How does the nervous system recruit muscle fibres to control force in sport?
The motor unit and the all-or-none law, the recruitment of motor units and wave summation to grade force, and the proprioceptors and reflexes that control movement and protect muscle.
A focused answer to OCR A-Level PE on the neuromuscular system: the structure of a motor unit, the all-or-none law, the recruitment of motor units and wave (spatial and temporal) summation to grade force, and the muscle spindles, Golgi tendon organs and the stretch reflex used in PNF stretching.
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What this dot point is asking
OCR wants you to describe the structure of a motor unit, state the all-or-none law, explain how force is graded by recruiting motor units and by wave summation, and explain the roles of the proprioceptors (muscle spindles and Golgi tendon organs) and the reflexes that protect the muscle, including their use in PNF stretching.
The motor unit and the all-or-none law
Grading force: recruitment and wave summation
Because each fibre is all-or-none, the body controls how much force a whole muscle produces in two ways.
A games player uses light recruitment for a gentle pass and full recruitment with wave summation for a maximal shot or tackle. This is why strength training improves force partly through better neural recruitment before any muscle growth appears.
Proprioceptors and protective reflexes
Applying it: PNF stretching
Proprioceptive neuromuscular facilitation (PNF) is an advanced flexibility method that uses these reflexes. The performer stretches the target muscle to its limit, then isometrically contracts it against a partner or immovable resistance for several seconds, then relaxes and stretches further. The strong contraction loads the tendon, firing the Golgi tendon organs, which inhibit (relax) the muscle. With the muscle inhibited, the next stretch can reach a greater range than before, which is why PNF is one of the most effective ways to develop static flexibility.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR 20184 marksExplain how a weightlifter recruits muscle fibres to lift a near-maximal load, using the terms motor unit, all-or-none law and recruitment.Show worked answer →
A Component 01 application question. One mark each for the motor unit definition, the all-or-none law, recruitment of more units and the link to greater force.
Award marks for: a motor unit is a single motor neuron and all the muscle fibres it stimulates. By the all-or-none law, when the impulse reaches the threshold every fibre in that unit contracts maximally, or none do. To lift a heavier load the lifter recruits more motor units (and larger ones with more fibres), so more fibres contract at once and total force rises. For a near-maximal lift, almost all available motor units are recruited.
Markers reward the idea that force is graded by how many units are recruited, not by each fibre contracting harder.
OCR 20226 marksExplain the roles of the muscle spindles and Golgi tendon organs, and how a performer uses the stretch reflex during PNF stretching to improve flexibility.Show worked answer →
A Component 01 extended-response question. Markers reward accurate proprioceptor roles (AO1) and the PNF application (AO2).
Award marks for: muscle spindles detect the rate and degree of stretch in a muscle and trigger the stretch reflex, causing the muscle to contract to protect it from over-stretching. Golgi tendon organs detect tension in the tendon and, when tension is very high, trigger the inverse stretch reflex (autogenic inhibition), causing the muscle to relax to protect it from injury. In PNF stretching the performer stretches the muscle, then isometrically contracts it against a partner for several seconds; this strongly stimulates the Golgi tendon organs, which inhibit the muscle so that on relaxing it can be stretched further than before, improving the range of movement.
A top answer contrasts the protective contraction triggered by spindles with the protective relaxation triggered by Golgi tendon organs, and uses that relaxation to explain why PNF works.
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Sources & how we know this
- OCR A Level Physical Education (H555) specification — OCR (2016)