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Muscle recruitment, or motor unit recruitment, is a measure of how many motor neurons are activated in a particular muscle, and subsequently, how many muscle fibres are activated. The higher the number of motor neurons activated, the stronger the muscle contraction. When a motor neuron is activated, all of the muscle fibres stimulated by the neuron are contracted. Motor units are generally recruited in order of size, from smallest to largest, as contraction increases. This is known as Henneman's size principle.
| Characteristics | Values |
|---|---|
| Definition | Motor unit recruitment is the activation of additional motor units to accomplish an increase in contractile strength in a muscle. |
| Motor unit | A motor unit consists of one motor neuron and all of the muscle fibres it stimulates. |
| Motor unit recruitment order | Motor units are generally recruited in order of smallest to largest. This is known as Henneman's size principle. |
| Motor unit categorization | Burke categorized motor unit types into three general groups: S (slow – slow twitch), FR (fast, resistant – fast twitch, fatigue-resistant), and FF (fast, fatigable – fast twitch, fatigable). |
| Muscle contraction | The activation of more motor neurons will result in more muscle fibres being activated, and therefore a stronger muscle contraction. |
| Muscle recruitment percentage | If a muscle contains 1000 muscle fibres and 500 of them are being engaged, there is 50% muscle recruitment. |
| Muscle activation | Muscle activation refers to the force generated by the individual recruited muscle fibres or their ability to contract and generate a given force. |
| Muscle stiffness | Stiff muscles can lead to a decrease in overall muscle recruitment and activation. |
| Muscle fibre restructuring | Restructuring muscle fibres back to their original anatomical state allows proper recruitment and activation of individual muscle fibres, enhancing overall performance. |
| Neurophysiological recruitment | Neurophysiological recruitment refers to the stimulation of nerve and muscle activity to help differentiate neuropathy from myopathy. |
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What You'll Learn
Motor unit recruitment
Henneman proposed that the mechanism underlying the size principle was that the 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 (EPSPs) 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.
The force produced by a single motor unit is determined in part by the number of muscle fibres in the unit. Another important determinant of force is the frequency with which the muscle fibres are stimulated by their innervating axon. The rate at which the nerve impulses arrive is known as the motor unit firing rate and may vary from frequencies low enough to produce a series of single twitch contractions to frequencies high enough to produce a fused tetanic contraction.
The central nervous system can increase the strength of muscle contraction by increasing the number of active motor units (spatial recruitment) and increasing the firing rate (firing frequency) at which individual motor units fire to optimise the summated tension generated (temporal recruitment). Both mechanisms occur concurrently.
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Muscle activation
The force produced by a single motor unit is determined by the number of muscle fibres in the unit and the frequency with which they are stimulated. The rate at which nerve impulses arrive is known as the motor unit firing rate, which may vary from frequencies that produce a series of single twitch contractions to frequencies high enough to produce a fused tetanic contraction.
In an EMG study, the term "size" of a motor unit usually refers to the amplitude of the motor unit action potential (MUAP). The later recruited type II fibres, especially the FF type, have larger diameter muscle fibres generating higher potentials than the smaller, slow-twitch type I units.
The central nervous system can increase the strength of muscle contraction by increasing the number of active motor units (spatial recruitment) and increasing the firing rate at which individual motor units fire (temporal recruitment). Both mechanisms occur concurrently. When nearly all motor units are recruited, the increase in firing frequency becomes the primary mechanism to increase motor strength.
It is important to note that muscle activation and recruitment are not the same. Muscle recruitment refers to the number of muscle fibres being engaged, while activation refers to the force generated by the individual recruited muscle fibres and their ability to contract and generate force. Ideally, there would be 100% muscle recruitment and 100% muscle activation, but this is often not the case. For example, repetitive use of muscle tissue can cause muscle fibres to become tight, hardened, and immobile, resulting in decreased overall muscle recruitment. Breaking up fibrotic tissue and restructuring the muscle back to its original anatomical state can enhance function by improving recruitment and activation of individual muscle fibres.
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Henneman's size principle
Motor unit recruitment is the activation of additional motor units to achieve a stronger muscle contraction. A motor unit consists of a motor neuron and all the muscle fibres it stimulates.
According to Henneman's size principle, motor units are recruited in a specific order based on their force output, with smaller units being recruited first, followed by larger units as more force is required. This principle is known as "smallest to largest" recruitment, with the smallest motor neurons being activated first, resulting in a weak but distributed muscle contraction. As more force is needed, larger motor neurons are recruited, leading to a stronger muscle contraction.
The size principle has two important physiological benefits. Firstly, it minimises fatigue by using fatigue-resistant muscle fibres first and only recruiting fatigable fibres when necessary. Secondly, it maintains a relatively constant relative change in force produced by additional recruitment, ensuring that the increase in force remains proportional to the number of active motor units.
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Reverse recruitment order
Motor unit recruitment is the activation of additional motor units to achieve a greater contractile strength in a muscle. A motor unit is made up of a motor neuron and all the muscle fibres it stimulates. When a motor neuron is activated, all the muscle fibres connected to it contract. The activation of more motor neurons results in a stronger 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, under some circumstances, the normal order of motor unit recruitment may be altered, with smaller motor units ceasing to fire and larger ones being recruited. This reversal of recruitment order can be achieved through cutaneous stimulation during voluntary muscle contraction.
In a study by Stephens, Garnett, and Buller, published in Nature in 1978, it was found that the recruitment threshold of motor units could be changed by cutaneous stimulation. The study involved eight experiments with five subjects, and the results showed that in every subject tested, there were examples where cutaneous stimulation was sufficient to reverse the normal order of recruitment of two units recorded simultaneously.
This reversal of recruitment order has implications for understanding the physiology of motor unit recruitment and the mechanisms underlying Henneman's size principle. It also highlights the potential for therapeutic interventions that target the cutaneous stimulation to modify motor unit recruitment patterns and potentially enhance muscle performance or rehabilitation.
Furthermore, understanding the reversal of recruitment order can inform strategies for breaking up fibrotic tissue and restructuring muscles to their original anatomical state. This process allows for proper recruitment and activation of individual muscle fibres, leading to enhanced muscle function and performance.
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Muscle recruitment patterns
Motor units are generally recruited in order of size, from smallest to largest, as contraction increases. This is known as Henneman's size principle. According to this principle, the smallest units, S (slow-oxidative), are recruited first, followed by larger FR (fast-resistant) units, and lastly the largest FF (fast-fatigable) units, which are reserved for high-energy tasks requiring additional motor unit recruitment.
The force produced by a single motor unit is determined by the number of muscle fibres in the unit and the frequency with which these fibres are stimulated by their innervating axon. This frequency is known as the motor unit firing rate, which may vary from frequencies that produce a series of single twitch contractions to frequencies high enough to produce a fused tetanic contraction.
The normal order of motor unit recruitment may be altered under certain circumstances, such as when small motor units cease to fire and larger ones are recruited. This is thought to be due to the interaction of excitatory and inhibitory motoneuronal inputs.
Research has been conducted to examine muscle recruitment patterns during specific physical activities. For example, one study analysed muscle recruitment patterns during shoulder flexion, comparing activity levels and recruitment patterns of shoulder flexor, scapular lateral rotator, and rotator cuff muscles. The results indicated that certain muscles, such as the anterior deltoid, pectoralis major, supraspinatus, and infraspinatus, were activated at similar moderate levels.
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Frequently asked questions
Muscle recruitment refers to the activation of motor units to increase the contractile strength of a muscle. A motor unit is made up of a motor neuron and the muscle fibres it stimulates.
A motor unit consists of a motor neuron and all the muscle fibres it stimulates. Motor units are recruited in a specific order, with smaller units generally being recruited first, followed by larger units.
The order of motor unit recruitment is known as Henneman's size principle, which states that smaller motor units are recruited first as they are highly fatigue-resistant and produce small amounts of force. Once greater force production is needed, larger motor units are activated.
Muscle recruitment patterns refer to the way in which specific muscles turn on and off to allow for movement. Efficient muscle recruitment patterns allow for smooth, effective, and efficient movement.
Muscle recruitment refers to the number of muscle fibres that are engaged, while muscle activation refers to the force generated by those fibres and their ability to contract.
