The system with no time limit, and how all three work at once

The aerobic system completes the breakdown that glycolysis begins. Where the glycolytic system stops early and leaves most of the fuel molecule intact, the aerobic system takes the products all the way, inside the mitochondria, using oxygen — and the difference in what it extracts is not marginal.

Two features of that table deserve to be pulled out, because they explain most of what follows.

It depends on other systems, which is why it is slow to start. Phosphocreatine is already in the fibre and glucose is already in the muscle; oxygen is not. It has to be breathed in, loaded onto haemoglobin, pumped through the circulation and diffused into the fibre — which is Lessons 4, 5 and 7 doing their work before a single aerobic ATP can be made. The aerobic system is the only energy system that cannot be faster than the respiratory and circulatory systems supplying it.

Its fatigue is a supply problem, not a chemistry problem. Nothing accumulates to a damaging level. What fails is that the tank empties, the fluid runs low, or the body overheats — all of which are addressable by planning, which is exactly why marathon runners are so preoccupied with fuelling, drinking and pacing, and sprinters are not.

The aerobic system can burn either fat or carbohydrate, and it does not choose at random. The mix shifts with intensity.

Read the last two rows together and Card 01 answers itself. Fat is the bigger tank but the narrower pipe. As intensity rises, the rate at which energy is needed exceeds the rate at which fat can be broken down and supplied, so the body shifts progressively towards carbohydrate — which it can use faster and which costs less oxygen per unit of energy released.

So at 32 kilometres the marathon runner has not run out of energy. They have run out of the only fuel that can be metabolised fast enough to hold race pace. Glycogen is gone; fat remains in abundance, but fat alone cannot support the required rate, so the pace collapses to one that fat can support. The runner does not stop moving. They slow dramatically, which is exactly what "the wall" looks like.

The aerobic system's dependence on delivery creates a problem at both ends of a session.

At the start there is an oxygen deficit. Demand rises the instant you begin; supply cannot. Heart rate, stroke volume and ventilation take a minute or two to rise to the level the work requires, so for that period oxygen delivery lags behind oxygen demand, and the shortfall is covered by the two anaerobic systems. This is why the first few minutes of a run feel disproportionately hard, why they feel easier once you settle, and why a warm-up is physiologically useful rather than merely traditional: it raises heart rate, ventilation and muscle temperature in advance, reducing the deficit when the real work starts.

Once supply catches demand the athlete is in a steady state, and the activity is sustainable — by definition, since the energy is being supplied as fast as it is being used. Raise the intensity above what oxygen delivery can cover and there is no steady state available; the anaerobic contribution keeps rising, by-products accumulate, and the effort now has a time limit.

At the end, oxygen consumption stays high. You stop running and keep breathing hard for minutes. This is excess post-exercise oxygen consumption, and it is the answer to the second half of Card 01. The extra oxygen is being used to:

• resynthesise the phosphocreatine that Lesson 9 said was depleted, which is the largest early component;
• restore the small store of ATP in the muscle;
• metabolise the lactate produced, converting it back to glucose or oxidising it for energy;
• reload oxygen onto myoglobin within the muscle and haemoglobin in the blood;
• support the elevated heart rate, ventilation, body temperature and hormone levels, which return to resting values gradually rather than instantly.

The older name for this was the oxygen debt, and the change of name is meaningful. Recovery is not the absence of activity; it is a set of processes that require energy and oxygen of their own. That is the reason an active recovery — light jogging or easy cycling — clears by-products faster than sitting down: blood flow to the muscles stays high, and active tissue consumes the lactate, whereas sitting down reduces the very circulation the recovery depends on.

The syllabus asks for the interplay of the energy systems, and it is a separate requirement from analysing the three of them. Three statements define it, and all three are examinable.

• All three systems operate at all times. There is no activity supplied by one system alone. Even a two second maximal lift produces a measurable rise in blood lactate, and even sitting still uses a small amount of phosphocreatine turnover.
• What changes is the proportion each contributes. The correct language is predominance, not switching. "The ATP-PCr system predominates for the first ten seconds" is right; "the ATP-PCr system is used for the first ten seconds, then the glycolytic system takes over" is the relay-race error.
• The proportions are set by intensity and duration together, and they move continuously. Every change of pace in a match changes the mix within seconds, in both directions.

Applied to real activity, the interplay takes two characteristic shapes.

That final cell is the most useful sentence in the lesson, and it settles a question that puzzles a lot of students. Why does a footballer need aerobic fitness when nothing they do lasts more than about six seconds? Because the aerobic system is what recovers them between those six second efforts. A better-conditioned player restores phosphocreatine faster and clears by-products faster in the walking and jogging periods, so they arrive at the next sprint with more of the fast fuel available. Aerobic fitness in an intermittent sport is not about the running; it is about the recovery between the running — and it shows up as the ability to repeat efforts in the last ten minutes rather than as a faster single sprint.

Everything from Lesson 9 and this lesson, in the form an exam question will ask for it.

With that table, several questions that looked separate turn out to be the same question:

• Why can nobody sprint a marathon? Because the fast systems have small capacities, and the system with the capacity cannot supply energy quickly enough for sprint pace.
• Why does a 400 metre runner suffer more than a 100 metre runner? Because the duration falls in the glycolytic window, so fatigue is by accumulation rather than depletion.
• Why does an endurance athlete eat during the event and a sprinter does not? Because glycogen depletion is the endurance limiter, and nothing can be eaten quickly enough to matter over ten seconds.
• Why does a team sport athlete train aerobically? Because the aerobic system restores the anaerobic ones between efforts.
• Why do coaches prescribe rest intervals so precisely? Because the rest period, not the work, determines which system is trained — the point Lesson 9's Activity 2 measured.

Choose a sport or event you play. If it is a team sport, choose a single position, because a goalkeeper and a midfielder have different profiles in the same game.

Then compare with a partner who plays a different sport or position. Where the two profiles differ most, explain the difference from the timings alone — not from the reputations of the sports involved.

Each of the following is something a real coach, teammate or online source has said. For each: decide whether it is right, wrong, or right for the wrong reason; correct it using this lesson and Lesson 9; and say what you would tell the athlete instead.

Finally, the harder question: each of these pieces of advice contains something that is true. Identify the grain of truth in each one, and explain what has gone wrong in getting from that truth to the recommendation. This is a better test of understanding than simply labelling them wrong, and it is closer to what you will actually have to do when someone gives you advice in a change room.