March 04, 2024

Top Line

Whether you’re striving to outperform your opponent, break a record or just want to be in best shape possible to run, lift, fight, swim, and have sex all in the same day - knowing how your body generates energy is paramount to optimizing performance.

Why it Matters

Many coaches and personal trainers prescribe conditioning without the demands for the sport or clients goal in mind. This blind approach to exercise prescription will only get you so far. 

Key Takeaways

  • Three energy systems work to replenish your bodies ATP
  • Knowing when to train and how much time to devote to training each energy system is an integral ingredient for success
  • Conditioning programs should carefully consider the requirements of the sport or goal

If you grew up playing team sports the first thing you likely picture when you hear the word ‘conditioning’ is endless bouts of sprints on the basketball court (‘suicides’) or laps around the football oval.

Conditioning allows you to be prepared for both the physiological and psychological demands of a task. How you prepare your body has huge implications on success. For example if you decided conquering Mt Everest was your life goal, without purposeful conditioning it is unlikely you will join the 29% of climbers who make it to the top!

To fully understand conditioning as it relates to sport and exercise you must first understand some basics around energy metabolism.

Energy metabolism is an interesting but potentially confusing topic. This article is not intended to be a substitute for a detailed exercise physiology textbook but rather to provide a basic introduction to the energy systems and suggestions as to their application to training. 

Every living cell in your body requires energy. Where does this energy come from?


Adenosine triphosphate (ATP) is a usable form of chemical energy utilized in cell function. Some ATP is stored in muscles however most is synthesized from food.

Macronutrients; carbohydrates, fats and proteins when digested into their simplest form (glucose, fatty acids and amino acids) are available for use by metabolic pathways. 

Our bodies can not store large quantities of ATP and need a continuous supply.

The Quest for ATP

Increases in movement (all exercise) will increase this demand for energy (ATP). Short-term intense exercise can require a 1,000-fold increase in the demand for ATP compared to that at rest. (1)

Muscle contraction is required for your eyes to track these words and your diaphragm to contract and fill your lungs with air.

Physiologically the key players involved in movement are the motor neuron, neuromuscular junction and muscle cells. Initiation of the contraction mechanism requires a process called excitation contraction coupling. This begins with an action potential travelling along an axon and through a series of intrigue biochemical pathways ultimately leading to cross-bridge cycling with myosin heads releasing stored energy (adenosine diphosphate + inorganic phosphate) allowing the thick filament to pull on the thin filament – contraction.

Energy System 101

Three energy systems work to replenish your bodies ATP: 

(1) ATP-PC System, (2) Glycolytic, and (3) Oxidative. The three systems differ in the substrates involved, products, rate and capacity of ATP regeneration. 

Go into any gym or sporting club and you will likely hear words ‘aerobic’ or ‘anaerobic’ thrown around. To clarify the ATP-PC and glycolytic energy systems are said to be anaerobic (oxygen is not involved in ATP production) and the aptly named oxidative system aerobic (oxygen is involved in ATP production).

As you increase intensity with exercise, replacing ATP involves a coordinated metabolic response from all three energy systems. The contribution of each is dependent on the intensity and duration of the exercise. One common myth is that the energy systems act like a light switch with changing intensity or duration instantly turning one system ‘off’ and another ‘on’. This is not a great analogy. With changing intensity or duration there is an interplay of the energy systems, behaving more like light dimmers then switches.

Knowing when to train and how much time to devote to training each of the three energy systems is an integral ingredient for success. Specificity of training is perhaps the most significant principle used in athlete preparation. (5) Conditioning programs should therefore carefully consider the requirements of the sport particularly as it relates to intensity and duration.

ATP-PC System

Our bodies fastest source for ATP is creatine phosphate which combines with adenosine diphosphate to form ATP. These creatine phosphate stores can provide energy for high-force contractions, lasting around 10 to 15 seconds. (2) This system is responsible for short, explosive bouts of energy and yields one net ATP. As many sports and activities require short-term or repeated intense efforts knowing how to train this system is important.


Targeting the ATP-PC system involves inclusion of exercises performed at very high intensities, with very low volume. Allow full recovery with a work to rest ratio of around 1:12+. You can train this system with short sprints, all out efforts on a cardio machine (anecdotally Versa Climber does the job) or with speed-strength training e.g. jump squats.

The Glycolytic System (Glycolysis)

When exercise continues longer than a few seconds, the energy to regenerate ATP is increasingly derived from blood glucose and muscle glycogen stores.4 The glycolytic system is the dominant energy system for moderate to high intensity and the next tool in line after ATP-PC system runs its course. It yields two net ATP if utilizing blood glucose, three net ATP if utilizing muscle glycogen. This system creates a buildup of H+ ions (resulting in a change in pH), which is responsible for that ‘sick’ feeling.


Targeting the glycolytic system involves inclusion of moderate to high intensity exercise, with a low to moderate volume. Exercise duration should be 30-90 seconds (crossover with ATP-PC from 10-30 sec) with a 1:3 work rest ratio. You have a variety of options for training this system. My personal favorite would have to be punching a boxing bag.

The Oxidative System

This system is responsible for ATP creation for long duration exercise and is utilized for any continuous event >90 seconds. It’s said to be the dominant energy system by the 30-s time period during the 400-, 800-, and 1500-m running events.5 It can produce ATP through either fat (fatty acids) or carbohydrate (glucose). ATP can be produced three ways;

The Krebs Cycle

Yielding a fruitful 28-30 net ATP, (34-36 if you include the two NADH2 from glycolysis). 

Targeting the oxidative system typically involves low intensity, high volume such as distance running with a work to rest ratio of 1:1 or even >1:1.

Knowing how your body generates energy is paramount to optimizing performance. By understanding the physiological demands of a sport or task and how best to target your body's energy systems you can have an unfair advantage over your gym partner or opponent.


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