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Muscle recruitment describes a motor neuron (a nerve) and all the individual muscle fibres that it innervates. The order in which we innovate different muscles is important for the quality and efficiency of movement. Motor units are generally recruited in order of smallest to largest, as contraction increases. This is known as Henneman's size principle. For example, when throwing a ball, the deltoid (shoulder) muscle works hard to produce force, but the dynamic stability provided by the rotator cuff would cancel out this upwards motion by applying a downwards force on the humerus, keeping it centred in the shoulder joint.
| Characteristics | Values |
|---|---|
| Definition | Muscle recruitment describes a motor neuron (a nerve) and all the individual muscle fibres that it innovates |
| Muscle recruitment patterns | Smaller muscles are recruited first as they are highly fatigue-resistant and produce small amounts of force. Once these motor units have been recruited and we need more force production, larger motor units will be activated as they are more prone to fatigue due to their high force output |
| Motor unit recruitment | 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. The higher the recruitment, the stronger the muscle contraction will be |
| Muscle groups | Some muscles, such as the quadriceps, have a high ratio of muscle fibres to motor neurons. This is because they are a powerful muscle group responsible for displaying force very quickly |
| Muscle groups | Other muscles, such as hand or eye muscles, have much lower ratios as they use more precise, refined movement |
| Muscle recruitment and movement | The order in which we innovate different muscles is important for the quality and efficiency of movement |
| Muscle recruitment and movement | Throwing a ball is a great example to illustrate the importance of muscle recruitment. During this motion, the deltoid (shoulder) muscle works hard to produce force in order to throw the ball |
| Muscle recruitment and movement | The dynamic stability provided by the rotator cuff would cancel out the upwards motion of the deltoid muscle by applying a downwards force on the humerus, keeping it centred in the shoulder joint |
| Muscle recruitment and movement | Without this recruitment, the head of the humerus can move around quite a bit, and pinch some important tissue |
| Muscle recruitment and movement | The order of recruitment is fixed for many movements, including resistance exercise; if the body position changes, however, the order of recruitment can also change and different muscle fibres can be recruited (e.g., in a flat vs. an incline bench press) |
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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. This means that smaller muscles are recruited first as they are highly fatigue-resistant and produce small amounts of force. Once these motor units have been recruited and we need more force production, larger motor units will be activated as they are more prone to fatigue due to their high force output.
The order in which we innovate different muscles is important for the quality and efficiency of movement. For example, during the motion of throwing a ball, the deltoid (shoulder) muscle works hard to produce force in order to throw the ball. Without the dynamic stability provided by the rotator cuff, the head of the humerus can move around and pinch some important tissue.
The order of recruitment is important from a practical standpoint for several reasons. First, in order to recruit Type II fibres and thus achieve a training effect in these fibres, the exercise must be characterised by heavy loading or demands for high power output. Second, the order of recruitment is fixed for many movements, including resistance exercise; if the body position changes, however, the order of recruitment can also change and different muscle fibres can be recruited (e.g. in a flat vs an incline bench press).
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Muscle recruitment patterns
Muscle recruitment describes a motor neuron (a nerve) and all the individual muscle fibres that it innovates. 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 will be.
Motor units are generally recruited in order of smallest to largest 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. Smaller muscles are recruited first as they are highly fatigue-resistant and produce small amounts of force. Once these motor units have been recruited and we need more force production, larger motor units will be activated as they are more prone to fatigue due to their high force output.
The order in which we innovate different muscles is important for the quality and efficiency of movement. For example, during the motion of throwing a ball, the deltoid (shoulder) muscle works hard to produce force in order to throw the ball. In an ideal recruitment pattern, the dynamic stability provided by the rotator cuff would cancel out this upwards motion by applying a downwards force on the humerus, keeping it centred in the shoulder joint. Without this recruitment, the head of the humerus can move around quite a bit, and pinch some important tissue.
The order of recruitment is fixed for many movements, including resistance exercise; if the body position changes, however, the order of recruitment can also change and different muscle fibres can be recruited. For example, in a flat vs an incline bench press. This ability to rest motor units when submaximal force is needed also helps to delay fatigue. When velocities are very slow and loads are very light, this type of recruitment may predominate during the exercise, leaving many muscle fibres unstimulated and thus primarily promoting endurance.
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Muscle fibres
Muscle recruitment describes a motor neuron (a nerve) and all the individual muscle fibres that it innervates. The number of motor neurons activated in a particular muscle is a measure of 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). This is known as Henneman's size principle. Smaller muscles 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 production is needed, larger motor units will be activated as they are more prone to fatigue due to their high force output.
The order of recruitment is important from a practical standpoint for several reasons. First, in order to recruit Type II fibres and thus achieve a training effect in these fibres, the exercise must be characterised by heavy loading or demands for high power output. Second, the order of recruitment is fixed for many movements, including resistance exercise; if the body position changes, however, the order of recruitment can also change and different muscle fibres can be recruited (e.g. in a flat vs. an incline bench press).
Muscle recruitment patterns are important for the quality and efficiency of movement. For example, during the motion of throwing a ball, the deltoid (shoulder) muscle works hard to produce force in order to throw the ball. In an ideal recruitment pattern, the dynamic stability provided by the rotator cuff would cancel out this upwards motion by applying a downwards force on the humerus, keeping it centred in the shoulder joint. Without this recruitment, the head of the humerus can move around quite a bit, and pinch some important tissue.
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Muscle contraction
Motor units are generally recruited in order of smallest to largest, as contraction increases. This is known as Henneman's size principle. Smaller muscles 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 will be activated. This is because they are more prone to fatigue due to their high force output.
The order in which we innovate different muscles is important for the quality and efficiency of movement. For example, when throwing a ball, the deltoid (shoulder) muscle works hard to produce force. In an ideal recruitment pattern, the dynamic stability provided by the rotator cuff would cancel out the upwards motion of the humerus by applying a downwards force, keeping it centred in the shoulder joint.
The order of recruitment is fixed for many movements, including resistance exercise. However, if the body position changes, the order of recruitment can also change and different muscle fibres can be recruited. For example, in a flat vs. an incline bench press.
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Muscle fatigue
Muscle recruitment describes a motor neuron (a nerve) and all the individual muscle fibres that it innovates. The order in which we innovate different muscles is important for the quality and efficiency of movement. 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 as contraction increases. This is known as Henneman's size principle.
Smaller muscles 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 production is needed, larger motor units will be activated as they are more prone to fatigue due to their high force output. This ability to rest motor units when submaximal force is needed also helps to delay fatigue. When velocities are very slow and loads are very light, this type of recruitment may predominate during the exercise, leaving many muscle fibres unstimulated and thus primarily promoting endurance.
The quadriceps, for example, have a high ratio of muscle fibres to motor neurons. This is because they are a powerful muscle group responsible for displaying force very quickly. Other muscles, such as hand or eye muscles, have much lower ratios as they use more precise, refined movement.
Throwing a ball is a great example to illustrate the importance of muscle recruitment. During this motion, the deltoid (shoulder) muscle works hard to produce force in order to throw the ball. Due to the deltoid's origin and insertion, it produces an upwards translation of the humerus. In an ideal recruitment pattern, the dynamic stability provided by the rotator cuff would cancel out this upwards motion by applying a downwards force on the humerus, keeping it centred in the shoulder joint. Without this recruitment, the head of the humerus can move around quite a bit, and pinch some important tissue.
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Frequently asked questions
Muscle recruitment describes a motor neuron (a nerve) and all the individual muscle fibres that it innovates. The higher the recruitment, the stronger the muscle contraction will be.
Henneman's size principle states that smaller muscles are recruited first as they are highly fatigue-resistant and produce small amounts of force. Once these motor units have been recruited and we need more force production, larger motor units will be activated as they are more prone to fatigue due to their high force output.
During this motion, the deltoid (shoulder) muscle works hard to produce force in order to throw the ball. In an ideal recruitment pattern, the dynamic stability provided by the rotator cuff would cancel out the upwards motion by applying a downwards force on the humerus, keeping it centred in the shoulder joint.
