How do muscles contract, what fibre types do we have, and how does the body adapt to training?
The neuromuscular system, the sliding filament theory, slow and fast muscle fibre types, motor units and recruitment, and the acute responses and chronic adaptations of the body to exercise.
A focused WJEC A-Level PE answer on the neuromuscular system, covering motor units, the all-or-none law, the sliding filament theory, type I, IIa and IIx muscle fibres, and the acute and chronic adaptations to training.
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What this dot point is asking
WJEC wants you to explain how a muscle contracts using the sliding filament theory, describe the role of motor units and the all-or-none law, distinguish the muscle fibre types and match them to activities, and outline the acute responses and chronic adaptations of the body to training.
The motor unit and the all-or-none law
Because individual fibres are all-or-none, the body grades the force of a whole muscle in two ways. Multiple unit summation (spatial summation) recruits more motor units for a stronger contraction. Wave summation (temporal summation) sends impulses more frequently so contractions merge into a stronger, smoother force, leading to tetanic contraction. Larger forces also tend to recruit larger, faster motor units (the size principle).
The sliding filament theory
Skeletal muscle is built from myofibrils divided into repeating units called sarcomeres, each containing thin actin and thick myosin filaments. In the sliding filament theory:
- A nerve impulse releases calcium ions from the sarcoplasmic reticulum.
- Calcium exposes binding sites on the actin, so myosin heads form cross-bridges.
- The myosin heads swivel (the power stroke), pulling actin over myosin so the sarcomere shortens.
- ATP detaches the myosin head and re-cocks it, and the cycle repeats while calcium and ATP are present.
The filaments do not themselves shorten; they slide past each other, which shortens the whole muscle.
Muscle fibre types
Most people have a mix of fibre types, but the proportion is largely genetically determined and influences which events suit them. Training can shift type IIx fibres towards type IIa characteristics, but cannot convert slow to fast.
Acute responses and chronic adaptations
Acute (immediate) responses to a single bout of exercise include a rising heart rate and stroke volume, deeper and faster breathing, vasodilation to working muscle, increased body temperature and sweating, and a rise in lactate.
Chronic (long-term) adaptations to regular training include:
- Cardiovascular: cardiac hypertrophy (a larger, stronger heart), increased stroke volume and resting bradycardia, and capillarisation of muscle.
- Respiratory: stronger respiratory muscles and a greater diffusion capacity.
- Muscular: muscle hypertrophy, more and larger mitochondria, increased myoglobin and glycogen stores, and stronger tendons and ligaments.
Examples in context
Example 1. A 100 m sprinter. Elite sprinters have a high proportion of type IIx and IIa fibres, giving rapid, powerful contractions over a few seconds before fatigue sets in. Their training emphasises power and recruitment of fast units rather than aerobic capacity.
Example 2. Muscle hypertrophy from resistance training. Repeated heavy resistance training causes type II fibres in particular to thicken as more contractile proteins (actin and myosin) are laid down. This chronic adaptation increases maximum strength, a classic WJEC example of structure following function.
Try this
Q1. Define a motor unit. [1 mark]
- Cue. A single motor neurone and all the muscle fibres it stimulates.
Q2. Explain how the sarcomere shortens during a muscle contraction. [3 marks]
- Cue. Calcium exposes actin binding sites; myosin cross-bridges form and swivel (power stroke), pulling actin over myosin; ATP detaches and re-cocks the heads so the cycle repeats and the sarcomere shortens.
Q3. Give two chronic adaptations of skeletal muscle to endurance training and explain a benefit of each. [4 marks]
- Cue. More mitochondria (greater aerobic energy production); increased myoglobin and capillarisation (better oxygen delivery and storage so the muscle works aerobically for longer).
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC 20186 marksCompare the structural and functional characteristics of slow twitch (type I) and fast twitch (type IIx) muscle fibres, and link each to a suitable sporting activity.Show worked answer →
Type I (slow oxidative) fibres contract slowly with low force but are highly resistant to fatigue. They have many mitochondria, a rich capillary supply and high myoglobin content, so they rely on aerobic respiration. They suit endurance events such as a marathon or long-distance cycling.
Type IIx (fast glycolytic) fibres contract rapidly with high force but fatigue quickly. They have few mitochondria, low capillary density and large glycogen stores, relying on anaerobic respiration. They suit explosive events such as a 100 m sprint or shot put.
Type IIa (fast oxidative glycolytic) fibres sit between the two and suit events such as a 1500 m run.
Markers reward paired structural points (mitochondria, capillaries, myoglobin), paired functional points (speed, force, fatigue), and correct sporting examples.
WJEC 20204 marksExplain the role of the motor unit and the all-or-none law in producing a muscle contraction.Show worked answer →
A motor unit is a single motor neurone and all the muscle fibres it stimulates.
When a nerve impulse of sufficient size (above the threshold) arrives, every fibre in that motor unit contracts fully; if the impulse is below threshold, none of them contract. This is the all-or-none law.
The strength of a whole-muscle contraction is increased by recruiting more motor units (multiple unit summation) and by stimulating them more frequently (wave summation).
Markers reward the definition of a motor unit, the all-or-none law, and recruitment of more units to grade force.
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