Lesson 9 · What factors influence movement and performance?
Ten seconds, then a minute: the two systems that work without oxygen
A muscle holds about two seconds' worth of usable energy. Two seconds — less than a single sprint stride cycle. Everything beyond that is rebuilt while you move, and the body has three different ways of rebuilding it, each with a different fuel, a different speed and a different price. This lesson takes the two that work without oxygen: the one that powers a shot put and the one that makes a 400 metre race hurt more than any other event in athletics. It also settles the most persistent piece of misinformation in sport science, which almost every student arrives believing.
Watch the last twenty metres of any 100 metre final. Nobody is accelerating. Every finalist is slowing down, and the winner is the one who slows least.
These are the fastest people on earth, running for under ten seconds, and they still cannot hold top speed to the line. What has run out? It is not oxygen — you can hold your breath for far longer than ten seconds. Write down what you think it is. Then a second question, about a different event: why do 400 metre runners so often stagger, sit down or vomit at the finish when 100 metre runners almost never do, even though the 400 is run more slowly?
- Explain why ATP is the only fuel a muscle can actually use, and why the store is so small.
- Analyse the ATP-PCr system by fuel source, efficiency of ATP production, duration, intensity, rate of recovery and cause of fatigue.
- Analyse the glycolytic system against the same six criteria.
- Compare the two systems directly, and explain the trade-off between speed and yield that separates them.
- Explain what actually causes fatigue in each system, and why lactic acid is not the villain it is usually said to be.
- Match a sporting activity to the system that predominates in it, and justify the match from duration and intensity.
- ATP — adenosine triphosphate, the molecule whose breakdown releases the energy a muscle fibre uses to contract. The only immediately usable form of energy in the body.
- Energy system — a pathway that resynthesises ATP from another fuel so contraction can continue.
- Anaerobic — without oxygen.
- Aerobic — requiring oxygen.
- Phosphocreatine — a high-energy compound stored in small amounts in muscle, used to rebuild ATP almost instantly. Also written as creatine phosphate, and abbreviated PCr.
- Glycolysis — the breakdown of glucose to release energy; anaerobic glycolysis does this without oxygen.
- Lactate — the compound formed when glycolysis proceeds rapidly without oxygen. It is a usable fuel, not a waste product.
- Acidosis — the fall in muscle pH caused by accumulating hydrogen ions, which impairs contraction.
- Fatigue — the decline in the ability to produce force, whatever its cause.
- Rate of recovery — how quickly a system's capacity is restored once the effort stops.
- Yield — the amount of ATP produced from one unit of fuel.
- Delayed onset muscle soreness — the stiffness and tenderness appearing a day or two after unaccustomed exercise, caused by muscle damage and inflammation.
Muscle fibres cannot burn glucose, and they cannot burn fat. They can only use ATP. Every other fuel in the body is a way of making more of it.
ATP is adenosine bonded to three phosphate groups. Breaking the bond to the third phosphate releases energy:
ATP → ADP + P + energy, where ADP is adenosine diphosphate and P is the released phosphate group.
That energy drives the contraction described in Lesson 2. The catch is the size of the store. A muscle holds roughly one to two seconds' worth of ATP at maximal effort. Not minutes. Not one tenth of a race. About the length of a single stride cycle.
So the body does not store ATP, it rebuilds it — continuously, at the rate it is being used, by reattaching a phosphate to ADP. An energy system is simply a method of doing that, and the body has three because no single method is good at everything:
- The ATP-PCr system — extremely fast, tiny capacity, no oxygen required.
- The glycolytic system — fast, moderate capacity, no oxygen required, but it accumulates something.
- The aerobic system — slow, enormous capacity, oxygen required. This is Lesson 10.
The organising idea for the whole topic is a trade-off, and it is worth writing down now: the faster a system can supply ATP, the less total ATP it can supply. Nothing in this lesson or the next contradicts that sentence, and almost every question about energy systems is an application of it.
Why "the body switches from one system to the next" is wrong, and what to say instead
All three systems operate at all times. What changes is which one is contributing most, and the changeover is a gradual shift in proportion, not a switch being thrown. Saying that the ATP-PCr system "stops after ten seconds and the glycolytic system takes over" describes a relay race that does not happen; the accurate version is that PCr's contribution peaks in the first few seconds and falls away as glycolysis rises to meet the demand. Examiners see the relay version constantly, and it is the single clearest marker of a memorised rather than an understood answer.
Quick check: Why does the body need energy systems at all, rather than simply storing more ATP?
Stored alongside ATP in the muscle is phosphocreatine, a molecule whose only job is to hold a phosphate group ready to donate. When ATP is broken down, phosphocreatine hands its phosphate straight to the ADP:
PCr + ADP → ATP + creatine
That is a single reaction, in the muscle fibre itself, requiring no oxygen and no delivery of anything from anywhere. It is the reason this system is quick: there is nothing to wait for.
| Criterion | ATP-PCr system |
|---|---|
| Fuel source | Phosphocreatine stored within the muscle fibre |
| Efficiency of ATP production | One ATP per phosphocreatine molecule — a very low yield, but produced faster than by any other pathway |
| Duration | Around 10 to 12 seconds at maximal effort |
| Intensity | Maximal, 95 to 100 per cent of capacity |
| Rate of recovery | Fast: roughly half the store restored in about 30 seconds, and near-complete restoration in 2 to 3 minutes of rest |
| Cause of fatigue | Depletion of phosphocreatine. Nothing accumulates and nothing becomes acidic — the fuel simply runs out |
This is the answer to the first half of Card 01. The sprinter in the last twenty metres has not run out of oxygen, and is not full of anything. They have run out of phosphocreatine. Maximal speed is unsustainable because its power source has a ten second tank, which is also why the world record for 100 metres has always been run with a decelerating final phase and always will be.
Two consequences worth noticing:
- Recovery is fast because restoration is simple. Rebuilding phosphocreatine needs energy, which the aerobic system supplies during the rest period — so even this most anaerobic of systems depends on aerobic fitness for its recovery. A well-conditioned athlete restores PCr faster between repeated sprints, which is why aerobic training improves repeat-sprint ability in sports that look purely anaerobic.
- The system is defined by the rest as much as the work. A training set of six second sprints with three minutes rest trains the ATP-PCr system; the identical sprints with twenty seconds rest do not, because PCr has not been restored and the glycolytic system is forced to supply the rest.
True or false: Fatigue in the ATP-PCr system is caused by the accumulation of lactic acid in the working muscle.
When phosphocreatine can no longer keep up, the muscle turns to the fuel it has far more of: glucose, circulating in the blood and stored as glycogen in the muscle itself. Breaking glucose down releases energy in a sequence of reactions called glycolysis, and the early steps of that sequence do not require oxygen.
The trade is immediately visible. Glycolysis takes many steps rather than one, so it is slower to respond than the phosphocreatine reaction — but there is a great deal more glucose than there ever was phosphocreatine, so it lasts much longer.
| Criterion | Glycolytic system |
|---|---|
| Fuel source | Glucose from the blood, and glycogen stored in the muscle and liver |
| Efficiency of ATP production | Two ATP per glucose molecule when no oxygen is used — twice the yield of a phosphocreatine molecule, but only a small fraction of what the same glucose yields aerobically |
| Duration | The predominant supplier from roughly 10 seconds to around 2 minutes, with its peak contribution in the first minute |
| Intensity | High but submaximal, around 85 to 95 per cent of capacity |
| Rate of recovery | Slow: 30 minutes to an hour to clear the accumulated by-products, and considerably longer, up to a day or more, to fully replace the glycogen used |
| Cause of fatigue | Accumulation of hydrogen ions, which lowers the pH inside the muscle and interferes with the enzymes and the calcium binding that contraction depends on |
Now the second half of Card 01. A 400 metre runner finishes in a far worse state than a 100 metre runner precisely because the race is slower. At 100 metre pace the event is over before glycolysis has contributed much; at 400 metre pace the athlete runs near-maximally for 45 to 60 seconds, which is exactly the window in which the glycolytic system supplies most of the energy. They finish with the muscle's chemistry substantially disturbed, and that is what produces the staggering, the nausea and the several minutes of genuine distress. The 100 metre runner simply ran out of fuel and stopped; the 400 metre runner ran the system into the ground.
What "lactic acid" actually is, and why the name is a problem
Rapid glycolysis without oxygen produces lactate and hydrogen ions. The traditional name for this pathway is the lactic acid system, and the syllabus uses it, so you should too — but be clear about what is doing the damage. It is the hydrogen ions, not the lactate, that impair contraction, by lowering pH so that the enzymes driving glycolysis work less well and calcium binds less effectively to the contractile proteins. Lactate itself is a useful fuel: it is shuttled to other muscle fibres, to the heart and to the liver, where it is converted back to glucose or oxidised for energy. This is why an athlete's lactate rises during hard work and falls again within the hour, and why "flushing out the lactic acid" is a description of something that was never poison in the first place.
Match each feature to the system it belongs to.
- ATP-PCr fuel source
- Glycolytic fuel source
- ATP-PCr cause of fatigue
- Glycolytic cause of fatigue
- Phosphocreatine stored within the muscle fibre
- Glucose from the blood and glycogen stored in muscle and liver
- Depletion of the stored fuel, with nothing accumulating
- Accumulating hydrogen ions lowering muscle pH and impairing contraction
Placed against each other, the two anaerobic systems demonstrate the trade-off from Card 04 in its purest form.
| ATP-PCr | Glycolytic | |
|---|---|---|
| Speed of ATP supply | Fastest available | Fast, but slower to respond |
| Total capacity | Very small | Moderate |
| Oxygen required | No | No |
| By-product | None of consequence | Lactate and hydrogen ions |
| Recovery | 2 to 3 minutes | 30 minutes to an hour, longer for glycogen |
| Typical events | Shot put, 100 metres, a single tackle, a gymnastics vault, a maximal lift | 400 metres, 100 metre swim, a long rally, a repeated sprint burst in a team sport |
The handover between them deserves a precise description, because it is where the marks are.
The demand is what stays constant, not the supply. A sprinter running flat out needs ATP at a certain rate throughout. In the first few seconds phosphocreatine supplies nearly all of it. As the store depletes, its contribution falls — and the shortfall is made up by glycolysis, which has been running all along and now rises to fill the gap. Neither system is switched on or off. The contribution of each changes continuously, and the total simply has to add up to what the movement requires.
Two things follow from that, and both are examinable:
- The lactic system's rise is a consequence of PCr's decline, not a decision. This is why hydrogen ions begin accumulating well before the ten second mark in a maximal effort, and why even a 100 metre sprint produces a measurable rise in blood lactate.
- If the intensity drops, the mix changes again, because the aerobic system can now cover a larger share. A footballer who sprints, then jogs, then sprints again is moving up and down this continuum several hundred times a match, which is what Lesson 10's interplay section is about.
Two truths and a lie. Which statement is false?
The syllabus asks for causes of fatigue, plural, and they are genuinely different between the two systems. That difference is the most reliable way to tell a strong answer from a memorised one.
| ATP-PCr | Glycolytic | |
|---|---|---|
| What goes wrong | The fuel is used up | The internal environment is disturbed |
| Mechanism | Phosphocreatine stores are depleted, so ATP cannot be rebuilt at the required rate | Hydrogen ions accumulate, lowering pH, inhibiting the enzymes of glycolysis and interfering with calcium binding in the contractile process |
| What the athlete feels | Power simply falls away; no particular discomfort | Burning, heaviness, loss of coordination, and often nausea |
| How it resolves | Minutes of rest | Tens of minutes, and faster with light activity than with sitting still |
Now three claims that are repeated constantly in gyms, in change rooms and in poorly written notes. Each one is false, and each one appears in student answers every year.
- "Lactic acid causes muscle soreness the next day." It does not. Lactate is cleared within roughly an hour of exercise finishing, while delayed onset muscle soreness peaks 24 to 48 hours later — long after the lactate has gone. Soreness is caused by microscopic damage to the muscle fibres and the inflammation that follows, and it is produced overwhelmingly by eccentric contractions, which is why running downhill and lowering heavy weights leave you sore while cycling uphill does not. The timings alone disprove the claim.
- "Lactic acid is a waste product that has to be flushed out." Lactate is a fuel. It is transported to other muscle fibres, to the heart and to the liver, and either oxidised for energy or converted back to glucose. The reason a light cool-down clears it faster than sitting down is that active tissue consumes it.
- "You can train to stop producing lactic acid." Training does not stop production; it improves the ability to buffer hydrogen ions, to clear and reuse lactate, and to supply more of the energy aerobically at any given speed. The trained athlete produces lactate at a higher work rate, not at no work rate — which is a considerably more useful thing to say in an exam.
Fill the gaps. Type one word in each box.
The fuel used by the ATP-PCr system is stored in the muscle as . Fatigue in the glycolytic system is caused by the accumulation of hydrogen ions, which lowers the muscle's . Muscle soreness felt a day or two after unaccustomed exercise is caused by muscle and the inflammation that follows it.
Text description
- Above, ATP breaking down to ADP and phosphate at the top, with the ATP-PCr system and the glycolytic system side by side beneath it and an arrow from each pointing up into it: ATP-PCr rebuilds ATP from phosphocreatine and glycolysis rebuilds it from glucose and glycogen, with their yields and causes of fatigue labelled.
- Below, a graph with time from zero to two minutes along the horizontal axis and share of ATP supplied up the vertical axis, showing the ATP-PCr contribution peaking in the first seconds and falling away while the glycolytic contribution rises to fill the gap, with the two shaded areas overlapping rather than meeting at a boundary.
Activities · put the method to work
For each of the following, name the predominant energy system and justify it from the duration and the intensity, in that order. The word "predominant" is doing real work: no effort uses only one system, so an answer that says "ATP-PCr" and stops has not answered.
A shot put, from the start of the glide to the release.
A 400 metre freestyle swim, in about four and a half minutes.
A 45 second wrestling exchange at near-maximal effort.
Six maximal 30 metre sprints with three minutes rest between each.
The same six sprints, but with only fifteen seconds rest between each.
A single maximal deadlift.
A 90 second high-intensity rally in a badminton match.
Walking 3 kilometres to school.
Items 4 and 5 are the same sprints and are not the same task. Explain the difference in one sentence — it is the most useful sentence in this activity, and it is why coaches specify rest intervals as carefully as they specify the work.
Odd one out. Three of these are true of the glycolytic system. Which one is not?
This is a practical test of one claim from Card 05: that phosphocreatine is roughly half restored in 30 seconds and largely restored in two to three minutes. You need a stopwatch, a marked 30 metre course and a partner.
- Condition A. Run six maximal 30 metre sprints with three minutes of rest between each. Record every time.
- Condition B. On another day, run six maximal 30 metre sprints with twenty seconds of rest between each. Record every time.
- For each condition, calculate the drop-off: the difference between the fastest and the slowest sprint, as a percentage of the fastest.
Then the analysis, which is the point of the activity:
- Which condition showed the greater drop-off, and why? Answer in terms of phosphocreatine restoration, not in terms of tiredness.
- Which energy system was forced to supply more of the work in Condition B, and what would the athlete have felt as a result that they did not feel in Condition A?
- Which condition is the better training session? This is a trick question and the answer is "for what?" — Condition A develops maximal speed, Condition B develops the ability to repeat efforts with inadequate recovery, which is what a team sport actually demands. Say which sport you would prescribe each for.
- Name three things you did not control that could have affected the comparison, given the two conditions were run on different days.
Analyse the ATP-PCr and glycolytic energy systems, including fuel source, efficiency of ATP production, duration, intensity, rate of recovery and causes of fatigue.
Analyse means identify components and show the relationships between them. The question names six criteria, so the components are given — which means the marks are in the relationships, not in the six facts. The relationship to build the answer on is the trade-off: speed of supply against total capacity.
Plan: state the trade-off as a thesis; take each system across all six criteria; then compare directly and explain why the differences follow from the trade-off; close on how the two overlap in a real effort.
The ATP-PCr system uses phosphocreatine stored within the muscle fibre, which donates its phosphate directly to ADP in a single reaction requiring no oxygen, yielding one ATP per molecule of phosphocreatine. Because that reaction is immediate and requires nothing to be delivered, it supplies ATP faster than any other pathway, which suits it to maximal intensities of 95 to 100 per cent for about 10 to 12 seconds. Recovery is rapid, with roughly half the store restored within about 30 seconds and near-complete restoration within 2 to 3 minutes, and fatigue occurs simply through depletion of phosphocreatine, with no accumulation of by-products.
The glycolytic system breaks down glucose from the blood and glycogen stored in muscle and liver, producing two ATP per glucose molecule without oxygen. Because glycolysis requires a sequence of reactions rather than one, it responds more slowly, and it predominates at high but submaximal intensities of about 85 to 95 per cent for roughly 10 seconds to 2 minutes. Recovery is far slower, taking 30 to 60 minutes to clear by-products and up to a day to restore glycogen, and fatigue is caused not by fuel depletion but by accumulating hydrogen ions lowering muscle pH, inhibiting glycolytic enzymes and interfering with calcium binding in the contractile process.
The ATP-PCr and glycolytic systems both resynthesise ATP without oxygen, and the differences between them across all six criteria follow from a single trade-off: the faster a system can supply ATP, the less total ATP it can supply. The ATP-PCr system uses phosphocreatine stored within the muscle fibre itself, which donates its phosphate group directly to ADP in one reaction, yielding one ATP per phosphocreatine molecule. That yield is low, but because the reaction is immediate and nothing has to be delivered to the fibre, this system supplies ATP faster than any other pathway, making it the predominant source at maximal intensities of 95 to 100 per cent of capacity for approximately 10 to 12 seconds — the duration of a shot put, a maximal lift or a 100 metre sprint. Its recovery is correspondingly quick, with about half the store restored in 30 seconds and near-complete restoration in 2 to 3 minutes, using energy supplied aerobically during the rest period. Its fatigue has a single simple cause: the phosphocreatine store is depleted, so ATP can no longer be rebuilt at the required rate, and because nothing accumulates the athlete experiences a loss of power rather than discomfort. The glycolytic system uses a far larger fuel store — glucose from the blood and glycogen from muscle and liver — and yields two ATP per glucose molecule when no oxygen is used. Because glycolysis proceeds through a sequence of reactions rather than one, it responds more slowly and cannot support maximal intensity, predominating instead at 85 to 95 per cent of capacity from around 10 seconds to 2 minutes, which is why it supplies most of the energy for a 400 metre run or a 100 metre swim. Its recovery is much slower, requiring 30 to 60 minutes for by-products to be cleared and up to a day or more for glycogen to be replaced, and its fatigue arises from a completely different mechanism: the accumulation of hydrogen ions lowers pH within the muscle, inhibiting the enzymes that drive glycolysis and interfering with the binding of calcium in the contractile process, which the athlete experiences as burning, heaviness and loss of coordination. It is worth noting that lactate itself is not the cause of this fatigue and is not a waste product — it is shuttled to other fibres, the heart and the liver and used as a fuel. The two systems therefore differ in every criterion, and the differences are related rather than independent: the larger fuel store buys longer duration but only through a multi-step pathway, which costs speed and produces a by-product, which in turn explains both the lower sustainable intensity and the far slower recovery. In a real maximal effort the two overlap continuously, with the glycolytic contribution rising as the phosphocreatine contribution falls, so that the total supply matches a demand that has not changed — neither system is switched on or off at any point.
A 100 metre sprinter slows over the final twenty metres but recovers within minutes. A 400 metre runner finishes in obvious distress and takes far longer to recover. Explain the difference, referring to the energy systems.
Two athletes, two different predominant systems, and therefore two different causes of fatigue and two different recovery rates. The question is not about who worked harder, and an answer built on effort or fitness has missed it entirely.
Plan: duration of each event, which system therefore predominates, what fatigue mechanism that implies, what recovery rate that implies, and finally what each athlete visibly does.
A 100 metre race lasts about 10 seconds at maximal intensity, which is precisely the window in which the ATP-PCr system predominates. The slowing over the final twenty metres occurs because phosphocreatine stores are being depleted, so ATP cannot be resynthesised quickly enough to sustain maximal power. Because this system produces no significant by-products, the fatigue is a loss of power rather than a disturbance of the muscle's chemistry, and recovery is quick: about half the store is restored within 30 seconds and it is largely replenished within 2 to 3 minutes.
A 400 metre race lasts 45 to 60 seconds at near-maximal intensity, well beyond what phosphocreatine can supply, so the glycolytic system must supply the majority of the energy. Breaking glucose down without oxygen at that rate causes hydrogen ions to accumulate faster than the circulation can buffer or clear them, lowering muscle pH and impairing both the enzymes of glycolysis and the calcium binding on which contraction depends. This is a disturbance of the internal environment rather than an empty fuel tank, so it produces burning, heaviness, nausea and loss of coordination, and it takes 30 to 60 minutes to resolve.
The difference arises because the two events are supplied predominantly by different energy systems, which fail in different ways and recover at different rates. A 100 metre race lasts about 10 seconds at maximal intensity, which is the window in which the ATP-PCr system predominates, using phosphocreatine stored within the muscle fibre to rebuild ATP almost instantly. The sprinter slows over the final twenty metres because those stores are being depleted and ATP can no longer be resynthesised fast enough to maintain maximal power output. Crucially this system produces no by-product of consequence, so the fatigue is purely a loss of power rather than a disturbance of the muscle's internal chemistry, and recovery is rapid: roughly half the phosphocreatine store is restored within 30 seconds and it is largely replenished within 2 to 3 minutes, which is why a sprinter can contest a heat and a semi-final on the same afternoon. A 400 metre race, by contrast, lasts 45 to 60 seconds at near-maximal intensity, far beyond what phosphocreatine can supply, so the glycolytic system must supply the majority of the energy by breaking down glucose and muscle glycogen without oxygen. At that rate hydrogen ions accumulate faster than they can be buffered or removed by the circulation, lowering the pH within the muscle, inhibiting the enzymes that drive glycolysis and interfering with the binding of calcium on which contraction depends. This is a disturbance of the internal environment rather than an exhausted fuel store, which is why the 400 metre runner experiences burning, heaviness, nausea and loss of coordination, and why the visible distress at the finish is so much greater despite the race being run at a lower speed. Recovery is correspondingly slower, requiring 30 to 60 minutes for the accumulated hydrogen ions to be cleared and the muscle's pH to be restored, with glycogen replacement taking considerably longer still, and it is accelerated by light activity rather than by sitting down because active tissue consumes the lactate produced. The apparent paradox — that the slower race hurts far more — is therefore explained entirely by duration: the 400 metres is long enough to run the glycolytic system into the ground, and the 100 metres is over before it has the chance.
Multiple choice · 5 questions
MC 1. The fuel source for the ATP-PCr energy system is:
- A. glucose circulating in the blood
- B. phosphocreatine stored in the muscle
- C. fatty acids released from adipose tissue
- D. glycogen stored in the liver
MC 2. The ATP-PCr system predominates for approximately:
- A. 2 to 3 seconds
- B. 30 to 60 seconds
- C. 10 to 12 seconds
- D. 2 to 3 minutes
MC 3. Fatigue during a 400 metre run is caused principally by:
- A. complete depletion of muscle and liver glycogen
- B. an inability of the lungs to take in sufficient oxygen
- C. accumulating hydrogen ions lowering muscle pH and impairing contraction
- D. depletion of phosphocreatine stores within the muscle
MC 4. Muscle soreness felt 24 to 48 hours after unaccustomed exercise is caused by:
- A. lactic acid remaining in the muscle from the session
- B. microscopic muscle damage and the inflammation that follows, particularly after eccentric work
- C. depletion of phosphocreatine that has not yet been restored
- D. dehydration of the muscle fibres during the session
MC 5. An athlete performs six maximal 30 metre sprints with only fifteen seconds of rest between each. Compared with the same sprints taken with three minutes of rest, the shorter rest will:
- A. make no difference, because each sprint is identical
- B. reduce the contribution of the glycolytic system, because less total work is done
- C. increase the contribution of the glycolytic system, because phosphocreatine is not fully restored between efforts
- D. eliminate the need for the ATP-PCr system entirely
Short answer · exam style
SA 1. Remember Band 3 Outline the fuel source, duration, intensity and cause of fatigue of the ATP-PCr energy system. (4 marks)
SA 2. Understand Band 5 Explain why a 100 metre sprinter recovers within minutes while a 400 metre runner takes far longer, referring to the energy systems. (5 marks)
SA 3. Analyse Band 6 Analyse the ATP-PCr and glycolytic energy systems, including fuel source, efficiency of ATP production, duration, intensity, rate of recovery and causes of fatigue. (6 marks)
📖 Comprehensive answers (click to reveal)
Multiple choice: 1 → B. 2 → C. 3 → C. 4 → B. 5 → C.
Activity 1 guidance: Item by item: the shot put is ATP-PCr, under two seconds at maximal intensity; the 400 metre swim is predominantly aerobic at around four and a half minutes, with a significant glycolytic contribution at the start and in the final sprint; the wrestling exchange at 45 seconds near-maximal is squarely glycolytic; the deadlift is ATP-PCr; the badminton rally at 90 seconds is glycolytic with a rising aerobic contribution; and the walk to school is aerobic throughout. The two sprint conditions are where the learning is. With three minutes of rest, phosphocreatine is nearly fully restored before each effort, so every sprint is supplied predominantly by the ATP-PCr system and each is close to maximal — that is a speed session. With fifteen seconds of rest, phosphocreatine is only partly restored, so from the second or third repetition the glycolytic system must supply an increasing share, hydrogen ions accumulate across the set, and times fall away progressively — that is a repeat-sprint or lactate-tolerance session. Same sprints, different physiological stimulus, different training effect, and the variable that changed was the rest. This is also why an athlete can honestly say a session "felt much harder" without having run any faster.
Activity 2 guidance: Condition B reliably produces a much larger drop-off, typically several times that of Condition A. The explanation must be phosphocreatine restoration, not tiredness: twenty seconds restores well under half the store, so each successive sprint begins with less of the fastest fuel available and an increasing share must come from glycolysis, which cannot supply ATP at the rate maximal sprinting requires. The athlete will report burning and heaviness in Condition B that was absent in Condition A, which is the subjective signature of hydrogen ion accumulation. On prescription: Condition A suits a track sprinter or any athlete developing maximal speed, where full recovery between efforts is the point; Condition B suits a footballer, netballer or hockey player, whose sport demands repeated near-maximal efforts on incomplete recovery. Uncontrolled variables between days routinely include sleep, food and fluid intake, time of day, surface and footwear, weather and wind direction, warm-up duration, motivation and whether the athlete trained the previous day. Naming these is not a formality: a difference in drop-off smaller than the day-to-day variation of the test tells you nothing, which is the same reliability argument Lesson 15 develops formally.
SA 1 (4 marks): Fuel source — phosphocreatine stored within the muscle fibre, which donates its phosphate directly to ADP without oxygen [1]. Duration — approximately 10 to 12 seconds at maximal effort [1]. Intensity — maximal, around 95 to 100 per cent of capacity [1]. Cause of fatigue — depletion of the phosphocreatine store, so ATP cannot be resynthesised at the required rate, with no accumulation of by-products [1]. Writing "lactic acid" anywhere in this answer forfeits the fourth mark and signals the two systems have been conflated.
SA 2 (5 marks): The 100 metres classified by duration and intensity as predominantly ATP-PCr [1]. Its fatigue identified as depletion of phosphocreatine, with no significant by-product [1]. Recovery of that system given as roughly half in 30 seconds and near-complete in 2 to 3 minutes [1]. The 400 metres classified as predominantly glycolytic at 45 to 60 seconds near-maximal, with fatigue caused by accumulating hydrogen ions lowering pH and impairing enzyme function and calcium binding [1]. Its slower recovery, 30 to 60 minutes to clear by-products and longer for glycogen, explained as following from accumulation rather than depletion [1]. The strongest answers state the contrast explicitly as an empty tank against a disturbed environment, because that single distinction generates both the symptoms and the recovery times.
SA 3 (6 marks): ATP-PCr: fuel source and yield of one ATP per phosphocreatine [1]. ATP-PCr: duration, intensity, recovery rate and fatigue by depletion [1]. Glycolytic: fuel source and yield of two ATP per glucose anaerobically [1]. Glycolytic: duration, intensity, recovery rate and fatigue by hydrogen ion accumulation, with the mechanism named [1]. The trade-off between rate of supply and total capacity used to connect the criteria rather than list them [1]. The overlap in a real effort, with the glycolytic share rising as the phosphocreatine share falls and no system switching on or off [1]. A tabulated answer can earn the first four marks and structurally cannot earn the last two, which is worth knowing before choosing how to set the answer out.
Two seconds. That is how much ATP a muscle holds, and everything else in this topic follows from it. The body does not store energy in a usable form; it rebuilds it continuously, and the three energy systems are three methods of rebuilding, each trading speed against capacity.
ATP-PCr is the fast one: phosphocreatine in the fibre, one ATP per molecule, maximal intensity, 10 to 12 seconds, recovery in 2 to 3 minutes, and fatigue by pure depletion. Nothing accumulates, which is why a sprinter can race again within the hour.
Glycolytic is the middle one: glucose and glycogen, two ATP per glucose without oxygen, high but submaximal intensity, predominant from about 10 seconds to 2 minutes, recovery in 30 to 60 minutes, and fatigue by accumulation rather than depletion — hydrogen ions lowering pH until contraction itself is impaired.
Two habits to carry forward. First, never describe the systems as taking turns: they all run at once, and what changes is the proportion each contributes. Second, get the lactate story right — it is a fuel, the hydrogen ions are the problem, and next-day soreness comes from eccentric muscle damage, on a timescale that rules lactate out entirely.
Lesson 10 adds the third system, the one with almost unlimited capacity and no by-product worth worrying about, and then puts all three together as the interplay the dot point asks for.
Mark lesson as complete
Tick when you've finished the practice and review.