ATP: The Body’s Universal Energy Currency
All movement is powered by Adenosine Triphosphate (ATP). This high-energy molecule releases energy for muscle contraction when a phosphate bond is broken, converting it to ADP. The body’s key challenge is constantly rebuilding ATP, a process known as resynthesis.
The ATP-ADP Cycle
The body’s stores of ATP are tiny, lasting only 1-2 seconds. Therefore, the three energy systems must work continuously to resynthesise ATP from ADP and phosphate (Pᵢ) using energy from fuels.
The Three Energy Systems
To continue exercise, the body rebuilds ATP via three distinct pathways. Each has a trade-off between its rate (speed) and yield (amount) of energy production. Hover over the table rows to compare their key features.
| Feature | ATP-PC System | Anaerobic Glycolysis | Aerobic System |
|---|---|---|---|
| Rate (Speed) | Explosive / Fastest | Fast | Slow |
| Yield (Amount) | Very Small (~0.7 ATP) | Small (2-3 ATP) | Very Large (38+ ATP) |
| Dominant Duration | 0 – 10 seconds | 10 – 75 seconds | > 75 seconds |
| Cause of Fatigue | CP Depletion | H+ Ion Accumulation | Glycogen Depletion |
Fuel for the Fire
The energy systems use chemical fuels (stored in muscle) and food fuels (from our diet) to resynthesise ATP. The body’s choice of food fuel is largely dictated by exercise intensity and duration.
Carbohydrates (Glycogen)
The body’s preferred fuel during exercise, especially at higher intensities. Stored as glycogen in muscles and the liver, it can be broken down both with and without oxygen.
Fats (Triglycerides)
The main fuel source at rest and during low-to-moderate intensity exercise. Fats provide a huge energy yield but require more oxygen to break down than carbohydrates.
Protein (Amino Acids)
Makes only a minor contribution to energy production (5-10%), used primarily when glycogen stores are severely depleted during ultra-endurance events.
Energy System Interplay in Action
The three energy systems don’t work in isolation; they contribute simultaneously. Their relative contribution changes based on activity intensity and duration. Use the buttons to see how the interplay shifts between different events.
Acute Physiological Responses
When exercise begins, the body makes immediate adjustments to meet the increased demand for oxygen and fuel. Explore the key acute responses of the cardiovascular, respiratory, and muscular systems below.
Cardiovascular
▲ Heart Rate (HR): Increases linearly with intensity.
▲ Stroke Volume (SV): Volume of blood per beat increases.
▲ Cardiac Output (Q): Total blood pumped per minute rises.
▲ Blood Flow Redistribution: More blood is sent to working muscles.
Respiratory
▲ Respiratory Rate (RR): Breaths per minute increase.
▲ Tidal Volume (TV): Air per breath increases.
▲ Ventilation (V): Total air breathed per minute rises.
▲ Pulmonary Diffusion: Gas exchange becomes more efficient.
Muscular
▲ Motor Unit Recruitment: More muscle fibres activate.
▲ Muscle Temperature: Increases, improving pliability.
▲ Enzyme Activity: Speeds up metabolic reactions.
▼ Fuel Stores: ATP, PC, and Glycogen levels decrease.
Interpreting Performance Data
Analyzing physiological data provides deep insight into performance. These interactive charts show how training impacts oxygen uptake and lactate levels. Use the toggles to compare a trained vs. untrained individual.
Oxygen Uptake ($VO_2$)
Shows oxygen use during and after exercise. A trained athlete has a smaller O₂ deficit and faster recovery (EPOC).
Lactate Inflection Point (LIP)
The point where lactate production exceeds clearance. Training shifts this to the right, allowing higher sustainable intensity.
Fatigue and Recovery
Fatigue is specific to the activity performed. Understanding the cause is essential for choosing the correct recovery strategy to restore performance.
Activity: Maximal Efforts (0-10s)
FATIGUE CAUSE:
CP Depletion
OPTIMAL RECOVERY:
Passive Recovery
Activity: High Intensity (10-75s)
FATIGUE CAUSE:
H+ Ion Accumulation
OPTIMAL RECOVERY:
Active Recovery
Activity: Long Duration Endurance (>90 mins)
FATIGUE CAUSE:
Glycogen Depletion
OPTIMAL RECOVERY:
Nutritional Strategies
