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Motor Units, Recruitment
& Muscle Tension

Explore how your nervous system controls muscle force by activating motor units through recruitment and rate coding.

Anatomy & Physiology

What Is a Motor Unit?

A motor unit is the functional unit of muscle contraction. It consists of a single motor neuron and all of the muscle fibers it innervates. When the motor neuron fires an action potential, every fiber in that unit contracts simultaneously.

Anatomy of a Motor Unit

Spinal
Cord
Cell Body
Axon
NMJ
Muscle Fiber 1
Muscle Fiber 2
Muscle Fiber 3
Muscle Fiber 4
One Motor Unit = 1 Neuron + All Its Fibers

Why Do Motor Units Matter?

Your body must produce forces ranging from the gentle touch of threading a needle to the explosive power of a maximal jump. Motor units are the mechanism that lets the nervous system grade muscle force with precision.

Fine Control
Eye muscles have motor units with only 3-5 fibers per neuron, enabling extremely precise movements.
Gross Force
The quadriceps has motor units with 1,000+ fibers per neuron, generating massive force for locomotion.
Innervation Ratio
The number of fibers per motor neuron is called the innervation ratio: lower ratio = finer control.

The Size Principle of Recruitment

Henneman's Size Principle states that motor units are recruited in order from smallest to largest. Small motor units have lower activation thresholds and are recruited first. As more force is needed, progressively larger motor units are activated.

Force Demand (increasing)
Motor Units Active
Small
Type I
→
Medium
Type IIa
→
Large
Type IIx
Low threshold
Mid threshold
High threshold

How Recruitment Increases Force

Recruitment (also called spatial summation) is the process of activating additional motor units to increase total muscle force. The nervous system has two primary mechanisms for grading force:

Recruitment
Activating more motor units increases force. Going from 10 active units to 50 active units produces more total tension.
Rate Coding
Increasing the firing frequency of already-active motor units. Faster stimulation produces greater force per unit via temporal summation.

Twitch, Summation, and Tetanus

Time
Tension
Single Twitch
Unfused Tetanus
Fused Tetanus
S
S
S
S
S
Rapid S

Recruitment vs Rate Coding

Recruitment Rate Coding
Also called Spatial summation Temporal summation
Mechanism Activate more motor units Increase firing rate of active units
Force range Dominant at 0-80% max force Dominant above 80% max force
Analogy Adding more workers to a job Making each worker move faster
Governed by Size Principle (Henneman) CNS descending commands

Fiber Types Within Motor Units

All muscle fibers in a single motor unit are the same fiber type. The three main types correspond to different motor unit characteristics:

Type I (Slow Oxidative)
Small motor units, low threshold, fatigue-resistant. Used for posture and endurance activities.
Type IIa (Fast Oxidative)
Medium motor units, moderate threshold. Balance of speed and endurance: walking, moderate lifting.
Type IIx (Fast Glycolytic)
Large motor units, high threshold, fatigue quickly. Recruited for explosive power: sprinting, jumping.

The Force-Frequency Relationship

Stimulus Frequency (Hz)
Force Output
Twitches
Unfused Tetanus
Fused Tetanus (Max Force)

How to Analyze Recruitment Problems

1
Identify the task. Is the muscle action requiring fine control (low force) or gross power (high force)?
2
Apply the Size Principle. Small (Type I) motor units recruit first. Only add larger units (Type IIa, then IIx) as needed.
3
Consider rate coding. At higher force levels, existing units fire faster (temporal summation) alongside new recruitment.
4
Evaluate fatigue. Type IIx fatigues fastest. Sustained efforts rely more on Type I and IIa motor units.
5
Connect to output. Total muscle tension = sum of all active motor units' individual tensions.

Example 1: Holding a Coffee Cup

A person holds a coffee cup steadily for several minutes. Which motor units are primarily active, and why?

Type I
Type IIa (inactive)
Type IIx (inactive)
Force demand: ~10-15% of maximum
Motor unit recruitment bar
Low Force, Sustained Task
Only small, fatigue-resistant motor units needed

Example 2: Maximal Sprint Start

A sprinter explodes from the starting blocks. Describe the recruitment pattern in their quadriceps.

Type I
Type IIa
Type IIx
Force demand: ~95-100% of maximum
All motor unit types recruited + maximal rate coding
Maximal Force, Explosive Task
All motor units recruited: I → IIa → IIx

Example 3: Gradual Load Increase

A student picks up progressively heavier dumbbells: 2 kg, then 10 kg, then 25 kg. Describe the recruitment sequence in the biceps brachii.

1
2 kg dumbbell: Only a few small Type I motor units fire. Low threshold, low force demand. Minimal rate coding needed.
2
10 kg dumbbell: More Type I units are recruited, and Type IIa units begin to activate. Rate coding increases in already-active units.
3
25 kg dumbbell: Nearly all motor units are active, including Type IIx. Both recruitment and rate coding are near maximum. The muscle is approaching its force ceiling.

Common Mistakes

✕ Skipping the Size Principle
Claiming large motor units activate first for heavy tasks. The order is always small → large, even for maximal efforts: you just recruit through them faster.
✕ Confusing Summation Types
Mixing up temporal summation (rate coding within one unit) with spatial summation (recruitment of additional units). They are distinct mechanisms.
✕ All-or-None Misapplication
Thinking the whole muscle follows all-or-none. Only individual motor units do. The whole muscle grades force smoothly by varying how many units are active.

Summary of Motor Unit Properties

Property Type I Type IIa Type IIx
Motor unit size Small Medium Large
Recruitment threshold Low Moderate High
Contraction speed Slow Fast Very fast
Fatigue resistance High Moderate Low
Force per unit Low Medium High
Primary metabolism Oxidative Oxidative-glycolytic Glycolytic
Example activity Posture, walking Moderate lifting Sprinting, jumping

Question 1 of 5

According to Henneman's Size Principle, which motor units are recruited first during a voluntary contraction?

Question 2 of 5

What is the primary difference between recruitment and rate coding as mechanisms for increasing muscle force?

Question 3 of 5

A person is maintaining a steady posture while standing. Which statement best describes their motor unit activity?

Question 4 of 5

Fused tetanus occurs when:

Question 5 of 5

The extraocular muscles (eye muscles) have a very low innervation ratio (few fibers per motor neuron). What functional advantage does this provide?