ATP — the energy currency

Every muscle contraction is powered by adenosine triphosphate (ATP). When ATP loses a phosphate it becomes ADP and releases energy:

The body stores only enough ATP for about 2–3 seconds of maximal work, so it must constantly be resynthesised (ADP + P_i → ATP). Three energy systems rebuild ATP, and which one dominates depends on the intensity and duration of the activity.

Key idea: the systems never work alone — they overlap on the energy continuum. We label an activity by its predominant system.

The ATP-PC system

For short, explosive efforts the muscle uses stored phosphocreatine (PC) in the sarcoplasm. The enzyme creatine kinase breaks the PC bond and the released energy resynthesises ATP:

• Type: anaerobic and alactic — no lactic acid is produced.
• Duration: peaks for ~8–10 seconds until PC stores are depleted.
• Used for: maximal power — 100 m sprint, shot put, a smash in tennis.

Recovery: PC stores are fully replenished within ~2–3 minutes of rest using oxygen (the fast component of EPOC).

The anaerobic glycolytic system

Once PC runs low, muscle glycogen is broken to glucose and split anaerobically in glycolysis (in the sarcoplasm), controlled by the enzyme phosphofructokinase (PFK):

• By-product: lactic acid (lactate + H⁺), which lowers muscle pH and causes fatigue.
• Duration: dominant from ~10 seconds up to ~3 minutes; peak power at around 1 minute.
• Used for: the 400 m sprint, a 100 m swim, repeated high-intensity efforts.

Net vs gross: glycolysis uses 2 ATP to start and produces 4, so the net yield is 2 ATP per glucose.

Which system, which by-product?

The aerobic system

When oxygen is available, glucose (and later fats) are fully broken down in three stages for a huge ATP yield:

• Glycolysis (sarcoplasm): glucose → pyruvate, net 2 ATP — the same first step, but now aerobic.
• Krebs cycle (mitochondrial matrix): pyruvate is oxidised, producing 2 ATP, CO₂ and hydrogen carriers.
• Electron transport chain (cristae of mitochondria): hydrogen is oxidised to water, yielding 34 ATP.

Fuel: at low–moderate intensity fats (fatty acids, via beta-oxidation) yield even more ATP per molecule, but need more oxygen. By-products are only CO₂ and water, so long-duration work (marathon) is possible.

Which energy system?

Tap a characteristic, then tap the energy system it belongs to.

The energy continuum & thresholds

All three systems run at once; the predominant one changes with intensity and duration. A threshold is the point where one system hands over to the next:

• ATP-PC / lactic threshold: where PC is exhausted (~8–10 s) and the glycolytic system takes over.
• Lactic / aerobic threshold: where lactate levels off and the aerobic system dominates (after ~2–3 min).
• Lactate (anaerobic) threshold: the exercise intensity at which blood lactate begins to rise sharply — trained athletes reach it at a higher % of VO₂ max.

OBLA: the onset of blood lactate accumulation is usually taken at a blood lactate concentration of about 4 mmol·L⁻¹.

Where does it happen?

EPOC & the oxygen debt

EPOC — excess post-exercise oxygen consumption — is the extra oxygen taken in after exercise, above resting levels, to restore the body to its pre-exercise state. It has two components:

• Fast (alactacid) component: lasts ~2–3 minutes. Resynthesises ATP and PC stores and re-saturates myoglobin with oxygen. Needs ~1–4 litres of O₂.
• Slow (lactacid) component: can last hours (up to 24–48 h). Removes lactic acid (oxidised to CO₂ + H₂O, or reconverted to glycogen via the Cori cycle), and covers the raised heart rate, breathing, temperature and hormone levels.

Oxygen debt is the older term for the volume of oxygen consumed in recovery above what is needed at rest — essentially the EPOC.

Match each term to its meaning

Tap a term on the left, then its matching definition on the right.

VO₂ max & the factors affecting it

VO₂ max is the maximum volume of oxygen that can be taken in, transported and used by the body per minute during exhaustive exercise. It is the best single measure of aerobic capacity.

• Absolute VO₂ max is in litres/min (or ml/min); relative VO₂ max divides by body mass to give ml·kg⁻¹·min⁻¹, allowing fair comparison between performers.
• Factors that raise it: aerobic training (↑ by ~10–20%), youth, larger heart/stroke volume, more haemoglobin, greater capillary and mitochondrial density.
• Factors that lower it: increasing age, sedentary lifestyle/smoking, and (on average) smaller body size — females typically record lower absolute values than males.

Test: the multi-stage fitness test (bleep test) predicts VO₂ max; direct measurement uses a gas analyser during a maximal treadmill test.

Your turn — relative VO₂ max

Maximum heart rate & training zones

To set the intensity of aerobic training, coaches estimate maximum heart rate and work at a percentage of it:

Training near the upper end approaches the anaerobic/lactate threshold, improving the ability to work hard before lactate accumulates. The lower end builds an aerobic base.

Remember: 220 − age is an estimate that ignores individual variation, but AQA expects you to use it to set target zones.

Your turn — maximum heart rate

Your turn — target intensity

Training methods & principles

Methods are chosen to match the energy system being developed:

• Continuous: steady, sustained aerobic work (30+ min, no rest) — develops aerobic capacity/VO₂ max.
• Interval / HIIT: bouts of high-intensity work with recovery periods — develops both anaerobic and aerobic systems.
• Fartlek ("speed play"): continuous running with varied pace and terrain — trains aerobic and anaerobic together, ideal for games players.
• Plyometric: bounding, hopping and depth jumps using the stretch-shortening cycle (eccentric then concentric) — develops explosive power.

Principles guide programme design: SPORT (Specificity, Progression, Overload, Reversibility, Tedium) and FITT (Frequency, Intensity, Time, Type). Periodisation divides the year into macro-, meso- and microcycles to peak for competition.

Choose the method

The fast component

The big ideas to know

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