Energy Systems: How Your Body Uses Energy When You Exercise


The minute you begin to exert your body, it starts producing energy and burning through your fuel stores, helping you finish your workout. This applies whether you lift weights, ride waves, spin wheels, or hit the pavement. However, the method your body depends on to produce energy will be dependent on the physical activity you’re engaging in, more particularly, the activity’s duration and intensity.

Understanding these energy systems and using that knowledge for your workouts could help you level up your training. And, enjoy cheat days with that keto ice cream for sale you’ve been craving—considering that you’ve got your diet in check.

ATP and The Three Metabolic Pathways

Everybody’s process, from muscular contraction to cellular signaling, runs on ATP or adenosine triphosphate. This molecule is the basic energy unit for all living things. And to produce it, your body depends on three metabolic pathways or energy production systems: glycolytic, phosphagen, and oxidative.

These pathways utilize different fuel sources and are activated under specific conditions. To understand exactly how your body runs when you exercise, you need to understand how your metabolic systems work.

The Phosphagen System

Once you start exercising, this system activates and metabolizes phosphocreatine, a molecule used for producing ATP. This system doesn’t need oxygen, which makes it an anaerobic pathway. It also functions fast, but since the cells don’t have a lot of phosphocreatine stores, its ability to produce ATP is limited.

This means that the phosphagen system maxes out after 10 or so seconds of exercising. But if you’re working out in high-intensity, brief, repeated bursts, like with short sprints and pumping iron, it stays as the dominant system in your entire workout.

Ever wonder why people supplement with creatine? It helps the phosphagen system run for longer. Your body turns creatine into phosphocreatine and supplementing with creatine will aid in you completing more super-charged reps.

The Glycolytic System


The body transitions to this energy system after 10 seconds of exercising. This will remain the dominant energy production system for roughly two minutes of your workout. For people who do HIIT or run intervals, this is their body’s main metabolic system.

This pathway utilizes carbohydrates in stored glycogen and blood glucose forms for producing ATP. Similar to the phosphagen pathway, the glycolytic system starts with anaerobic energy production. But as the two-minute mark approaches, your body will need more and more oxygen to keep up with your actions.

The Oxidative System

About two minutes into your workout is when your oxidative system kicks in. Because oxygen’s crucial in producing energy in this system, it’s known as the aerobic pathway. This pathway utilizes both fat and carbs aside from oxygen for producing ATP.

Although the oxidative system can’t produce ATP as quickly as the other energy systems, it’s capable of producing a ton of it, which makes this pathway the body’s preferred system for relatively low-intensity and longer-duration cardiovascular activities like biking and state running.

Other Crucial Things to Remember

Take note that none of these energy pathways ever turn off. They are all functioning every minute of every single day to some extent. But the system that is emphasized changes according to your body’s energy requirements, which are decided by your training’s duration and intensity. Knowing this, you can optimize your training results and performance.

Additionally, if you focus mainly on lifting weights, consider taking creatine supplements for the reasons discussed above. Because while there are tons of performance-enhancing or ergogenic supplements out on the market, creatine is among the few that are backed by science.

Likewise, regardless of your main training method, consider tweaking your routine to make all your energy system work more intentionally.

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