The Three Energy Systems of Exercise

by Richard Lempicki

CrossFit methodology employs cardio and strength exercises that vary in intensity and duration to maximize overall fitness by constantly stimulating the various metabolic energy systems. Understanding how this works requires knowledge of the three major energy systems of exercise. Muscles are motors that use chemical energy to generate force through contractile movement. The primary fuel to power muscle is adenosine triphosphate (ATP). Unfortunately, our bodies cannot store appreciable amounts of ATP and must generate it on demand as needed. Three very different, yet interconnected energies systems have evolved to produce ATP: the phosphagen, glycolysis (anaerobic) and oxidative (aerobic) pathways. Each of these pathways use different fuel precursors to make ATP and are always active during exercise, with our bodies deciding what percentage of each pathway to draw energy from based on the intensity and duration of the exercise. Training targeting each energy system increases the efficiency, duration, and replenishment of each system.

Weekly WODs are designed to train across the various energy pathways for best overall fitness. The given stimulus for each WOD and rest time, when programmed, play a critical role in maximizing conditioning of a given energy pathway. Blowing the stimulus in either direction misses the goal of the workout and may cause under- or over-training of pathways – thereby not meeting the big-picture programming goal.

Phosphagen Pathway
The phosphagen pathway is used most used for max-effort instantaneous, high intensity and explosive exercises. This pathway uses stored creatine, a non-protein amino acid, as the precursor fuel source which is stored in muscle and mostly derived from red meat, poultry, fish, and supplements but also produced by our bodies from other amino acid. Creatine can be rapidly converted to ATP to fuel high intensity excises such as a 100-meter sprint or working up to a 1-3-rep max lift. This energy source only lasts for 10-15 seconds before becoming depleted and partial creatine replenishment occurs within 2-3 minutes with rest or light exercise, and can be used for subsequent burst activity, but for reduced time.

Anaerobic Glycolysis Pathway
The anaerobic glycolysis pathway is predominantly active during moderate-heavy exercise such as a 400-800 meter runs and is significantly activated in exercises lasting in the 30 sec to 4-minute range. This pathway uses glucose (directly or broken down from glycogen) in the absence of oxygen to produce ATP. Production of ATP is of an intermediate rate, but of lower efficiency in that less ATP is generated per glucose molecule. An unfortunate by-product of this pathway is lactic acid and other inhibitory by-products which causes pain and fatigue and inhibits ATP production from the other two pathways.

Aerobic Oxidative Pathway
The oxidative pathway refers to the specific use of oxygen to aid in the use of fat or glucose to produce ATP. This pathway is highly efficient in that a high amount of ATP is produced from each molecule of fat or glucose but proceeds quite slowly. This characteristic makes the pathway most suitable for moderate to long endurance exercises from a mile run to hero WOD Murph. Depletion of usable precursor sources can occur in endurance events if “refueling” is not maintained at an adequate level which leads to a person “hitting the wall” or “bonking”.

Tying the Three Energy Pathways Together and How it Applies CrossFit
Our performance in each WOD is determined by a multitude of factors that include genetics, training, mental fortitude, coaching, injuries, and even our emotional well-being. One additional factor we can regulate is how to approach a given WOD. Each of these pathways work in an overlapping and interactive manner based on intensity and duration. Regular training in one or more of these zones condition the specific pathway to work more efficiently and recover more quickly. An athlete needs gauge the workout requirements and know when to press hard when in a certain zone and back off in others. For example, going out too hard on a 10-minute AMRAP will deplete creatine within 15-30 seconds and greatly stimulate anaerobic glycolysis resulting high levels of lactic acids within 5 minutes thus inhibiting the already slower ATP generating oxidative pathway –> “crash and burn!”. Learning how to appropriately pace and scale a WOD based on your capabilities and strengths, without overtly taxing any one energy pathway, will help achieve your highest level of performance.

Pastino, Jessica. Understanding the Three Energy Systems Used During Exercise, March 23, 2022. article.

Biostrap. How The Phosphagen System Supports Your Strength Training, February 25, 2022. whitepaper.

Webster, Drake. Energy Systems: How They Work and When They are in Use, March 4, 2012. blog.