How does muscle structure allow movement, and how is it powered?
The structure of skeletal muscle and the sliding filament model of contraction, the role of ATP and calcium ions, and the difference between slow and fast twitch fibres.
An Edexcel A-Level Biology B (Salters-Nuffield) answer on muscles and movement, covering skeletal muscle structure, the sliding filament model, the role of ATP and calcium ions in contraction, and slow and fast twitch fibres.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
Jump to a section
What this dot point is asking
Edexcel wants you to describe the structure of skeletal muscle, explain the sliding filament model of contraction, explain the role of ATP and calcium ions, and contrast slow and fast twitch fibres. The sliding filament sequence and matching fibre type to activity are the most common exam questions.
Muscle structure
The sarcomere has named regions you must be able to read on a diagram: the A band (the full length of the myosin), the I band (actin only, either side, which narrows during contraction), the H zone (myosin only in the middle, which narrows during contraction) and the Z lines (which move closer together). The A band stays the same length, which is the proof that filaments slide rather than shorten.
The sliding filament model
The steps are:
- A nerve impulse triggers the release of calcium ions from the sarcoplasmic reticulum.
- Calcium ions cause tropomyosin to move, exposing binding sites on the actin.
- Myosin heads bind to actin, forming cross-bridges, and pull the actin inwards (the power stroke).
- ATP binds to the myosin head, breaking the cross-bridge; the head is reset using energy from ATP hydrolysis, ready to bind again.
This repeats, so the sarcomere shortens and the muscle contracts.
Slow and fast twitch fibres
- Slow twitch fibres contract slowly, resist fatigue, have many mitochondria and a rich blood supply, and rely on aerobic respiration. They suit endurance activity.
- Fast twitch fibres contract quickly and powerfully, fatigue rapidly, have fewer mitochondria and a poorer blood supply, store glycogen and rely more on anaerobic respiration (glycolysis). They suit short bursts of intense activity.
Most muscles contain a mix of both fibre types, and the proportions vary between people and can shift slightly with training, which is why athletes specialise in endurance or power events.
Examples in context
Example 1. Rigor mortis. After death, respiration stops so no ATP is made. ATP is needed to break the actin-myosin cross-bridges, so without it the cross-bridges stay locked and the muscles become stiff (rigor mortis). This is a striking demonstration of the exam point that ATP is required to detach the myosin head, not just to power the stroke.
Example 2. Postural muscles. The muscles that hold you upright (such as the soleus in the calf) are rich in slow twitch fibres because they must contract continuously for hours without fatiguing, relying on aerobic respiration. This links fibre type to a clear physiological role and reinforces why fibre proportions differ between muscles.
Try this
Q1. Explain the role of calcium ions in muscle contraction. [2 marks]
- Cue. They cause tropomyosin to move, exposing the binding sites on actin so myosin heads can attach.
Q2. State two differences between slow and fast twitch fibres. [2 marks]
- Cue. Slow twitch contract slowly and resist fatigue using aerobic respiration; fast twitch contract quickly and fatigue rapidly, using more anaerobic respiration.
Exam-style practice questions
Practice questions written in the style of Pearson Edexcel exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Edexcel 20186 marksDescribe the sequence of events in the sliding filament model that leads to the shortening of a sarcomere when a muscle contracts.Show worked answer →
Markers want an ordered mechanism from calcium release to power stroke.
A nerve impulse causes calcium ions to be released from the sarcoplasmic reticulum into the sarcoplasm. Calcium ions bind to troponin, moving tropomyosin away from the binding sites on the actin filament. Myosin heads (carrying ADP and inorganic phosphate) bind to the exposed sites forming cross-bridges. The myosin heads tilt, performing the power stroke that pulls the actin filaments towards the centre of the sarcomere, and ADP and phosphate are released. ATP binds to the myosin head, breaking the cross-bridge. ATP is hydrolysed to ADP and phosphate, providing energy to recock the head ready to bind again. The cycle repeats, the actin and myosin slide past each other and the sarcomere shortens (the I band and H zone narrow).
Award marks for: calcium released and binds troponin; tropomyosin moves exposing sites; cross-bridge forms; power stroke pulls actin; ATP breaks cross-bridge; ATP hydrolysis recocks head; sarcomere shortens.
Edexcel 20214 marksExplain why a long-distance runner is likely to have a high proportion of slow twitch muscle fibres, while a sprinter has more fast twitch fibres.Show worked answer →
Markers want fibre properties matched to the demand of each activity.
Slow twitch fibres contract slowly but resist fatigue because they have many mitochondria, a rich blood (capillary) supply and high myoglobin, so they rely on aerobic respiration and can keep contracting for a long time, which suits the sustained, lower-power demand of distance running. Fast twitch fibres contract quickly and powerfully but fatigue rapidly because they rely more on anaerobic respiration (glycolysis) and accumulate lactate, which suits the short bursts of high power needed for sprinting.
Award marks for: slow twitch aerobic, fatigue resistant, many mitochondria, suit endurance; fast twitch anaerobic, fast and powerful, fatigue quickly, suit sprinting.
Related dot points
- The light-dependent and light-independent reactions of photosynthesis, the role of chloroplast structure, the products of each stage, and the factors that limit the rate of photosynthesis.
An Edexcel A-Level Biology B (Salters-Nuffield) answer on photosynthesis, covering the light-dependent and light-independent reactions, the structure of the chloroplast, the role of ATP and NADP, and limiting factors.
- The stages of aerobic respiration (glycolysis, the link reaction, the Krebs cycle and oxidative phosphorylation), the role of ATP and the mitochondrion, and anaerobic respiration in animals and yeast.
An Edexcel A-Level Biology B (Salters-Nuffield) answer on respiration, covering glycolysis, the link reaction, the Krebs cycle and oxidative phosphorylation, the role of ATP and the mitochondrion, and anaerobic respiration in animals and yeast.
- The structure of a neurone and the transmission of a nerve impulse, the events at a synapse, the action of hormones, and the differences between nervous and hormonal coordination.
An Edexcel A-Level Biology B (Salters-Nuffield) answer on nervous and hormonal coordination, covering neurone structure, the resting potential and action potential, synaptic transmission, the action of hormones, and how nervous and hormonal coordination differ.
- The principle of homeostasis and negative feedback, the control of body temperature and blood glucose, and the structure and function of the kidney in osmoregulation and excretion.
An Edexcel A-Level Biology B (Salters-Nuffield) answer on homeostasis and the kidney, covering negative feedback, the control of temperature and blood glucose, and the structure and function of the kidney in excretion and osmoregulation.
- The structure of the mammalian heart and the cardiac cycle, the structure of arteries, veins and capillaries, and the role of the circulatory system in transporting substances around the body.
An Edexcel A-Level Biology B (Salters-Nuffield) answer on the heart and circulation, covering heart structure, the cardiac cycle and pressure changes, the structure of arteries, veins and capillaries, and the mammalian double circulatory system.
Sources & how we know this
- Pearson Edexcel A-Level Biology B (9BN0) specification — Pearson Edexcel (2015)