Introductory exercise physiology presents energy systems as a relay: ATP-PCr runs for 10 seconds, hands the baton to glycolysis which runs for 2 minutes, then oxidative metabolism takes over for the remaining distance. This model is pedagogically convenient and biologically inaccurate. All three systems are active from the first second of exercise. Only the relative contribution shifts as intensity and duration change.
---
The Three Systems and Their Characteristics
ATP-Phosphocreatine (ATP-PCr) — Alactic Anaerobic System:
- Energy source: stored ATP + phosphocreatine (PCr) hydrolysis
- Peak power output: ~1,200–1,500 W in trained athletes
- Time to peak: 1–2 seconds
- Duration at maximum contribution: 0–10 seconds
- ATP resynthesis rate: highest of all three systems
- Byproduct: none (alactic — no lactate or H⁺ produced)
- Recovery: PCr resynthesis requires 3–5 minutes for near-complete restoration
- Energy source: glucose and glycogen → pyruvate → lactate
- Peak power: 600–900 W
- Dominant contribution: approximately 10–120 seconds of maximal effort
- ATP resynthesis rate: faster than oxidative, slower than PCr
- Byproduct: H⁺ (acidosis), inorganic phosphate — responsible for peripheral fatigue in efforts of 30–90 seconds
- Can sustain high power output for 1–2 minutes before acidosis becomes limiting
- Energy source: glucose, glycogen, free fatty acids, amino acids (minor)
- Peak power contribution: 200–400 W (limited by O₂ delivery and mitochondrial density)
- Onset: active within seconds; dominant contributor from approximately 75 seconds of maximal exercise (Gastin, 2001)
- Duration: unlimited substrate at submaximal intensities
- Byproduct: CO₂ and H₂O — removed by respiratory and renal systems
The Gastin Correction: Oxidative Dominance at 75 Seconds
A landmark review by Paul Gastin (2001, *Sports Medicine*) re-examined the energy system contribution data and found that the textbook models dramatically underestimated oxidative contribution at short durations. Using oxygen deficit methodology and published exercise data:
- At 10 seconds maximal effort: oxidative system already contributes approximately 15–25% of total energy
- At 30 seconds (e.g. Wingate sprint): oxidative contributes approximately 35–45% (not 5–10% as commonly taught)
- At 75 seconds: oxidative metabolism becomes the dominant contributor (>50%) of total energy
- At 2 minutes: oxidative provides approximately 70% of energy for maximal effort
---
Why This Changes Training Programme Design
For 400m runners and 400m swimmers: These athletes need both a high glycolytic capacity (for the race's second half) and a substantial aerobic base (which supplies 35–55% of energy and clears lactate between races in multi-round tournaments). Pure glycolytic training without aerobic base development will limit performance in both the individual effort and tournament recovery.
For team sport athletes (rugby, football, basketball, hockey): Efforts of 3–15 seconds are common throughout match play — but there are hundreds of them. The cumulative energy demand of repeated sprint activity is predominantly aerobic — the aerobic system is what determines recovery speed between sprints and total sprint count over 80 minutes. Building VO₂max in team sport athletes is not "endurance training" — it is repeat-sprint recovery training.
For combat sports: A 3×5-minute round format at maximal effort draws approximately 65–70% of energy from oxidative metabolism over the bout. An athlete who trains exclusively on "anaerobic" circuits is leaving the majority of their energy system undertrained.
---
Pacing, Race Prediction, and Energy System Planning
For endurance athletes, understanding energy system crossover points provides the theoretical basis for pacing strategy: the point at which glycolytic rate must drop (to prevent H⁺ accumulation exceeding buffering capacity) determines critical speed or power — the maximal intensity sustainable without progressive lactate accumulation.
For runners modelling race pace across distances — and calculating optimal split strategies that keep effort within the aerobic-dominant zone for target race distances — the marathon race predictor at winsport.uk/tools/performance/marathon-race-predictor estimates race times across distances from existing performance data using the Riegel model, providing the framework for energy-system-appropriate pacing targets.
In your coaching — do you design energy system training based on event-specific contribution data, or does training zone allocation still follow the simplified relay model that underestimates aerobic contribution in short-duration events?