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Muscle fiber recruitment is a process that involves activating specific motor units within a muscle to produce varying degrees of force. Efficient recruitment is essential for optimising strength, endurance, and coordination during physical activities. The order of recruitment is governed by Henneman's size principle, which states that smaller motor units are recruited first due to their fatigue resistance and lower force output. As the demand for force increases, larger motor units are sequentially activated, leading to a synergistic activation that enables smooth and efficient movement patterns. This process is influenced by the mechanics of muscle contraction and is crucial for achieving top fitness and improving performance in activities such as running.
| Characteristics | Values |
|---|---|
| Definition | Muscle fibre recruitment is a measure of how many motor neurons are activated in a particular muscle, and therefore is a measure of how many muscle fibres of that muscle are activated. |
| Muscle contraction | The higher the recruitment, the stronger the muscle contraction. |
| Motor units | 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 rate at which nerve impulses arrive and stimulate muscle fibres. The motor unit firing rate of each individual motor unit increases with increasing muscular effort until a maximum rate is reached. |
| Motor unit size | There is an inverse relationship between the number of motor units and the force each generates. There are many small motor units and progressively fewer larger motor units. |
| Abnormal recruitment patterns | If the recruitment ratio approaches 10, motor units are too few for the greatest firing frequency and force produced (decreased recruitment). If it is reduced to less than 4 or 5, motor units are too many for the highest firing rate (early recruitment). |
| Selective recruitment | During locomotion, faster motor units are selectively recruited within the MG muscle in response to increasing muscle fascicle strain rates. |
| Myoelectric signals | When a muscle is active, the faster fibres generate higher frequencies within the myoelectric spectra than slow fibres. |
| Muscle fibre types | Type I oxidative fibres, Type IIa fast twitch, fatigue-resistant fibres, and Type IIx fast-twitch fibres. |
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What You'll Learn
Motor unit recruitment
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. This is known as Henneman's size principle. 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 three main types of motor units, which have different physiologic and staining properties, are:
- Type I or Type S (Slow) - slow-twitch, fatigue-resistant units with the smallest force or twitch tension and slowest contraction; contain oxidative enzymes.
- Type IIa or Type FR (Fast, Resistant) - fast-twitch, fatigue-resistant units with larger forces and faster contraction times; contain oxidative and glycolytic enzymes.
- Type IIb or Type FF (Fast, Fatigable) - fast-twitch, easily fatigable units with the largest force and fastest contraction; contain glycolytic enzymes.
The recruitment sequence is thought to begin with Type I motor units analogous to Type S units, progress to Type II units that first include Type FR (Type IIa), and end with units analogous to Type FF (Type IIb), which are active only at relatively high force output. Under some circumstances, the normal order of motor unit recruitment may be altered, such that 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.
The primary mechanism at lower levels of muscle contraction strength is the addition of more motor units, but the firing rate of the initially recruited motor units also increases. When nearly all motor units are recruited, the increase in firing frequency becomes the predominant mechanism to increase motor strength.
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Muscle fibre types
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. The higher the recruitment, the stronger the muscle contraction. Motor units are generally recruited in order of smallest to largest (smallest motor neurons to largest motor neurons and, thus, slow to fast twitch).
Skeletal muscle fibres can be classified based on two criteria: how fast the fibres contract relative to others, and how the fibres regenerate ATP. Using these criteria, there are three main types of skeletal muscle fibres: slow oxidative (Type I), fast oxidative (Type IIa), and fast glycolytic (Type IIx). Slow oxidative fibres use aerobic metabolism to produce low-power contractions over long periods and are slow to fatigue. They contract relatively slowly and use aerobic respiration (oxygen and glucose) to produce ATP.
Fast oxidative fibres use aerobic metabolism to produce ATP but generate higher tension contractions than slow oxidative fibres. They have relatively fast contractions and primarily use aerobic respiration to generate ATP.
Fast glycolytic fibres use anaerobic metabolism to produce powerful, high-tension contractions but fatigue quickly. They have a large diameter and possess large volumes of glycogen, which is used in glycolysis to generate ATP quickly. Because of their reliance on anaerobic metabolism, these fibres do not possess substantial numbers of mitochondria, have a limited capillary supply, and have low amounts of myoglobin, resulting in a white coloration for muscles containing large numbers of these fibres.
Most skeletal muscles in the human body contain all three types of fibres, although in varying proportions. The speed of contraction is dependent on how quickly myosin's ATPase hydrolyzes ATP to produce cross-bridge action.
During locomotion, the skeletal muscles are excited by impulses from the α-motorneurons that originate in the spinal cord. The smallest α-motorneurons have the lowest thresholds for excitation and innervate the slowest muscle fibres. Thus, a weak stimulus to the motorneuron pool results in the slowest muscle fibres being recruited. The faster motor units are sequentially recruited as the stimulus strength increases in a graded fashion known as the size principle of motor unit recruitment.
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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 of that muscle are activated. The higher the recruitment, the stronger the muscle contraction. 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. This is known as Henneman's size principle.
The rate at which nerve impulses arrive is known as the motor unit firing rate. This rate increases with increasing muscular effort until a maximum rate is reached. This smooths out the incremental force changes which would otherwise occur as each additional unit was recruited. The motor unit firing rate 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 primary mechanism at lower levels of muscle contraction strength is the addition of more motor units, but the firing rate of the initially recruited motor units also increases. When nearly all motor units are recruited, the increase in firing frequency becomes the predominant mechanism to increase motor strength. At this level and beyond, motor units may be driven to fire in their secondary range to rates greater than 50 Hz.
In a prolonged contraction of constant strength, only continuously firing motor units are active. On rapid accelerations, however, both continuously and intermittently firing motor units are active and play similar roles. It is suggested that continuously firing low-frequency motor units have type I muscle fibres, and intermittently firing high-frequency units have type II muscle fibres. The order of recruitment and the relative roles of the two motor unit types are adapted to the mode of contraction.
In some circumstances, the normal order of motor unit recruitment may be altered, such that 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.
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Muscle contraction
Muscle fibre recruitment is a process that involves activating specific motor units within a muscle to produce varying degrees of force. 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. The higher the recruitment, the stronger the muscle contraction.
Motor units are recruited in a specific order, starting with the smallest motor neurons and progressing to the largest as contraction increases. This is known as Henneman's size principle. According to Henneman, the smaller motor neurons have a smaller surface area, resulting in higher membrane resistance. This means that the current generated by an excitatory postsynaptic potential (EPSP) will lead to a higher voltage change across the neuronal membrane of the smaller motor neurons.
During locomotion, the skeletal muscles receive impulses from α-motorneurons that originate in the spinal cord. The smallest α-motorneurons have the lowest thresholds for excitation and innervate the slowest muscle fibres. As the stimulus strength increases, faster motor units are sequentially recruited, following the size principle.
The firing rate of individual motor units also plays a role in muscle contraction. As muscular effort increases, the firing rate of each motor unit increases until a maximum rate is reached. This helps smooth out the incremental force changes that would otherwise occur with the recruitment of each additional unit. The force produced by a single motor unit is influenced by the number of muscle fibres in the unit and the frequency of stimulation by its innervating axon.
In certain circumstances, the normal order of motor unit recruitment may be altered. For example, in muscle diseases like polymyositis or muscular dystrophies, muscle fibres are damaged, reducing the force output of each motor unit. To compensate, multiple motor units may begin firing simultaneously. Similarly, in acute nerve injuries, a lesion may result in a reduced number of functional motor neurons, leading to decreased recruitment and requiring an increased firing rate to achieve the same force.
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Muscle activation
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. This is known as Henneman's size principle. The smallest units, S (slow) (Slow-Oxidative), are recruited first, followed by larger FR (fast, resistant) (Fast-Oxidative) units, and lastly the largest FF (fast, fatigable) (Fast-Glycolytic) units, reserved for high-energy tasks that require additional motor unit recruitment.
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 primary mechanism at lower levels of muscle contraction strength is the addition of more motor units, but the firing rate of the initially recruited motor units also increases. When nearly all motor units are recruited, an increase in firing frequency becomes the predominant mechanism to increase motor strength.
In some circumstances, the normal order of motor unit recruitment may be altered, such that 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. In muscle diseases such as polymyositis or muscular dystrophies, muscle fibres are damaged. A number of motor units are unaffected but the muscle fibre content of each motor unit is reduced; therefore, the force output of each unit is diminished. The number of units required to maintain a given force increases in proportion to the inefficiency of the individual motor unit discharge. Compensation occurs by having multiple motor units begin firing simultaneously.
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Frequently asked questions
Muscle fibre recruitment is the process by which the nervous system activates specific motor units within a muscle to produce different levels of force.
Motor units are recruited in a specific order, from smallest to largest, as the contraction increases. This is known as Henneman's size principle.
Proposed by Henneman, the 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 when an excitatory postsynaptic potential (EPSP) is generated.
Training to recruit muscle fibres efficiently and powerfully can help runners increase their speed and VO2 max. Recruiting and strengthening all three types of muscle fibres is essential for optimal performance.
Muscle fibres can be categorized into three types: slow-twitch (Type I), intermediate, and fast-twitch. Slow-twitch fibres are fatigue-resistant and produce small amounts of force, while fast-twitch fibres generate higher levels of force but fatigue more quickly.
