How The Body Produces Energy During Exercise

In this blog series, I've been driving home the idea that to live your best life, you must find a purpose, set goals, and make a strategy to improve and achieve them.

With the right mindset, consistent purpose-driven barbell training will build muscle and improve your self-image, enhance your effectiveness in day-to-day life, and even improve the most essential psycho-spiritual elements of your being- Decisiveness and Perseverance, the components of your Will. You have everything to gain, but you have to expend some energy to get these gains. Read on to better understand how your body fuels your ascent.

To improve, we use various movements to apply stress to our bodies to drive specific adaptations that ensure more effective performance in sports competitions and life. Every action in training and competition requires engagement of the body's energy systems, and recovery processes from these actions do as well. Understanding the body's energetic process is crucial for optimizing your stress-recovery-improvement process; the process we call TRAINING.

The body utilizes three primary energy systems, each defined by the duration and intensity of the activity:

• Short-Term System (ATP & CP / Phosphagen System): Powers high-intensity, explosive movements lasting only a few seconds. This system relies on stored ATP and creatine phosphate (phosphagens) and is dominant in short bursts like sprints or heavy lifts.

  
  

  

  
  

• Intermediate-Term System (Glycogen / Anaerobic Glycolytic System): Supports moderate-to-high intensity efforts lasting from around 10 seconds to 2 minutes. It primarily uses muscle glycogen and produces lactate as a byproduct.

  
  

  

  
  
  
  

• Long-Term System (Glycogen & Fats / Oxidative System): Fuels low-intensity, sustained activity for several minutes or more. This aerobic system taps into glycogen and fat stores and is crucial for endurance performance.

  
  

  

  
  

In reality, at any given instant, all three systems functionally overlap and interact to produce the energy we need for action.

Figure 1: An example of the Interaction of the energy systems at a given instant of physical effort.

Understanding how they function in isolation and collectively is essential for improving performance.

Let’s examine how the body produces energy during exercise and how this applies to different types of training and sports.

⚡The Short-Term,  High Intensity System

📌 Duration: Short-Term 

🔋 Fuel Source: ATP & CP

Also called the ATP-PCr system, this is the fastest-acting energy system. It fuels extremely intense efforts lasting up to 20–30 seconds using ATP (Adenosine Triphosphate) and CP (Creatine Phosphate) already stored in the muscle cells to regenerate ATP quickly.

🧪 Biochemical Classification:

• System: Phosphagenic

• Fuel: ATP & CP

• Time Frame: Short-Term

🔋 The Short-Term Energy System: ATP & CP in Action

Every powerful movement begins with a rapid release of energy. The body’s Short-Term Energy System is the first responder — delivering immediate fuel for high-intensity actions in two rapid phases.

Figure 2: The Phosphagenic (ATP, CP, ADP) Energy System. *Ref. Supertraining, (1999), Verkhoshansky and Siff

⚡ Phase 1: Stored ATP Fuels the First Few Seconds

For the first 2–4 seconds of all-out effort, the body relies entirely on ATP (adenosine triphosphate) already stored in the muscle. Through a reaction catalyzed by the enzyme ATPase, ATP is broken down into ADP (adenosine diphosphate) and inorganic phosphate (Pi), releasing a burst of energy for muscular contraction.

This reaction is fast and powerful — but short-lived. Once ATP levels drop, the system must adapt quickly or performance will stop.

🔁 Phase 2: CP Resynthesizes ATP to Extend Performance

After those first few seconds, creatine phosphate (CP) becomes essential. CP donates its phosphate group to ADP, rapidly regenerating ATP via the enzyme creatine kinase. This keeps energy flowing for continued explosive output.

Since there is approximately 3–5 times more CP than ATP stored in the muscle, this process can support high-intensity work for up to 20–30 seconds — ideal for sprints, lifts, or sport-specific bursts like tackles and jumps.

🧠 The Cellular Signal to Transition

As ADP accumulates, it signals the muscle cell that more energy is needed. At this point, the Intermediate-Term Energy System must begin stepping in — breaking down muscle glycogen through anaerobic glycolysis. This transition allows the effort to continue, but only if the intensity is reduced.

🔥 The Intermediate-Term, Moderate Intensity System

📌 Duration: Medium-Term 

🔋 Fuel Source: ATP & CP & Glycogen, then Glycogen & Lactic Acid

As the ATP-PCr system depletes, the glycolytic system (glycolysis) takes over. It breaks down glycogen without oxygen, producing lactic acid as a byproduct. This system is dominant in efforts lasting 30 seconds to ~2 minutes, depending on intensity. Glycogen is a form of stored carbohydrate found in both muscle cells and the liver, derived from carbohydrates in your diet.

🧪 Biochemical Classification (two overlapping subphases):

• System: Anaerobic Glycolysis

• Phase 1 Fuel: ATP & CP & Glycogen (early glycolysis)

• Phase 2 Fuel: Glycogen & Lactic Acid (later glycolysis during OBLA)

• Time Frame: Medium-Term

🌀 The Long-Term, Low Intensity System

📌 Duration: Long-Term 

🔋 Fuel Source: Glycogen & Fatty Acids (and Proteins in extreme cases)

This system produces energy aerobically — meaning with oxygen — and can last for hours. It relies on the breakdown of glycogen and fats to fuel sustained, low-intensity activity. It is important to remember that even though most lifting, throwing, and jumping training, and most competitive sporting competitions are primarily fueled by the short-term (and secondarily by the intermediate) system, all of the body's recovery processes (like protein synthesis-muscle growth) are driven by the long-term system; this is why some light (around 60% of Max Heart Rate for 30 minutes) aerobic cardio training done 2-3 times per week is good for power and strength athletes.

🧪 Biochemical Classification:

• System: Aerobic / Oxidative System

• Fuel: Glycogen & Fatty Acids (oxidative phosphorylation)

• Time Frame: Long-Term

🔄 The Krebs Cycle: Aerobic Energy in Action

At the heart of the Long-Term Energy System is the Krebs cycle — a key aerobic process that takes place inside the mitochondria (the "Powerplant") of your cells. Once oxygen is available, the pyruvate produced from glycolysis (and even fats and proteins) enters this cycle to be broken down further for energy.

The Krebs cycle doesn’t work alone — it's part of a more extensive system that includes the Electron Transport Chain (ETC). Together, they generate large amounts of ATP over extended periods, making them essential for endurance sports, sustained training, and even daily low-intensity movement.

🪵 Think of it like a slow-burning campfire: once it's lit and has steady oxygen, it provides consistent, lasting energy. It's not explosive like a match (short-term system), but it's dependable — ideal for marathons, hikes, and recovering from heavy workouts/competitions.

While it’s not fast enough to fuel sprints or heavy lifts, the Krebs cycle keeps you going when your body needs efficient, long-lasting effort.

🏅 Energy System Demands in Sport

Every sport places unique demands on the body’s energy systems, depending on the intensity, duration, and performance structure. The body blends the three systems to meet energy needs from explosive sprints to prolonged endurance events.

The Short-Term System (ATP & CP) dominates in sports requiring quick, powerful bursts such as weightlifting, wrestling, or sprinting.

The Intermediate-Term System (Anaerobic Glycolysis) becomes more relevant in efforts lasting 30 seconds to 2 minutes, such as a 400m sprint, a long tennis rally, or repeated soccer drives.

For sustained efforts beyond 2–3 minutes — like rowing, marathon running, or cycling — the Long-Term System (Aerobic) takes over, tapping into glycogen and fat stores for efficient, steady energy.

The table below shows how each system contributes across various sports. Scroll and find your sport; entries are listed alphabetically.

Table 1: Energy production in sports. *Adapted from Fox & Matthews D (1974) Interval training of Sports and General Fitness, WB Saunders Co.

The information in table 1 provides a useful guide for training programming design. You can organize training activities to precisely perfect your energy systems' efficiency according to your sport.

💪How The Body Produces Energy During Exercise: Developing Abilities

We develop the physiological abilities of performance in training using certain movements (Means) and methods (way of sequencing and organizing effort).

Figure 3: The Physiological Abilities Spectrum

 The abilities lie on a spectrum with two continuums, with Absolute Strength as the fulcrum ability between the two. One continuum ranges from absolute speed to absolute strength. The abilities of this continuum are speed and power-based:

Velocity-Power-Force (V-P-F) Continuum

• Reactive Ability (Unloaded {Absolute} Speed, particularly in reaction to a signal)

• Speed Strength

• Power

• Strength Speed

• Absolute Strength

The other continuum is Force and Endurance-based:

Force-Structure-Vegetative (F-S-V) Continuum

• Absolute Strength

• Muscular Endurance

• Anaerobic Endurance

• Aerobic Endurance

The abilities of the V-P-F continuum and Muscular Endurance are primarily developed in training with the major lifts. Barbell Squats, Bench Presses, and Deadlifts are the primary means for developing Speed Strength, Strength Speed, Absolute Strength, and Muscular Endurance.

Global (Absolute) Power is developed with Snatches and Clean & Jerks. Regional (sub-absolute) Power is developed with Barbell Squats, Bench Presses, and Deadlifts.

The different abilities have particular ROFDs (Rate of Force Development) and energetic demands.

The crucial actions in your sport also have particular ROFDs and energetic demands; understanding these variables for your sport will allow you to optimally periodize the development of the abilities through training. Understanding what contributions each system makes to performance in your sport and to each ability greatly facilitates effective training programming.

The following table shows how different intensity (as % of One Rep Maximum) and rep combinations performed with Squats, Presses, and Deadlifts develop different Abilities and the contributions of the three energy systems.

Table 2: The Contribution of the Energy Systems and Ability developed in various Squat, Bench Press, and Deadlift training sets.

By referencing your sport in table 1 and matching it with the ability that has the same energy profile allows you to design training sets to precisely perfect your bodies energy utilization process for success in competition.

🏁 Conclusion: Train the System, Not Just the Muscle

Understanding how the body produces energy isn't just science — it's strategy. Every rep, sprint, or mile taps into one or more of the body’s energy systems: Short-Term (ATP & CP), Intermediate-Term (Glycolytic), and Long-Term (Aerobic). These systems don’t work in isolation; they blend and shift depending on the intensity and duration of activity.

By aligning your training with the dominant energy system used in your sport or goal, you can program smarter, recover better, and perform at a higher level. Whether you're training for power, endurance, or aesthetics, the key isn't just movement — it's metabolic precision.

👉 To reach your Peak, you need Training NOT Exercise — Training designed according to how the body works. Contact Us Today to Get Started or to Learn More!