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Muscle fibres are recruited according to the size principle, which states that motor units are always recruited in an order from smallest to largest. This is because slow motor units are naturally smaller in size and have the lowest activation threshold, so they are the first to be recruited during muscle movement. Large motor units are only recruited when slow motor units are unable to produce enough force, or when a high level of force is needed, such as during heavy resistance training or sprinting.
| Characteristics | Values |
|---|---|
| Muscle fibres are activated following two principles | The all-or-none law and the size principle |
| Motor units are recruited | From smallest to largest |
| Slow motor units | Are smaller in size and have the lowest activation threshold |
| Large motor units | Are activated when slow motor units cannot produce enough force |
| Large motor units | Contract with more force and fatigue sooner |
| Large motor units are recruited | When the body needs more force to finish a task |
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The size principle
Muscle fibres are activated following two principles: the all-or-none law, and the size principle. The size principle states that motor units are always recruited in an order from smallest to largest. This is because slow motor units are naturally smaller in size, which means they have the lowest activation threshold. Therefore, these motor units are the first ones to be recruited during muscle movement.
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Motor units
Slow motor units are activated by a weak stimulus to the motor neuron pool. They are smaller in size and have a lower strength capacity, but they are also more resistant to fatigue. This is why postural muscles, which must remain constantly active to maintain an upright position, have a higher number of slow motor units.
When slow motor units cannot provide the necessary force or fatigue, larger motor units are recruited. These contract with more force but also fatigue sooner, so they are only recruited when needed, such as during heavy resistance training or sprinting.
The most common way to increase motor unit activation is to lift heavier weights. An increased load placed on a muscle will cause a greater number of motor units to activate more fibres to generate the force necessary to overcome the resistance.
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Muscle fatigue
Motor units are recruited in an order from smallest to largest, known as the size principle. Slow motor units are naturally smaller in size, which means they have the lowest activation threshold. Therefore, these motor units are the first ones to be recruited during muscle movement.
Large motor units contract with more force, but they also fatigue sooner. This is why they are only recruited when needed, for example, during heavy resistance training or sprinting.
To increase motor unit activation, you can lift heavier weights. An increased load placed on a muscle will cause a greater number of motor units to activate more fibres to generate the force necessary to overcome the resistance.
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Muscle contractions
The size principle is also known as the Henneman size principle, named after the scientist who discovered it. It states that 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.
The size principle is significant for human movement because it means that it takes a relatively long time (0.5-2.5 seconds) for a muscle to reach its maximum strength output. This is because large motor units contract with more force, but they also fatigue sooner. Hence why they are only recruited when needed, for example during heavy resistance training or sprinting.
The size principle also explains why postural muscles have a higher number of slow motor units. Postural muscles must remain constantly active to maintain an upright position. To ensure these types of muscles do not tire too easily, they have a higher number of slow motor units for improved fatigue resistance.
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Muscle strength
The size principle also explains why there is a short lag before a muscle can reach its maximal strength output. If the body requires more force to finish a task, it will recruit fast motor units with a higher strength capacity. Large motor units contract with more force but also fatigue sooner, which is why they are only recruited when needed, such as during heavy resistance training or sprinting.
The most common way to increase motor unit activation is to lift heavier weights. An increased load on a muscle will cause a greater number of motor units to activate more fibres to generate the force necessary to overcome the resistance.
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Frequently asked questions
Muscle fibres are recruited according to the size principle, which means that motor units are always recruited in an order from smallest to largest.
Motor units are the combination of a nerve and the muscle fibres it controls.
There are two types of motor units: type I and type II. Type I motor units are smaller and slower, while type II motor units are larger and faster.
Type I motor units are recruited first when a muscle requires force. They are slower but have improved fatigue resistance.
Type II motor units are recruited when type I motor units cannot provide the necessary force or fatigue. They are faster but fatigue sooner, so they are only recruited when needed, such as during heavy resistance training or sprinting.
