FSHP Academy
Exercise Physiology — all lessons

Muscle Physiology

Muscle physiology: excitation–contraction and the sliding filament modelConcentric, eccentric and isometric muscle actionsMotor units, recruitment and rate codingNeuromuscular adaptations to training

Energy Systems

Energy systems overview: how the body makes ATPThe ATP-PC (phosphagen) systemAnaerobic glycolysis and lactateThe aerobic oxidative system

Cardiovascular Physiology

Heart anatomy and the cardiac cycleStroke volume, heart rate and cardiac outputBlood components and oxygen transport

Aerobic Fitness and Intensity

Aerobic fitness concepts and adaptationsVO₂ maxLactate thresholdHeart rate zones and RPE-based intensity control

Motor units, recruitment and rate coding

How the nervous system grades muscle force — the motor unit, orderly recruitment by size, firing rate, and what this means for load, effort and fatigue in training.

Intermediate Evidence: Established consensus 3 min read Reviewed 2026-10-31 Status: draft

Learning objectives

  • Define a motor unit and innervation ratio
  • Explain recruitment by the size principle and force grading by rate coding
  • Describe how fatigue changes recruitment during sets to failure
  • Apply these mechanisms to load selection and effort prescription

Before this lesson

Why this matters

Whether a set builds strength, size or endurance depends partly on which motor units it activates. This lesson connects neural control to practical decisions about load and effort.

Fundamental concepts

A motor unit = one alpha motor neuron + all the fibres it supplies. When the neuron fires, all its fibres contract.

  • Innervation ratio — fibres per neuron: small in muscles needing precision (eye, hand), large in big postural and locomotor muscles (thousands of fibres per unit).
  • All fibres in a unit share the same type (see fibre types).

Recruitment: the size principle

The size principle of motor unit recruitment Horizontal bars represent eleven motor units ordered from small to large. Each bar begins at the force level at which that unit is recruited and continues to maximal force. Small, low-threshold units are recruited at low forces; progressively larger units are added as force increases. Bar thickness grows with unit size. Beyond the last recruitment threshold, further force comes from increased firing rates (rate coding). rate coding only 0%20%40%60%80%100% Force (% of maximal voluntary contraction) Small unitsmostly type Ifatigue-resistant Large unitsmostly type IIhigh force, fatigable Illustrative. Upper recruitment limit varies by muscle: ~50 % MVC in small hand muscles, up to ~85–90 % in large limb muscles.
Figure 1. Orderly recruitment. Each bar starts at the force at which a unit is recruited; small units join first, larger units as force rises. Beyond the last threshold, force increases by rate coding. — Original diagram, FSHP Academy (schematic, not to scale).

Small motor neurons are more excitable, so they reach threshold first. As required force rises, progressively larger units are added [henneman1957]. Recruitment order is remarkably consistent across tasks, though fast ballistic contractions lower recruitment thresholds.

Rate coding

Each impulse causes a twitch. If impulses arrive before relaxation is complete, twitches summate; at high frequencies they fuse into a smooth maximal contraction (tetanus). Increasing firing rate can raise a unit’s force several-fold. In small hand muscles, recruitment is complete at roughly half of maximal force and rate coding provides the rest; in large limb muscles recruitment continues to much higher forces [enoka2017].

Fatigue and recruitment

During a set taken toward failure, active low-threshold units fatigue; to maintain force, the nervous system recruits higher-threshold units and raises firing rates. This is why sets with moderate loads taken close to failure can stimulate high-threshold units, and why trembling appears near failure as units drop in and out.

Practical application

Common mistakes and misconceptions

Case study

A client with knee pain cannot tolerate heavy squats. The trainer uses split squats with moderate loads for sets of 15–20 taken to 1–2 RIR, explaining that effort near failure still recruits high-threshold motor units while keeping joint loads tolerable.

Further learning: video lessons

Video links are only marked “verified” after a human has watched them. Unverified entries open a YouTube search for that channel and topic rather than a guessed link.

Technical needs verification — opens a YouTube search

Motor units and the size principle

Ninja Nerd

You will understand: recruitment and rate coding in detail

Why: Thorough lecture explanation.
Introductory needs verification — opens a YouTube search

How muscles grade force

Khan Academy

You will understand: the basics of recruitment and summation

Why: Clear foundational content.

Knowledge check

Knowledge check · 0/5

1. Muscles controlling fine movements (e.g., eye muscles) have…

Answer B. Few fibres per neuron allow precise force control.

2. According to the size principle, motor units are recruited…

Answer B. Henneman’s principle — small, low-threshold units are recruited first.

3. Increasing the firing frequency of already-active motor units is called…

Answer B. Higher firing rates cause twitch summation and greater force.

4. During a set of 15 reps to failure with a moderate load…

Answer B. As low-threshold units fatigue, larger units are recruited to maintain force.

5. An explosive intent to move a light load quickly…

Answer B. Rapid, ballistic contractions recruit units at lower force thresholds with high initial firing rates.

Short-answer & application questions

  1. Explain how both a 3-rep heavy set and a 20-rep set to failure can recruit high-threshold units.
  2. Why does a muscle tremble near failure?
  3. Why might very light, easy sets far from failure provide a weak stimulus for type II fibres?

Key takeaways

  • A motor unit is one motor neuron and all the muscle fibres it innervates; all its fibres are the same type.
  • Force is graded by recruiting more units (in order of size) and increasing their firing rate.
  • In large limb muscles, recruitment continues to high forces; in small hand muscles it is complete at about half of maximum.
  • Sets taken close to failure progressively recruit higher-threshold units even with moderate loads.

Connected topics

anatomy

Muscle fibre types

physiology

Neuromuscular adaptations to training

physiology

Concentric, eccentric and isometric muscle actions

strength conditioning

Progressive overload and programming variables (sets, reps, tempo)

References

  1. Henneman E (1957). Relation between size of neurons and their susceptibility to discharge. Science, 126(3287):1345–1347. doi:10.1126/science.126.3287.1345 Classic study check pending
  2. Enoka RM, Duchateau J (2017). Rate coding and the control of muscle force. Cold Spring Harbor Perspectives in Medicine, 7(10):a029702. Narrative review check pending

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