Energy systems: big idea
· The body uses three energy systems to resynthesize ATP for life and physical activity: phosphagen system, glycolytic system and oxidative system.
· ATP is the immediate energy currency for muscle contraction; the body must constantly resynthesize it because stores are limited.
· The systems differ in fuel source, rate of ATP production, capacity, recovery needs, benefits and limitations.
· They work on an energy continuum: all systems contribute at all times, but the dominant system changes with exercise intensity and duration.
· Exam focus: match the activity demands to the dominant energy system and justify using intensity, duration and recovery.
This graph shows the energy continuum clearly: the phosphagen system dominates early, glycolysis rises after the initial burst, and oxidative metabolism becomes more important as exercise continues. Use it to explain why different activities rely on different dominant energy systems rather than one system working alone. Source
Phosphagen system
· Also called the ATP-PC or ATP-phosphocreatine system.
· Main fuel/source: stored ATP and phosphocreatine (PCr/CrP) in muscle.
· Dominates in very short, maximal/high-intensity efforts, especially sudden bursts or rapid increases in intensity.
· Benefits: fastest ATP resynthesis, no oxygen required, ideal for explosive power.
· Limitations: very limited capacity because ATP and PCr stores are small; fatigue occurs quickly if intensity is maintained.
· Recovery: requires time to restore ATP-PC stores, so repeated sprints need appropriate rest.
· Example activities: 100 m sprint start, shot put, jumping, maximal lift, short acceleration in games.
Glycolytic system
· Also called anaerobic glycolysis when operating without enough oxygen for full aerobic metabolism.
· Main fuel/source: carbohydrate, especially glucose or muscle glycogen.
· Dominates during short to moderate duration, high-intensity activity when ATP demand is too high for the oxidative system alone.
· Benefits: produces ATP quickly and supports intense work after the phosphagen system declines.
· Limitations: lower ATP capacity than the oxidative system; associated with increased lactate and hydrogen ion accumulation during high-intensity work.
· Important exam point: lactate is not simply “waste”; it can be metabolized when production and removal are balanced.
· Example activities: 200–400 m sprint, repeated high-intensity game efforts, hard intervals.
This diagram shows glycolysis as the breakdown of glucose into pyruvate, helping students connect carbohydrate fuel to ATP production. IB students do not need detailed enzyme steps for A.2.3, but the visual supports the idea that glycolysis is a pathway for rapid ATP resynthesis. Source
Oxidative system
· Also called the aerobic system or oxidative phosphorylation.
· Main fuel/source: mainly carbohydrates and lipids; proteins contribute much less in normal exercise contexts.
· Requires oxygen and is dominant at rest and during extended periods of submaximal intensity.
· Benefits: largest ATP capacity, supports long-duration activity and recovery between intense efforts.
· Limitations: slower rate of ATP production than anaerobic systems, so it cannot meet sudden maximal energy demand alone.
· Example activities: distance running, cycling, swimming endurance events, steady-state exercise.
· IB boundary: detailed biochemistry of the Krebs cycle and electron transport chain is not assessed.
This labelled educational diagram shows how oxygen-linked oxidative phosphorylation produces ATP in mitochondria. For IB SEHS, use it only to understand the broad idea that the oxidative system depends on oxygen and has high ATP capacity; detailed Krebs cycle and electron transport chain steps are not assessed. Source
Energy continuum and activity analysis
· The energy continuum describes how the relative contribution of each energy system changes with activity demands.
· Key variables for exam answers: duration, intensity, rest/recovery periods, fuel availability and whether the activity involves steady-state or repeated bursts.
· Short + maximal intensity = greater reliance on phosphagen and anaerobic glycolysis.
· Long + submaximal intensity = greater reliance on the oxidative system.
· Team and intermittent sports require all systems because athletes repeatedly sprint, jog, recover and change intensity.
· High-scoring exam phrasing: “The dominant system is ___ because the activity is ___ intensity and lasts approximately ___, but the other systems still contribute.”
VO₂ max
· VO₂ max = the maximum rate at which the body can take in, transport and use oxygen during intense exercise.
· It is a key indicator of aerobic fitness and helps explain endurance performance.
· Influenced by age, sex differences, body composition, lifestyle factors and level of fitness.
· Higher VO₂ max generally supports better endurance potential, but performance also depends on efficiency of movement.
· Example: running economy affects endurance performance because two athletes with similar VO₂ max values may use different amounts of energy at the same pace.
· Exam link: endurance performance = not just VO₂ max; include movement efficiency/economy for stronger answers.
HL only: lactate inflection point (LIP)
· Lactate inflection point (LIP) = the maximum intensity at which the body can metabolize lactate at the same rate as it is produced.
· Below LIP: lactate production and clearance are balanced, so exercise can usually be sustained for longer.
· Above LIP: lactate production exceeds clearance, so lactate and associated acidity rise more quickly, contributing to fatigue.
· A higher LIP usually indicates better ability to sustain a higher exercise intensity aerobically.
· Practical interpretation: on a blood lactate graph, identify where lactate begins to rise more steeply as intensity increases.
· Exam link: explain LIP using the balance between lactate production and lactate removal/metabolism, not just “lactate appears”.
HL only: excess post-exercise oxygen consumption (EPOC)
· EPOC = the extra oxygen consumed after exercise to help the body return to homeostasis.
· EPOC depends on the oxygen deficit incurred during exercise: higher intensity usually creates a larger oxygen deficit and greater EPOC.
· Fast component: early recovery processes such as replenishing ATP-PC stores and restoring oxygen stores.
· Slow component: longer recovery processes linked to elevated temperature, ventilation, heart rate, hormones and metabolic disturbance.
· EPOC is greater after high-intensity exercise than after low-intensity steady exercise of similar duration.
· Exam skill: identify oxygen deficit, EPOC, fast component and slow component on an oxygen uptake graph.
Common exam comparisons
· Phosphagen vs glycolytic: both are anaerobic, but phosphagen is faster and shorter-lasting, while glycolytic supports intense exercise for longer.
· Glycolytic vs oxidative: glycolytic is faster and suits high intensity; oxidative is slower but has much greater capacity for sustained work.
· VO₂ max vs LIP: VO₂ max reflects maximal oxygen use; LIP reflects the highest intensity where lactate production and clearance are balanced.
· Oxygen deficit vs EPOC: oxygen deficit occurs when oxygen uptake initially cannot meet demand; EPOC is the recovery oxygen needed after exercise.
· Dominant system vs exclusive system: never say only one system is used; say the system is dominant.
Checklist: can you do this?
· Identify the dominant energy system from an activity’s intensity and duration.
· Compare fuel sources, ATP production rate, capacity, benefits and limitations of the three systems.
· Explain how the energy continuum changes from rest to high-intensity and prolonged exercise.
· Interpret the role of VO₂ max and movement efficiency in endurance performance.
· For HL, interpret LIP and EPOC graphs using correct terminology.
High-grade exam sentence starters
· “The dominant energy system is likely to be ___ because the activity is ___ intensity and lasts ___.”
· “Although ___ is dominant, all three energy systems contribute along the energy continuum.”
· “The oxidative system is suited to this activity because it has a high ATP capacity and supports prolonged submaximal work.”
· “The athlete’s endurance performance depends on VO₂ max and efficiency of movement, such as running economy.”
· “The lactate inflection point occurs when lactate production begins to exceed the body’s ability to metabolize it at the same rate.”
· “EPOC reflects the oxygen required after exercise to restore homeostasis following the oxygen deficit.”