Muscle hypertrophy increases the force each recruited motor unit can produce, so fewer motor units may be required for the same absolute force. Muscle atrophy reduces force per motor unit, so more available units may be recruited for the same task.
A motor unit consists of one motor neuron and all the muscle fibres it innervates. Recruitment normally follows the size principle: small, fatigue-resistant units are recruited first, followed by progressively larger, more powerful units as force requirements rise.
During hypertrophy, fibres increase their cross-sectional area as contractile protein synthesis exceeds breakdown. Larger fibres generate greater tension, increasing force from each active motor unit. For the same submaximal absolute load, the nervous system may recruit fewer units or use a lower motor neuron firing rate. During maximal effort, the performer still attempts to recruit all available units and maximize firing rate.
Atrophy decreases fibre cross-sectional area and contractile capacity. A fixed load then represents a greater percentage of reduced maximum force, so more units and potentially higher firing rates are required. Severe disuse, ageing, or denervation may also impair activation, limiting force despite high recruitment demand.
| Muscular change | Recruitment effect |
|---|---|
| Hypertrophy | Greater force per unit; fewer units for the same absolute submaximal task |
| Atrophy | Less force per unit; more available units for the same absolute task |
| Maximal effort | All units approached; hypertrophied muscle produces greater total force |
A common misconception is that hypertrophy automatically increases the number of motor units. It mainly enlarges fibres within existing motor units; it does not normally create new motor neurons.
Exam technique: For B.1.3 Muscular function, link fibre cross-sectional area to force production, then apply the size principle. Distinguish the same absolute workload from the same because recruitment depends strongly on task intensity.
