Logan Steele
Muscle recruitment is a measure of how many motor neurons are activated in a particular muscle and, therefore, how many muscle fibers are activated. The more muscle fibers that are recruited, the stronger the muscle contraction. Different activities require different levels of muscle recruitment. For example, an easy run requires less muscle recruitment than a maximum-effort race. Similarly, lifting a broomstick requires less muscle recruitment than lifting a barbell. The order in which we innovate different muscles is important for the quality and efficiency of movement. Muscle recruitment patterns can also help prevent injuries. For instance, during the motion of throwing a ball, the rotator cuff should apply a downward force on the humerus to prevent it from moving around and causing injury.
| Characteristics | Values |
|---|---|
| Muscle activation vs. muscle recruitment | Muscle activation and muscle recruitment are different: activation refers to the degree of engagement of the muscle fibers, while recruitment is the number of fibers being engaged |
| Muscle activation | The force generated by individual muscle fibers |
| Motor unit recruitment | The activation of additional motor units to increase contractile strength in a muscle |
| Motor units | Consist of one motor neuron and all the muscle fibers it stimulates |
| Motor unit firing rate | The rate at which nerve impulses arrive, increasing with muscular effort |
| Motor unit recruitment order | Motor units are generally recruited from smallest to largest |
| Mechanical loading | The primary mechanism for muscle growth, which can occur with or without motor unit recruitment |
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What You'll Learn
Motor unit recruitment
Motor units are generally recruited in order of size, from smallest to largest, as contraction increases. This is known as Henneman's size 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, 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 and the frequency with which the muscle fibres are stimulated. The rate at which nerve impulses arrive is known as the motor unit firing rate.
The central nervous system can increase the strength of muscle contraction in two ways: by increasing the number of active motor units (spatial recruitment) and by increasing the firing rate (firing frequency) at which individual motor units fire (temporal recruitment). Both mechanisms occur concurrently. 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.
Under some circumstances, the normal order of motor unit recruitment may be altered, with small motor units ceasing to fire and larger ones being recruited. This is thought to be due to the interaction of excitatory and inhibitory motoneuronal inputs.
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Muscle fibre types
Slow oxidative fibres, also known as Type I or SO fibres, contract relatively slowly and use aerobic respiration (oxygen and glucose) to produce ATP. They produce low-power contractions over long periods and are slow to fatigue. The legs and thighs of a turkey contain slow oxidative fibres, which is why the meat is dark in colour.
Fast oxidative fibres, also known as Type IIa or FO fibres, have relatively fast contractions and primarily use aerobic respiration to generate ATP. They produce higher-tension contractions than slow oxidative fibres.
Fast glycolytic fibres, also known as Type IIx or FG fibres, have fast contractions and primarily use anaerobic glycolysis to generate ATP. They have a large diameter and possess large volumes of glycogen, which allows them to generate ATP quickly. However, they fatigue quickly and are only suitable for short periods of use.
The speed of contraction of a muscle fibre depends on how quickly myosin's ATPase hydrolyzes ATP to produce cross-bridge action. Fast fibres hydrolyze ATP approximately twice as rapidly as slow fibres, resulting in much quicker cross-bridge cycling. The number of slow and fast-twitch fibres in an individual varies and is determined by their genetics.
Motor unit recruitment refers to the activation of additional motor units to increase contractile strength in a muscle. A motor unit consists of one motor neuron and all the muscle fibres it stimulates. When a motor neuron is activated, all the muscle fibres it innervates are stimulated and contract. The activation of more motor neurons will result in a stronger muscle contraction. Motor units are generally recruited from smallest to largest, known as Henneman's size principle.
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Muscle contraction mechanics
Muscle contraction is the process by which muscles fibres shorten and relax to move the body. This process is triggered by messages from the nervous system, specifically the brain, which sends electrochemical signals through the somatic nervous system to motor neurons that innervate muscle fibres.
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, as contraction increases. This is known as Henneman's size principle.
The sequence of events that result in the contraction of an individual muscle fibre begins with a signal from the motor neuron innervating that fibre. The local membrane of the fibre will depolarize as positively charged sodium ions enter, triggering an action potential that spreads to the rest of the membrane, including the T-tubules. This triggers the release of calcium ions from storage in the sarcoplasmic reticulum. The calcium ions then bind to troponin, causing conformational changes in the sarcomere, leading to the interaction of thick and thin filaments of the sarcomere and resulting in muscle contraction.
The sliding filament theory of muscle contraction proposes that actin filaments actively attach to and pull on myosin filaments. The contraction of a striated muscle fibre occurs as the sarcomeres, linearly arranged within myofibrils, shorten as myosin heads pull on the actin filaments. The region where thick and thin filaments overlap is very important to muscle contraction, as it is the site where filament movement starts.
Over time, through repetitive use, muscle fibres can become tighter, hardened, and immobile, resulting in decreased muscle recruitment and activation. This can lead to a feeling of weakness, despite the absence of true weakness, but rather tightness causing the perceived weakness.
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Training and exercise
To improve muscle recruitment and activation, it is important to understand the concept of motor unit recruitment. A motor unit consists of a motor neuron and all the muscle fibres it stimulates. During physical activity, the activation of motor neurons leads to the stimulation and contraction of muscle fibres. The more motor neurons that are activated, the stronger the muscle contraction. Motor units are typically recruited from smallest to largest, known as Henneman's size principle, where smaller motor neurons have higher membrane resistance.
Furthermore, addressing muscle tightness and stiffness is crucial for effective muscle recruitment. Techniques such as precise, direct pressure into the muscle fibres can help separate and loosen contracted fibres, improving their ability to be recruited and engaged during exercise. This approach is different from conventional stretching or foam rolling, as it targets deep muscle tightness. By combining specific exercises with targeted muscle recruitment techniques, individuals can enhance their strength, speed, and overall performance while reducing the risk of injury.
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Muscle coordination
During multi-joint movements, a muscle's action can impact joints and segments it is not directly attached to. For example, the gastrocnemius muscle can accelerate the knee into extension during upright standing. The coordination of multiple muscles is essential to achieve specific objectives, such as minimising energy expenditure, increasing stability, or reducing joint loading in individuals with osteoarthritis.
Motor unit recruitment plays a crucial role in muscle coordination. Motor units consist of a motor neuron and the muscle fibres it stimulates. When a motor neuron is activated, the corresponding muscle fibres contract. The activation of additional motor units leads to an increase in contractile strength. Motor units are typically recruited from smallest to largest, following Henneman's size principle, which suggests that smaller motor neurons have a higher membrane resistance. However, under certain circumstances, this order may be altered, with larger motor units being recruited first.
The human body has muscular redundancy, meaning there are often more muscles than necessary to produce a particular movement. This redundancy allows the central nervous system to optimise muscle coordination based on specific performance goals. For example, individuals may adopt a ""gastrocnemius avoidance" gait pattern to reduce knee contact force during walking.
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Frequently asked questions
Muscle recruitment is the activation of additional motor units to increase the strength of a muscle contraction. A motor unit consists of one motor neuron and all the muscle fibres it stimulates.
To recruit more muscle fibres, you need to generate more force. This can be done through exercises such as heavy weightlifting, which requires you to activate more fibres to lift the weight.
Muscle activation and recruitment are often used interchangeably, but there are differences. Muscle recruitment refers to the number of muscle fibres being engaged during a contraction, whereas muscle activation refers to the force generated by those individual fibres and their ability to contract.
Yes, people recruit muscles differently depending on the activity and the muscle group. For example, the quadriceps have a high ratio of muscle fibres to motor neurons, whereas the hand or eye muscles have a lower ratio.
Muscle recruitment patterns are important for quality movement and injury prevention. To improve muscle recruitment, you can focus on specific exercises that target the recruitment of certain muscle groups, such as the deltoids and rotator cuffs for throwing a ball.
