Victor Forge
Muscle recruitment is a measure of how many motor neurons are activated in a particular muscle and, therefore, how many muscle fibres are activated. The higher the recruitment, the stronger the muscle contraction. Motor units are generally recruited in order of smallest to largest, with contraction increasing. This is known as Henneman's size principle. The process of muscle recruitment is important to understand in the context of movement injury prevention, as the order in which muscles are recruited is important for quality and efficiency of movement.
| Characteristics | Values |
|---|---|
| Definition | Muscle recruitment is a measure of how many motor neurons are activated in a particular muscle, and therefore is a measure of how many muscle fibers of that muscle are activated. |
| Motor Unit Firing Rate | The firing rate of individual motor units increases with increasing muscular effort until a maximum rate is reached. |
| Motor Unit Size | Motor units are generally recruited in order of their size, from smallest to largest. |
| Motor Unit Types | Type I or Type S (Slow) - Slow twitch, fatigue-resistant units with the smallest force or twitch tension and slowest contraction; Type II or Type FR (Fast, Resistant) - Fast-twitch, fatigue-resistant units; Type FF (Fast, Fatigable) - Fast-Glycolytic units, reserved for high-energy tasks. |
| Henneman's Size Principle | Smaller motor neurons are recruited first as they are highly fatigue-resistant and produce small amounts of force. Once these motor units have been recruited and more force is needed, larger motor units are activated. |
| Muscle Activation | Muscle activation refers to the degree of activation of the recruited muscle fibers and their ability to contract and generate force. |
| Muscle Fibrosis | With repetitive use, muscle fibers can become fibrotic, leading to decreased muscle recruitment and activation. |
| Exercise and Training | Exercise and training can influence motor unit activation and recruitment patterns. |
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What You'll Learn
Motor unit recruitment patterns
Motor unit recruitment is a measure of the number of motor neurons activated in a particular muscle and, therefore, the number of muscle fibres activated. Motor units are recruited in order of size, from smallest to largest, as contraction increases. This is known as Henneman's size principle.
However, there are exceptions to the size-ordered activation of motor units. Motor unit recruitment patterns vary depending on the task and other factors, such as the mechanical function of the muscle, sensory feedback, and central control. For example, in some cases, smaller motor units may cease to fire and larger ones are recruited due to the interaction of excitatory and inhibitory motoneuronal inputs.
Research has also shown that motor unit recruitment patterns can be influenced by changes in locomotor velocity and incline. For example, faster locomotor velocities lead to a relative increase in high-frequency components, while greater inclines lead to a relative increase in low-frequency components. This indicates that motor unit recruitment is a multi-factorial phenomenon that is not yet fully understood.
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Henneman's size principle
In 1957, Dr. Elwood Henneman discovered the "size principle", which describes the relationships between the properties of motor neurons and the muscle fibres they innervate and control. Motor neurons are recruited in order of size, from low-threshold motor units containing 1-10 muscle fibres to high-threshold motor units containing hundreds to thousands of muscle fibres. This is important in the context of strength, hypertrophy, and the rate of force development.
The size principle can be observed in the soleus and gastrocnemius muscles. The soleus muscle is composed of "red" muscle, indicating that the muscle fibres are fatigue-resistant but create small forces when contracting. On the other hand, the gastrocnemius muscle is heterogeneous, composed of both "red" and "pale" muscle, and thus containing fast-twitch high-force fibres.
Henneman and his colleagues observed that the neurons innervating the soleus muscle produced smaller electrical signals when measuring electrical activity, reflecting the diameter of the motor neuron. Additionally, they found that motor neurons innervating the soleus muscle fired first when the afferent nerves in the dorsal root were electrically stimulated.
The size principle has been supported by various experiments, including one that examined the quadriceps femoris. This study found that motor units are recruited in an orderly manner, with the size of motor units increasing linearly with increased force production. As the force generated increases, the amplitude (strength) and frequency (firing rate) also increase.
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Motor unit categorisation
- S (slow-twitch): small motor units with a high surface-to-volume ratio, allowing for greater metabolic activity and optimised for the "highest duty cycles" of motor neurons. These are the first to be recruited during muscle activation.
- FR (fast-twitch, resistant): larger motor units that are recruited after S units.
- FF (fast-twitch, fatigable): the largest motor units, reserved for high-energy tasks requiring additional motor unit recruitment.
This categorisation is based on Henneman's size principle, which states that motor units are recruited from smallest to largest based on the size of the load. However, modern research indicates that human motor units are more complex and may not directly fit this model. Additionally, there are exceptions to the size-ordered activation of motor units, as the recruitment patterns vary depending on factors such as the mechanical function of the muscle, sensory feedback, and central control.
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Muscle activation vs. muscle recruitment
Muscle activation and muscle recruitment are often used interchangeably, but they are distinct concepts. Muscle activation is the neurological impulse that causes muscle fibres to contract and generate movement. Muscle recruitment, on the other hand, refers to the number of muscle fibres that are engaged during a muscle contraction.
To illustrate the difference, consider a muscle containing 1000 muscle fibres. If 500 of them are engaged during contraction, that would be 50% muscle recruitment. However, this does not indicate the degree of activation of those fibres. The force generated by the individual fibres is a measure of activation, or their ability to contract and produce force.
In terms of muscle contraction, the force generated increases with the number of muscle fibres recruited. Motor unit recruitment refers to the process by which different motor units are activated to produce a given level and type of muscle contraction. Motor units are generally recruited in order of their size, from smallest to largest, as contraction increases. This is known as Henneman's size principle.
The size principle dictates that smaller, endurance-oriented fibres are recruited first, with larger, more powerful fibres joining in as demand increases. This ensures a smooth increase in muscle strength. However, there are exceptions to the size-ordered activation of motor units, as the recruitment pattern can vary depending on factors such as the mechanical function of the muscle and sensory feedback.
Understanding the distinction between muscle activation and recruitment is important for maximising the effectiveness and efficiency of workouts. By manipulating intensity and load, different muscle fibres can be targeted, promoting balanced development and reducing the risk of injuries.
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Muscle recruitment and movement quality
Muscle recruitment is a measure of how many motor neurons are activated in a particular muscle and, therefore, how many muscle fibres are activated. Motor units are generally recruited in order of smallest to largest, with contraction increasing. This is known as Henneman's size principle.
During slow and ballistic increases in force, motor units are recruited in rank order of their size. However, there are exceptions to the size-ordered activation of motor units. Motor unit recruitment patterns vary for different movement tasks, depending on factors such as the mechanical function of the muscle, sensory feedback, and central control.
The quality of movement is dependent on muscle recruitment, or the way in which specific muscles will turn on and off to allow movement. For example, the quadriceps have a high ratio of muscle fibres to motor neurons because they are a powerful muscle group responsible for displaying force very quickly. On the other hand, hand or eye muscles have much lower ratios as they use more precise, refined movement. The order in which we innovate different muscles is important for the quality and efficiency of movement.
When it comes to movement, quality is more important than quantity. The quality of movement is dependent on the muscle recruitment pattern, or the way in which specific muscles will turn on and off to allow movement. The muscles will use the easiest way to produce the least amount of force required.
Muscle recruitment patterns are important in movement injury prevention. The synergistic activation between smaller and larger motor units allows smooth, efficient, and effective movement patterns. For example, when throwing a ball, the deltoid (shoulder) muscle works hard to produce force to throw the ball.
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Frequently asked questions
Muscle recruitment describes the way in which specific muscles turn on and off to allow movement. It is a measure of how many motor neurons are activated in a particular muscle, and therefore how many muscle fibres are activated.
The Henneman size principle, or size principle, states that motor units are recruited in order of their size, from smallest to largest. This is because smaller motor neurons have a smaller surface area and therefore a higher membrane resistance.
The order in which muscles are recruited is important for the quality and efficiency of movement. The force output of a muscle is graded by the order of motor unit recruitment.
