How does the neuromuscular system control and produce muscular contractions during physical activity?
The structure and recruitment of motor units, the all-or-none law, slow and fast twitch muscle fibre types and their characteristics, and the role of proprioceptors in the prevention of injury through PNF stretching.
A focused answer to AQA A-Level PE applied anatomy on the neuromuscular system, covering motor units, the all-or-none law, the three muscle fibre types and their characteristics, proprioceptors and PNF stretching.
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What this dot point is asking
AQA wants you to describe a motor unit and how motor units are recruited, state the all-or-none law, compare the structure and function of slow twitch and fast twitch muscle fibres, and explain the role of proprioceptors and how PNF stretching uses them to improve flexibility and prevent injury.
Motor units and the all-or-none law
The all-or-none law states that a motor unit either contracts completely or not at all: if the stimulus reaches the threshold, every fibre in the unit contracts maximally; if it does not, none contract. This raises an obvious question, if each unit is all-or-none, how does a muscle produce graded force, from gently lifting a pencil to a maximal lift? The answer is three mechanisms. Spatial summation recruits more, or larger, motor units to increase force. Wave summation delivers a higher frequency of impulses so individual twitches merge into a stronger, smoother contraction, and at maximal frequency this produces a sustained tetanic contraction. Motor unit recruitment follows the size principle, with small slow-twitch units recruited first and large fast-twitch units added only as force demand rises.
Muscle fibre types
Slow twitch fibres have many mitochondria, a rich capillary supply and high myoglobin, so they resist fatigue by producing ATP aerobically. Fast glycolytic fibres have large stores of phosphocreatine and glycogen, high glycolytic enzyme activity and few mitochondria, so they generate force quickly via the anaerobic systems but tire fast as lactate accumulates. Type IIa (fast oxidative glycolytic) fibres sit between the two: they can use both aerobic and anaerobic pathways, produce high force and resist fatigue better than type IIx, which suits them to middle-distance and games play. Fibre type is largely genetically determined, but training can shift the characteristics of type IIa and IIx fibres towards the demands placed on them (for example, endurance training improves their oxidative capacity).
Proprioceptors and PNF
Proprioceptors give the body information about position and tension. Muscle spindles detect the rate and extent of stretch and trigger the stretch reflex to protect the muscle. Golgi tendon organs (GTOs) detect tension in the tendon.
PNF (proprioceptive neuromuscular facilitation) stretching is the most effective form of flexibility training. The performer stretches a muscle, then isometrically contracts it against a partner for about 6 to 10 seconds, then relaxes and stretches further. The contraction stimulates the GTO, which causes autogenic inhibition: the stretch reflex is overridden so the muscle relaxes and a greater range of movement is achieved.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20184 marksCompare the structural and functional characteristics of slow twitch (type I) and fast glycolytic (type IIx) muscle fibres, and explain why a marathon runner and a 100 m sprinter have different fibre proportions.Show worked answer →
AO1/AO2 comparison. Slow twitch (type I): many mitochondria, high myoglobin, dense capillary network, small fibre size, slow contraction, low force, very fatigue-resistant, aerobic. Fast glycolytic (type IIx): few mitochondria, low myoglobin, large stores of phosphocreatine and glycogen, high glycolytic enzyme activity, large fibre size, very fast and forceful contraction, fatigues quickly, anaerobic. Application: a marathon runner has a high proportion of type I fibres suited to sustained low-intensity aerobic work, while a sprinter has a high proportion of type IIx fibres for rapid, powerful, short-duration efforts. Markers reward paired comparisons (a feature of one set against the other) and a clear link to the event demands.
AQA 20213 marksExplain how PNF stretching uses a proprioceptor to achieve a greater range of movement than static stretching.Show worked answer →
Three linked points. The performer stretches the muscle, then isometrically contracts it (typically against a partner) for about 6 to 10 seconds. This contraction increases tension in the tendon, which is detected by the Golgi tendon organ (GTO). The GTO triggers autogenic inhibition, overriding the stretch reflex from the muscle spindle, so the muscle is inhibited and relaxes, allowing it to be stretched further. Reward naming the GTO (not the muscle spindle) and the term autogenic inhibition. The common error is attributing the effect to the muscle spindle, which actually opposes the stretch.
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Sources & how we know this
- AQA A-level Physical Education (7582) specification — AQA (2016)