Neuromuscular stimulation

· Acetylcholine (ACh) = neurotransmitter that stimulates skeletal muscle contraction.
· A nerve impulse reaches the muscle at the neuromuscular junction, causing the muscle fibre to become activated.
· Exam focus: link nervous system signalling to muscle contraction and movement production.

This diagram shows how a motor neuron communicates with a skeletal muscle fibre. It is useful for remembering that acetylcholine crosses the synaptic cleft and binds to receptors on the sarcolemma to trigger contraction. Source

Motor units and all-or-none principle

· Motor unit = one motor neuron plus the muscle fibres it innervates.
· Motor units contract using the all-or-none principle: once stimulated above threshold, the fibres in that motor unit contract fully.
· Small motor units = fine control, precision movements.
· Large motor units = greater force, gross/powerful movements.
· Recruitment = activating more motor units to increase total force production.

This image helps visualize the idea that a motor unit is the functional link between the nervous system and skeletal muscle. It supports exam explanations of recruitment and force control. Source

Muscle fibre types and recruitment

· Motor units differ by muscle fibre type and motor neuron diameter.
· Main fibre types: Type I, Type IIa, Type IIx.
· Type I fibres: slower, more fatigue-resistant, suited to endurance activity.
· Type IIa fibres: fast, moderately fatigue-resistant, useful for repeated high-intensity work.
· Type IIx fibres: fastest, highest force/power, fatigue quickly.
· Recruitment varies with activity demand: low-intensity endurance uses more Type I; high-force/high-speed actions recruit more Type II fibres.

Hypertrophy, atrophy and recruitment patterns

· Hypertrophy = increase in muscle size, often linked to training adaptations.
· Atrophy = decrease in muscle size, often linked to disuse, injury or inadequate stimulus.
· Changes in muscle size can alter motor unit recruitment patterns.
· Exam link: explain how training or inactivity may change the muscle’s ability to produce force efficiently.

Types of muscular contraction

· Isometric contraction = muscle produces tension with no visible change in muscle length; useful for posture and joint stability.
· Isotonic concentric contraction = muscle shortens while producing force; example: upward phase of a biceps curl.
· Isotonic eccentric contraction = muscle lengthens under tension; example: controlled lowering phase of a biceps curl.
· Isokinetic contraction = muscle contracts at a constant speed, usually using specialist equipment.
· Exam focus: identify contraction type from a sporting movement and explain its function.

This diagram clearly compares how muscle length changes during different contraction types. It is especially useful for applying contraction terminology to sporting movements. Source

Agonist, antagonist and reciprocal inhibition

· Muscles usually work in pairs.
· Agonist = main muscle responsible for producing a movement.
· Antagonist = muscle that opposes or controls the movement.
· Reciprocal inhibition = when the agonist contracts, the antagonist relaxes to allow smooth movement.
· Example: during elbow flexion, biceps brachii = agonist and triceps brachii = antagonist.

ATP and muscle contraction

· Muscular contraction requires ATP metabolism within muscle cells.
· ATP provides energy for contraction and repeated cross-bridge cycling.
· Lack of ATP contributes to reduced contraction efficiency and fatigue.
· Exam link: connect energy systems to the ability to sustain muscle contraction.

This diagram shows that actin and myosin do not shorten; instead, they slide past each other. It is ideal for explaining how sarcomere shortening produces muscle contraction in HL answers. Source