Energy Sources and Muscle Fibre Types Revision Notes | A Level AQA Biology

1. Why Muscles Need a Rapid Energy Supply

A sprinter leaves the blocks in under 0.15 seconds. A marathon runner sustains effort for over two hours. Both are using skeletal muscle — yet the energy demands could not be more different. Understanding how muscles meet those demands is the core of this lesson.

Every muscle contraction requires ATP (adenosine triphosphate) — the universal energy currency of the cell. When myosin heads pull on actin filaments during the sliding filament mechanism, they hydrolyse ATP to ADP and inorganic phosphate, releasing the energy needed for movement. The problem is that muscle cells store only a tiny amount of ATP at any one time — enough for roughly two seconds of maximal effort.

This creates an urgent question: how does a muscle keep contracting once that initial ATP store is exhausted? The answer lies in two complementary systems — phosphocreatine and aerobic respiration — and in the specialised fibre types that are built around each system.

2. ATP and Phosphocreatine: The Immediate Energy System

Think of ATP as the only currency a muscle can actually spend. No matter how much glucose or fat is available, none of it can power a myosin head directly — it must first be converted into ATP. So the cell maintains a small reserve of ATP ready to use, and then has several mechanisms to replenish it the moment levels begin to fall.

Phosphocreatine is the muscle's emergency reserve. The moment ATP concentration drops, creatine kinase catalyses the transfer of the phosphate group from phosphocreatine onto ADP, instantly regenerating ATP. This reaction is so fast that it can sustain maximal contraction for approximately 8–10 seconds — exactly the duration of a 100 m sprint.

Once phosphocreatine stores are depleted, the muscle must switch to slower ATP-generating pathways: anaerobic glycolysis (producing lactate) and, for sustained activity, aerobic respiration in the mitochondria. Recovery of phosphocreatine stores requires oxygen and takes several minutes, which is why athletes need rest periods between explosive efforts.

Here is where it gets interesting: the relative importance of each energy source depends not on the individual but on the type of muscle fibre doing the work. Different fibres are structurally and biochemically specialised to favour different energy pathways — and that specialisation is the key to understanding muscle physiology at A Level.

3. Slow-Twitch Muscle Fibres (Type I)

Slow-twitch fibres — also called Type I fibres — are built for endurance. Picture the postural muscles of your back, which must maintain tension for hours without rest. These fibres are the workhorses of sustained, low-to-moderate intensity activity.

Their defining structural feature is a high density of mitochondria, which allows them to generate ATP continuously through aerobic respiration. They also contain large amounts of myoglobin — the red, oxygen-binding protein that stores oxygen within the muscle cell itself. Myoglobin gives slow-twitch fibres their characteristic dark red colour and ensures a steady oxygen supply to the mitochondria even when blood flow fluctuates.

• Many mitochondria — high capacity for aerobic ATP production
• High myoglobin content — stores oxygen within the fibre, giving a dark red colour
• Rich capillary network — efficient oxygen and glucose delivery
• Slow contraction speed — myosin ATPase activity is lower
• High resistance to fatigue — aerobic pathway produces ATP continuously
• Smaller fibre diameter — shorter diffusion distance for oxygen
• Found predominantly in postural muscles (e.g. soleus, erector spinae)

The trade-off is force. Slow-twitch fibres generate relatively low peak tension compared with fast-twitch fibres. They are not suited to explosive movements — but for walking, maintaining posture, or running a marathon, they are precisely the right tool.

4. Fast-Twitch Muscle Fibres (Type II)

Fast-twitch fibres — Type II fibres — are the opposite design philosophy. Where slow-twitch fibres prioritise endurance, fast-twitch fibres prioritise speed and power. They contract rapidly and forcefully, but they fatigue quickly.

Structurally, fast-twitch fibres have fewer mitochondria and lower myoglobin content, giving them a pale, whitish appearance. They rely heavily on anaerobic glycolysis and the phosphocreatine system for rapid ATP generation. They also store more glycogen than slow-twitch fibres, providing the substrate for glycolysis. Their myosin ATPase enzymes are more active, meaning they can hydrolyse ATP — and therefore cycle the cross-bridge — at a much higher rate.

• Fewer mitochondria — limited aerobic capacity
• Low myoglobin content — pale/white colour
• High glycogen stores — fuel for anaerobic glycolysis
• Fast myosin ATPase activity — rapid cross-bridge cycling
• Large fibre diameter — generates greater force per contraction
• Fatigues rapidly — lactate and H⁺ accumulation inhibits enzyme activity
• Found in muscles used for explosive movements (e.g. gastrocnemius, biceps brachii)

It is worth noting that AQA recognises two subtypes of fast-twitch fibres at A Level: Type IIa (fast oxidative-glycolytic, with some aerobic capacity) and Type IIb (fast glycolytic, almost entirely anaerobic). For most exam questions, the key distinction is between slow-twitch (Type I) and fast-twitch (Type II) as a group, but being aware of the subtypes demonstrates higher-level understanding.

5. Comparing Slow and Fast Fibres: A Side-by-Side Analysis

Now that both fibre types are established, it is worth placing them directly side by side. Exam questions frequently ask you to 'compare' — meaning you must address both similarities and differences, and link structural features to functional consequences.

Notice the coherent logic running through each column. Every structural feature of the slow-twitch fibre serves aerobic endurance: more mitochondria to produce ATP aerobically, more myoglobin to store oxygen, denser capillaries to deliver it. Every feature of the fast-twitch fibre serves rapid force production: larger diameter for more myofilaments, more glycogen for anaerobic fuel, faster myosin ATPase for quicker cross-bridge cycling.

6. Linking Energy Sources to Fibre Types: The Full Picture

Earlier, we established that phosphocreatine provides immediate ATP for roughly 8–10 seconds, anaerobic glycolysis sustains effort for up to two minutes, and aerobic respiration supports prolonged activity. These three systems map directly onto the fibre types you have now studied.

Fast-twitch fibres are the primary users of phosphocreatine and anaerobic glycolysis. Their large stores of phosphocreatine and glycogen, combined with fast myosin ATPase, make them ideal for explosive, short-duration efforts. Slow-twitch fibres, by contrast, are almost entirely dependent on aerobic respiration, which is why their mitochondrial density and oxygen-delivery infrastructure are so highly developed.

Worked example: Applying Fibre Type Knowledge to an Exam Scenario

A 400 m runner experiences significant fatigue in the final 100 m of a race. Using your knowledge of muscle fibre types and energy sources, explain why this occurs.

1. 400 m running requires rapid, powerful contractions sustained over approximately 45–50 seconds. This recruits predominantly fast-twitch (Type II) muscle fibres. — Fast-twitch fibres are recruited for high-intensity efforts. Slow-twitch fibres alone cannot generate sufficient force or speed for competitive 400 m pace.
2. Phosphocreatine stores are depleted within the first ~8–10 seconds. The remainder of the race relies heavily on anaerobic glycolysis. — Aerobic respiration cannot produce ATP fast enough to meet the demand at 400 m race pace, so anaerobic glycolysis becomes the dominant pathway.
3. Anaerobic glycolysis produces lactate and H⁺ ions. Accumulation of H⁺ lowers intracellular pH, inhibiting enzyme activity (including myosin ATPase) and interfering with calcium ion binding to troponin. — This is the biochemical basis of the 'burn' felt in muscles during intense exercise. It is the H⁺ accumulation, not lactate itself, that primarily inhibits contraction.
4. By the final 100 m, glycogen stores in fast-twitch fibres are significantly depleted and H⁺ accumulation is high. ATP regeneration can no longer keep pace with demand, so contraction force and speed decline. — This explains the characteristic 'tying up' seen in 400 m runners approaching the finish line — a direct consequence of fast-twitch fibre fatigue.

Answer: Fatigue in the final 100 m occurs because fast-twitch fibres, which dominate 400 m running, have exhausted their phosphocreatine and glycogen stores. Anaerobic glycolysis has produced significant H⁺ accumulation, lowering pH and inhibiting myosin ATPase and calcium binding to troponin, reducing the rate and force of cross-bridge cycling.

7. Exam Technique: Securing Full Marks

Questions on this topic appear across a range of command words. The most common errors come not from not knowing the content, but from imprecise language or incomplete comparisons. Here is how to avoid losing marks.

Taken together, the two specification requirements for this topic form a coherent whole: the roles of ATP and phosphocreatine explain the immediate energy chemistry of contraction, while the structural and functional differences between slow and fast fibres explain how muscles are architecturally matched to different physiological demands. Examiners expect you to move fluently between these two levels of explanation.