Every movement the body makes, from a single explosive jump to an ultra marathon, is fuelled by energy. The body cannot store energy in a usable form, so it must constantly regenerate adenosine triphosphate (ATP), the universal energy currency. According to the new IB SEHS specification, students must understand the three energy systems that resynthesise ATP: the ATP creatine phosphate system, the anaerobic glycolytic system, and the aerobic system. Each operates with different fuels, at different speeds, and for different durations, and understanding their function is key to analysing and improving performance.

The ATP creatine phosphate system, also known as the phosphagen system, is the fastest source of ATP resynthesis. It is used during short-duration, high-intensity efforts lasting up to around ten seconds, such as sprinting, jumping, or weightlifting. This system relies on creatine phosphate stored in the muscles, which donates a phosphate group to regenerate ATP rapidly. It does not require oxygen, produces no harmful byproducts, and activates instantly. However, it is limited by the small supply of creatine phosphate, and recovery takes several minutes. Despite its brevity, this system is critical in sports requiring maximal power and explosive strength.

The anaerobic glycolytic system breaks down glucose to produce ATP without oxygen, generating lactic acid as a byproduct. It is dominant in activities lasting from around ten seconds to two minutes, such as a 400-metre sprint or a sustained bout of high-intensity effort in team sports. This system produces ATP more slowly than the phosphagen system but faster than the aerobic system. The accumulation of lactate and hydrogen ions can cause fatigue and limit performance, but the system allows sustained high-intensity output beyond the immediate energy reserves. Training adaptations can enhance lactate tolerance and delay the onset of fatigue.

The aerobic system is the most sustainable and complex of the three. It requires oxygen and uses carbohydrates, fats, and, in extreme cases, proteins to regenerate ATP through a series of chemical reactions, including glycolysis, the Krebs cycle, and the electron transport chain. While the aerobic system is relatively slow to activate, it can provide energy for hours, depending on substrate availability and intensity. It is the primary system used in endurance events, and it dominates at rest and during low to moderate intensity activity. Aerobic fitness enhances the body’s ability to deliver oxygen and utilise it efficiently in the muscles.

All three systems are active at all times, but their contribution varies depending on intensity, duration, and recovery periods. This is known as the energy continuum. For example, a 100-metre sprint is driven primarily by the ATP creatine phosphate system, but by the final metres, the anaerobic system also contributes. A 1500 metre runner relies heavily on the anaerobic glycolytic system and transitions to the aerobic system over time. A marathon runner primarily relies on aerobic metabolism, with the ability to utilise fats becoming crucial in later stages.

Training can shift the balance between systems. Sprint training improves phosphagen recovery and neural recruitment. Interval training enhances anaerobic capacity and lactate clearance. Endurance training improves mitochondrial density, oxygen transport, and fat oxidation.

In summary, energy systems are not isolated engines but an integrated network that responds dynamically to physical demands. Mastery of how they work, and how they can be trained, underpins intelligent programme design and peak athletic performance.