The Three Energy Systems
The body uses three distinct but integrated systems to resynthesise ATP. Each is suited to different demands of intensity and duration. Explore their unique characteristics below.
ATP-PC System
Immediate, explosive power.
- Fuel: Phosphocreatine (PC)
- Rate: Explosive
- Yield: Very Small (~0.7 ATP)
- Dominance: 0-10 sec
Anaerobic Glycolysis
Sustained high-intensity.
- Fuel: Glycogen (CHO)
- Rate: Fast
- Yield: Small (2-3 ATP)
- Dominance: 10-75 sec
Aerobic System
Endurance engine.
- Fuel: Glycogen, Fats
- Rate: Slow
- Yield: Very Large (38+ ATP)
- Dominance: >75 sec
System Comparison: Rate vs. Yield
A core principle is the inverse relationship between how *fast* a system produces ATP (rate) and how *much* it can produce in total (yield).
Energy System Interplay
The energy systems are always working together, with one being ‘predominant’ depending on activity intensity and duration. This is the energy continuum.
The Energy Continuum
Use the slider to see how the relative contribution of each system changes as the duration of a maximal-effort activity increases.
At 10 seconds…
The ATP-PC system is dominant, providing explosive power. The Anaerobic Glycolysis system is rapidly increasing its contribution.
Acute Physiological Responses
When we start exercising, our body makes immediate adjustments to meet the increased demand for oxygen and energy. Explore the key responses below.
Cardiovascular & Respiratory Responses: Trained vs. Untrained
Chronic training leads to significant physiological adaptations. This chart compares the acute responses of a trained athlete versus an untrained individual.
Heart Rate (HR)
Heart rate increases linearly with intensity. The trained athlete has a lower resting HR and maintains a lower HR at any given sub-maximal workload, demonstrating greater cardiac efficiency.
Oxygen Dynamics
The body’s oxygen consumption ($VO_2$) follows a dynamic pattern during exercise and recovery, involving three phases: oxygen deficit, steady state, and EPOC.
Oxygen Consumption: Deficit & EPOC
This chart illustrates the body’s oxygen usage during the transition from rest to exercise and back to rest.
Oxygen Deficit: The initial period where anaerobic systems cover the energy demand while the aerobic system ramps up. EPOC: After exercise, oxygen consumption remains elevated to “repay the debt” and restore the body to its resting state.
Fatigue & Lactate Inflection Point
Fatigue is an inability to continue at a given intensity. LIP is the point where lactate production exceeds clearance, a key predictor of endurance performance.
Lactate Inflection Point (LIP)
A higher LIP is a key indicator of endurance performance. Training shifts the LIP to the right, allowing an athlete to maintain a higher intensity before fatigue-inducing by-products accumulate rapidly.
Recovery Strategies
Effective recovery must target the specific cause of fatigue. Click on a “Cause of Fatigue” to see its optimal recovery method.
Matching Fatigue with Recovery
Cause of Fatigue
⚡️ PC Depletion
Limiting factor for the ATP-PC system (<10s).
🏃♂️ H+ Ion Accumulation
From anaerobic glycolysis in high-intensity events (10-75s).
💨 Glycogen Depletion
Key factor in endurance events (>90 mins).
Optimal Recovery
Passive Recovery
Rest or very light activity to rapidly replenish PC stores.
Active Recovery
Low-intensity exercise to clear H+ ions and lactate.
Nutritional Recovery
Consuming high-GI carbs and fluids to refuel and rehydrate.
