP yield amongst the three systems.
The aerobic system does not produce any fatiguing by products; however, the rate of ATP resynthesis is slower compared to the other two anaerobic systems.

During exercise, all three energy systems contribute to ATP resynthesis, but the relative contribution of each system depends on the intensity and duration of the activity.
Brief, high-intensity activities primarily use the ATP-PCr and lactic acid system, while prolonged, low-to-moderate intensity activities rely heavily on the aerobic system.

Fatigue occurs when the energy systems can no longer meet the demand for ATP.
Adequate recovery time and appropriate nutritional intake are necessary to replenish energy stores and continue ATP production for long-duration or high-intensity physical activities.