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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, as contraction increases. This is known as Henneman's size principle. However, there are exceptions to the size-ordered activation of motor units, and the recruitment patterns vary for different movement tasks.
| Characteristics | Values |
|---|---|
| Definition | Motor unit 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. |
| Muscle Activation vs. Muscle Recruitment | Muscle activation and muscle recruitment are often used interchangeably, but they have distinct differences. Muscle recruitment is determined by the number of fibers that are actually being engaged within a muscle contraction. |
| Motor Unit Categorization | Type I or type S (slow) – slow twitch, fatigue-resistant units with the smallest force or twitch tension and slowest contraction; contain oxidative enzymes. |
| Henneman's Size Principle | Motor units are generally recruited in order of smallest to largest (smallest motor neurons to largest motor neurons, and thus slow to fast twitch) as contraction increases. |
| Motor Unit Firing Rate | The firing rate at which individual motor units fire to optimize the summated tension generated (i.e., temporal recruitment). |
| Motor Unit Recruitment Patterns | Motor unit recruitment patterns vary for different movement tasks, depending on factors including the mechanical function of the muscle, sensory feedback, and central control. |
| Muscle Fibrosis | Repetitive use of muscle tissue can lead to muscle fibrosis, resulting in decreased muscle recruitment and activation. |
| Exercise and Training | Exercise and training can influence motor unit activation and recruitment, with specific techniques to separate and loosen tight muscle fibers for proper recruitment. |
| Movement Quality | Quality of movement is more important than quantity, emphasizing the importance of muscle recruitment patterns for efficient and injury-free movement. |
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What You'll Learn
Motor unit recruitment patterns
Generally, motor units are recruited in order of their size, following the "Henneman size principle" or simply the "size principle." When a muscle is initially activated, small motor units with low tension-generating capacity are the first to fire. As the strength of muscle contraction increases, larger motor units are progressively recruited, resulting in a smooth increase in muscle strength. This size-ordered activation ensures an orderly addition of motor units, with the smallest and weakest units activated first and the largest and strongest units activated last.
However, there are exceptions to the size-ordered activation. For instance, after nerve injury, the relationship between motoneuron size and the reinnervation of muscle fibers is temporarily lost. Over time, the size-ordered organization of motor units can be restored through size-dependent branching of axons. Additionally, the normal recruitment order may be altered under certain circumstances, with larger motor units being recruited before smaller ones due to the interaction of excitatory and inhibitory motoneuronal inputs.
The recruitment patterns can vary for different movement tasks and can be influenced by factors such as locomotor velocity and incline. Research on mammalian skeletal muscles has shown that motor units are not always recruited in a strictly orderly manner, suggesting that recruitment is a complex, multi-factorial process that is not yet fully understood.
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Muscle activation vs. muscle recruitment
Muscle activation and muscle recruitment are two distinct but interconnected concepts that are essential to understanding how muscles function and respond to exercise.
Muscle Activation
Muscle activation refers to the neurological impulse that triggers muscle fibres to contract and generate movement. This process is fundamental to all physical activity, from simple tasks to complex exercises. When a muscle is activated, it responds by contracting, producing movement and generating force. The force generated depends on the degree of activation and the ability of the muscle fibres to contract.
Muscle Recruitment
Muscle recruitment, on the other hand, is about the depth and breadth of the muscle's response. It determines the number of muscle fibres that are engaged and recruited during a muscle contraction. If a muscle contains 1000 muscle fibres, and only 500 are engaged, the muscle recruitment is 50%. However, this does not indicate the intensity or force generated by those recruited fibres. Muscle recruitment can be influenced by factors such as load and intensity, with heavier loads and higher intensities resulting in greater muscle recruitment.
The order of muscle fibre recruitment is important. Generally, motor units are recruited from smallest to largest as contraction increases, according to Henneman's size principle. This ensures that smaller, endurance-oriented fibres are recruited first, followed by larger, more powerful fibres as demand increases. However, there are exceptions to this order, and the recruitment pattern can vary depending on factors such as the mechanical function of the muscle and sensory feedback.
Interplay Between Activation and Recruitment
Maximising both muscle activation and recruitment is key to achieving a successful workout. By focusing on muscle activation, we ensure that the muscle responds to the stimulus. Meanwhile, muscle recruitment determines how many muscle fibres are engaged, impacting the overall force generated. Understanding this interplay allows individuals to tailor their workouts, promoting balanced development and reducing the risk of injuries.
Additionally, techniques such as varying tempos, progressive overload, compound movements, and adequate rest can enhance muscle activation and recruitment. For example, slowing down the eccentric phase of a lift can improve muscle activation, while increasing weight or intensity challenges the muscles, prompting greater muscle recruitment.
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Henneman's size principle
Motor unit recruitment is a measure of how many motor neurons are activated in a particular muscle and, therefore, how many muscle fibres of that muscle are activated. Motor units are recruited in order of their size, with the smallest motor units being activated first. This is known as Henneman's size principle.
Henneman proposed that the mechanism underlying the size principle was that smaller motor neurons had a smaller surface area and, therefore, a higher membrane resistance. He predicted that the current generated by an excitatory postsynaptic potential (EPSP) would result in a higher voltage change (depolarization) across the neuronal membrane of the smaller motor neurons and, therefore, larger EPSPs in smaller motor neurons.
There are exceptions to the size-ordered activation of motor units. Motor unit recruitment patterns vary for different movement tasks, depending on many factors, including the mechanical function of the muscle, sensory feedback, and central control.
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Motor unit firing rate
Motor unit 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.
At minimal levels of muscle contraction, muscle force is graded by changes in firing rate (rate coding) of individual motor neurons. The first motor unit usually begins to fire irregularly at 2-3Hz and then achieves a stable and fairly regular firing rate at 5-7Hz. This is the "onset frequency". When the patient minimally increases the force of contraction, the first unit increases the rate of firing to 6-10Hz. With further increase of muscle contraction, the second unit is recruited once the first unit achieves a firing rate of about 10Hz.
As a general rule, motor units are recruited in order of their size. When the muscle is activated initially, the first motor units to fire are small in size and weak in the degree of tension they can generate. Starting with the smallest motor units, progressively larger units are recruited with increasing strength of muscle contraction. The result is an orderly addition of sequentially larger and stronger motor units, resulting in a smooth increase in muscle strength. This is referred to as the "Henneman size principle," or simply "size principle."
There are exceptions to the size-ordered activation of motor units. Motor unit recruitment patterns vary for different movement tasks, depending on factors including the mechanical function of the muscle, sensory feedback, and central control.
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Muscle recruitment and movement quality
The quality of movement refers to the efficiency and effectiveness of muscle recruitment patterns. It involves the coordination of specific muscles turning on and off in a precise sequence to facilitate the desired movement. The nervous system acts as the conductor, signalling the muscles to contract, and the individual muscle fibres respond by contracting in a coordinated manner. This coordination is crucial for achieving smooth and efficient movements.
The order in which muscles are recruited plays a significant role in movement quality. According to Henneman's size principle, smaller muscles are recruited first as they are highly fatigue-resistant and produce smaller amounts of force. Once the initial small motor units are activated, larger motor units are subsequently recruited to generate higher levels of force. This sequential recruitment of motor units ensures a smooth increase in muscle strength, allowing for controlled and precise movements.
Additionally, the frequency of muscle contractions, also known as the motor unit firing rate, is another important factor influencing movement quality. As the strength of muscle contraction increases, the firing rate of the initially recruited motor units rises. When almost all motor units are recruited, an increase in firing frequency becomes the primary mechanism for enhancing muscle strength. This increase in firing frequency allows for more forceful contractions and contributes to the overall quality of movement.
Furthermore, the ratio of muscle fibres to motor neurons varies among different muscle groups, impacting movement quality. For example, the quadriceps have a high ratio of muscle fibres to motor neurons, enabling them to generate significant force rapidly. On the other hand, muscles responsible for more precise movements, such as the hand or eye muscles, have lower ratios. Understanding these variations in muscle recruitment patterns is essential for optimising movement quality and preventing injuries.
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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.
Motor units are generally recruited in order of smallest to largest, with contraction increasing as more units are added. This is known as Henneman's size principle.
The Henneman size principle states that smaller motor neurons have a smaller surface area and, therefore, a higher membrane resistance. This results in a higher voltage change across the neuronal membrane of the smaller motor neurons.
Muscle activation and muscle recruitment are often used interchangeably but there are differences. Muscle activation refers to the force generated by individual muscle fibres, whereas muscle recruitment is determined by the number of fibres that are engaged within a muscle contraction.
Muscle recruitment enhances function as there are more fibres being recruited and the force generated in each fibre is elevated, leading to an overall increase in performance.
