Energy System Development Part 1: The Energy Systems Explained

ATP and Energy Systems

Adenosine triphosphate (ATP) is the body’s fuel, the spark that powers every muscle contraction, breath, and heartbeat (Mc Ardle et al., 2010). If you’ve ever felt that burning sensation in your legs during a sprint or the exhaustion that hits after a heavy set in the gym, you’ve experienced your body running low on ATP and the challenge of replenishing it.

The purpose of the body’s energy systems is to maintain energy homeostasis by continuously producing ATP. This is done by converting the chemical energy from the food we eat into the fuel required for muscular work. Think of it as a highly efficient energy transformation system: metabolic energy is converted into physical effort.

How Does the Body Generate ATP?

There are two primary pathways for generating ATP:

Each of these processes feeds into ATP production to ultimately fuel muscular work. Whether you’re walking, lifting, or running, your body relies on a delicate balance between the aerobic and anaerobic systems to meet its energy demands.

Here’s the challenge: your body doesn’t have a limitless supply of ATP. Instead, it must rely on three distinct energy systems to produce it, with each one specialized for different types of activities. The way you train, the activities you perform, and even your fitness goals determine which energy system takes the lead.

QUICK EXAMPLE: If you’re a sprinter chasing a personal best on the 100m dash, you’ll rely on one energy system. But if you’re preparing for a marathon or a CrossFit competition, your energy system requirements shift dramatically.

The Three Main Energy Systems

Every time you move, you’re tapping into one (or more) of these systems:

1. Phosphagen System (Immediate Energy)
2. Glycolytic System (Short-Term Energy)
3. Oxidative System (Long-Term Energy)

Think of them as your body’s “engine,” where one system takes over when the other runs out of fuel. Let’s break each one down using examples to make sense of the science.

Phosphagen System: Immediate Energy

The Phosphagen System is your go-to for those all-out, explosive efforts. Think of it like the car’s turbo boost: It’s fast, powerful, but it burns out quickly. When you’re sprinting, lifting heavy, or performing high-intensity interval training (HIIT), your body taps into this system for quick bursts of energy.

How It Works:
Imagine you’re about to sprint for a 100-meter race. Your muscles already have a small reserve of ATP ready to go, but it won’t last long. To keep you moving, your body breaks down phosphocreatine (PCr)—a high-energy compound stored in the muscles – to regenerate ATP almost instantly. This reaction provides the ATP needed for the first few seconds of explosive activity.

Example in Action:
Picture an athlete like Usain Bolt in the 100m dash. His body relies almost entirely on the phosphagen system for the first few seconds as he explodes out of the starting blocks. It’s all about power, pure speed, and a short burst of energy.

Training the Phosphagen System:
If you’re aiming for maximum strength or speed:

• Train with short, intense efforts (e.g., sprint intervals, heavy resistance training).
• Prioritize adequate rest between sets to allow full PCr recovery.
• Examples: 5-10 second sprints with 2-3 minutes of rest or heavy back squats (1-5 reps) with 2-3 minutes of rest.

Glycolytic System: Short-Term Anaerobic Energy

When your Phosphagen System runs out of steam, the Glycolytic System takes over. This system is designed to fuel activities lasting 10 seconds to 2 minutes, like a fast 400-meter run or a tough WOD.

How It Works:
The glycolytic system breaks down glucose (or glycogen stored in your muscles) to produce ATP. Unlike the phosphagen system, this process doesn’t require oxygen, but it comes with a trade-off: it produces hydrogen ions (H⁺) as a byproduct (Brooks et al., 2005). These hydrogen ions accumulate in the muscle and lower the pH, leading to a condition called metabolic acidosis. This acidosis, not “lactic acid” is what causes the familiar burning sensation during intense exercise.

Lactate (commonly mistaken as lactic acid) actually plays a beneficial role in the process, helping to buffer hydrogen ions and providing fuel for other muscles or tissues (Gladden, 2004).

Example in Action:
Let’s say you’re doing a 500-meter rowing sprint. As you hit the halfway mark, your legs start to burn, your heart rate climbs, and your muscles begin to fatigue. This is the glycolytic system at work, supplying energy as quickly as possible while hydrogen ions accumulate in your muscle tissue, challenging your ability to sustain the effort.

Training the Glycolytic System:
To train this system:

• Incorporate high-intensity intervals (e.g., 30-60 second sprints with a 1:1 work-to-rest ration).
• Circuit-style training and tabata workouts are excellent methods.
• Goal: Improve your buffering capacity and your body;s ability to tolerate and clear hydrogen ions.

Oxidative System: Long-Term Aerobic Energy

When activities stretch beyond a few minutes, the oxidative system becomes your primary energy source. This system powers lower-intensity, longer-duration efforts, like running a marathon or cycling for an hour.

How It Works:
Unlike the glycolytic system, the oxidative system requires oxygen to generate ATP. This process breaks down glucose and fat through the Krebs Cycle and the Electron Transport Chain (Wilmore et al., 2004). The result? A steady, sustainable energy supply without the byproducts of the glycolytic system.

Example in Action:
Think of an Ironman triathlete. Their success relies on the oxidative system, as it allows them to fuel their muscles for hours by breaking down fat stores and carbohydrates efficiently.

Training the Oxidative System:
To develop this system:

• Focus on long, steady-state cardio (e.g., Cardiac output method).
• Incorporate tempo runs or extended aerobic sessions.
• Goal: Increase mitochondrial density, oxygen delivery, and fat utilization.

Energy System Characteristics at a Glance

Energy System	Primary Fuel Sour	Duration of Activity	Intensity Level	Byproducts	Key Limitation
Phosphagen System	ATP and Phosphocreatine (PCr)	0-10 seconds	Maximal (90-100% effort)	None (produces ATP directly)	Rapid depletion of PCr stores
Glycolytic System	Glucose and Glycogen	10 seconds to 2 minutes	High (75-90% effort)	Hydrogen ions (H+), Lactate	Accumulation of H+ causing fatigue
Oxidative System	Glucose, Glycogen, Fats	2 minutes and longer	Low to Moderate (50-75%)	Carbon Dioxide (CO2), Water (H2O)	Oxygen availability limits intensity

The Energy Systems in Action

Each of the Phosphagen, Glycolytic, and Oxidative systems has a key role in powering your body’s performance across all types of physical activity. These systems don’t work independently; rather, they overlap and shift depending on the activity, the intensity, and the duration. At any given moment, all three systems are contributing to energy production. However, the difference lies in which system is dominant based on the energy demand.

So, when you’re thinking about your next workout or race, take a moment to consider which energy system you’re targeting. A well-rounded fitness program takes into account all three systems, helping you develop more complete fitness and better overall performance.

By understanding how these systems work—and how to train them—you can design more effective workouts tailored to your goals. A complete fitness program should develop all three systems to ensure:

• More power and strength for short-duration efforts.
• Greater work capacity and resilience in moderate-duration efforts.
• Enhanced endurance for long-term performance and cardiovascular health.

Looking Ahead: Part 2 of Our Energy Systems Series

This post was just the beginning, and we’ve barely scratched the surface. Over the next few weeks, we’ll explore each of these energy systems in detail, providing you with practical training strategies for each one. In the next installment, we’ll dive deeper into the Phosphagen system—how to train it, common mistakes, and how it plays a huge role in short-term, explosive efforts.

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References

Brooks, G. A., Fahey, T. D., & Baldwin, K. M. (2005). Exercise physiology: Human bioenergetics and its applications (4th ed). McGraw-Hill.

Gladden, L. B. (2004). Lactate metabolism: A new paradigm for the third millennium. The Journal of Physiology, 558(Pt 1), 5–30. https://doi.org/10.1113/jphysiol.2003.058701

Mc Ardle, W. D., Katch, F. I., & Katch, V. L. (2010). Exercise physiology: Nutrition, energy, and human performance (7th ed). Wolters kluwer health – Lippincott Williams & Wilkins.

Wilmore, J. H., Costill, D. L., & Kenney, W. L. (2004). Physiology of sport and exercise (Vol. 20). Human kinetics Champaign, IL.