• Introduction
  • The basics of the size principle
  • Different types of motor units
  • Slow motor units
  • Fast motor units
  • Final thoughts 
  • Sources
  • Lactate: a byproduct of anaerobic respiration which can be used to generate more ATP.
  • Motor unit: a motor neuron and all the muscle fibers innervated by it.

Introduction

A muscle contraction starts from a motor neuron located in the brain. Once stimulated, the motor neuron sends a signal to the muscle fibers (a long, tubular muscle cell), causing them to contract. This contraction is also what generates movement in different parts of the body. The combination of muscle fibers and a single motor neuron is called a motor unit. The number of muscle fibers innervated by the same motor neuron varies greatly depending on the function of the muscle.

Skeletal muscles consists of thousands of muscle fibers. Their main function is to convert chemical energy into mechanical output in the form of muscle movement. However, not all muscle fibers in a muscle are fired simultaneously. By altering the firing rate and activating muscle fibers with different metabolic properties, the body is able to use energy more efficiently. This also allows each muscle to contract with varying levels of force. This phenomenon is known as multiple motor unit summation.

Muscle fiber are activated following two principles; the all-or-none law, and the size principle. This post focuses on the latter, and examines why it is so significant for human movement.

The basics of the size principle

Motor units are always recruited in an order from smallest to largest. This phenomenon is known as the size principle. Slow motor units are naturally smaller in size, which means they have the lowest activation threshold. Therefore, these motor units are the first ones to be recruited during muscle movement. 

Large motor units are activated when slow motor units are unable to produce enough force. Because of this, it takes a relatively long time (0,5s-2,5s) for a muscle to reach its maximum strength output. Since large motor units contract with more force, they also fatigue sooner. Hence why they are only recruited when needed (e.g. heavy resistance training, sprinting etc.).

The maximal strength and power exerted by a muscle is ultimately determined by three main factors; the number of motor units that are recruited, the rates at which they discharge action potentials (rate coding), and the amount of contractable muscle mass.

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The Size Principle

All motor units are activated from smallest to largestSmall motor units produce little force for an extended amount of timeLarge motor units are activated when increased force production is neededGives muscles the ability to generate varying amounts of force

Different types of motor units

Both motor units and motor neurons vary in size. Smaller motor neurons usually innervate fewer muscle fibers and therefore only produces a small amount of force. Large motor neurons tend to innervate larger and more powerful motor units, which results in a higher capacity to exert force.

Motor units also innervate different types of muscle fibers. In most skeletal muscles, smaller motor units innervate type I muscle fibers, and vice versa. Even though all motor units respond to stimuli, they have several distinct characteristics. The two biggest differences are; twitch time (how quickly the motor unit produces tension in the muscle) as well as fatigue resistance. This also divides motor units into two categories: Type I (slow motor units) and Type II (fast motor units).

The human body has roughly the same amount of both muscle fiber types, although their proportions may differ between individuals and muscles/muscle groups. As a result, each muscle is able to produce varying levels of force yet still maintain an adequate degree of muscular endurance.

Slow motor units

Slow (S) motor units, also known as type I motor units, are highly resistant to fatigue and focus on sustained physical activity and postural control. Due to their smaller size, slow motor units have a lower activation threshold. Thus, they are the first motor units to be activated during a muscle contraction. Slow motor units also produce less force than fast motor units, which are only activated when a stronger contraction is needed.

Slow motor units contain a high number of mitochondria (the powerhouse of the cell), which are an integral part of aerobic (with oxygen) energy production. Slow motor units also have an abundance of capillaries (tiny blood vessels found throughout the body). This provides oxygen and nutrients to the muscle while removing waste products. Additionally, slow motor units also contain myoglobin (a protein that binds iron and oxygen), giving them a deep red color. These aforementioned characteristics allow type I motor units to have a high oxygen capacity, making them well-suited for sustained muscle contraction.

Fast motor units

Fast motor units (type II motor units) produce a relatively high peak force in a short amount of time, making them essential in activities that rely on strength, power and speed. However, they also fatigue quicker than slow motor units.

This lower oxidative capacity is a result of having less mitochondria, fewer capillary beds, and lower myoglobin content. On the other hand, type II motor units have greater stores of muscle glycogen, which can be quickly used for anaerobic (without oxygen) energy production. This process also creates lactate as a side product.

Fast motor units are naturally larger than slow motor units, and therefore have a higher activation threshold. Because of this, fast motor units are recruited only when small motor units are unable to produce enough force. Fast motor units can be further divided into two subcategories; type IIa and type IIb/IIx. Both have specific characteristics and responses to physical activity.

Motor Unit Type

I

IIa

IIb/IIx

Contraction Speed

Slow
(90-140ms)

Fast
(50-100ms)

Very Fast
(40-90ms)

Fatigue Resistance

High

Medium

Low

Force Production

Low

High

Very High

Mitochondria Content
(powerhouse of the cell)

High

High

Low

Myoglobin Content
(a protein that binds iron & oxygen and gives blood its red colour)

High

High

Low

Capillary Content
(capillaries provide muscles with oxygen and nutrients while removing unwanted byproducts)

High

High

Low

Oxidative Capacity
(ability to use oxygen for energy production)

High

Medium

Low

Glycolytic Capacity
(ability to store and break down glycogen for intense exercises)

Low

High

High

Muscle Fiber Diameter

Small

Medium

Large

Muscle Fiber Color

Dark Red

Dark Red

Pale Red

Motor Neuron Size
(larger neurons provide faster activation)

Small

Large

Very Large

ATPase Level
(enzyme that controls glycogen breakdown and ATP synthesis)

Low

Medium

High

To put it simply, according to the size principle, small motor units are always activated first regardless of the activity. If your body realizes it needs more force to finish a task, it will start recruiting fast motor units with a higher strength capacity. This is also why there is a short lag before a muscle can reach its maximal strength output.

Motor units are always activated from smallest to largest. 

Final thoughts 

To understand why muscle fibers are recruited according to the size principle, you can think of the function of a specific muscle. For example, postural muscles must remain constantly active to maintain an upright position. To ensure these types of muscles do not tire too easily, they have a higher number of slow motor units for improved fatigue resistance. However, they are not able to generate high amounts of force required for demanding tasks or athletic performance.

When significant amounts of force is required, all motor units will be recruited; first type I, then type IIa, and finally type IIb/IIx. If you want to improve overall strength and power, you must train at high intensity and/or resistance to ensure consistent adaptation. Regular training enhances neural pathways, making them more efficient. This also helps them synchronize with other motor units to produce more force.

Did you learn anything new about the size principle? Let us know in the comments.

Sources

  • Henneman E, Somjen G, and Carpenter DO. Functional significance of cell size in spinal motoneurons. J Neurophysiol 28: 560–580, 196.
  • Henneman E and Olson CB. Relations between structure and function in the design of skeletal muscles. J Neurophysiol 28: 581–598, 1965.
  • Henneman E, Somjen G, and Carpenter DO. Excitability and inhibitability of motoneurons of different sizes. J Neurophysiol 28: 599–620, 1965.
  • McPhedran AM, Wuerker RB, and Henneman E. Properties of motor units in a homogeneous red muscle (soleus) of the cat. J Neurophysiol 28: 71–84, 1965.
  • Wuerker RB, McPhedran AM, and Henneman E. Properties of motor units in a heterogeneous pale muscle (m. gastrocnemius) of the cat. J Neurophysiol 28: 85–99, 1965.

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