Dominic Stone
Muscle fibres are responsible for controlling physical forces moving through the body. They can shorten to generate force and lengthen to control and decelerate a force. Muscle fibres can adapt to changing demands by changing size or fibre type composition. This plasticity is the basis for physical therapy interventions designed to increase a patient's force development or endurance. The three types of muscle fibres are slow oxidative (SO), fast oxidative (FO) and fast glycolytic (FG). 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.
| Characteristics | Values |
|---|---|
| Purpose | Control physical forces moving through the body |
| Muscle-shortening actions | Generate force to move resistance |
| Muscle-lengthening actions | Control and decelerate force |
| Contractile properties | Related to intrinsic speed |
| Skeletal muscles | Excited by impulses from α-motorneurons |
| α-motorneurons | Originate in the spinal cord |
| Excitability of α-motorneurons | Related to size of cell bodies |
| Motor unit recruitment | Faster motor units are sequentially recruited as stimulus strength increases |
| Types of muscle fibres | Slow oxidative (SO), fast oxidative (FO) and fast glycolytic (FG) |
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What You'll Learn
Muscle fibres and the control of physical forces
The primary purpose of muscle fibres is to control physical forces moving through the body. Muscle fibres can respond to the mechanics of muscle contraction, with faster fibres being recruited for faster tasks. This is known as the size principle of motor unit recruitment.
During locomotion, 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. As the stimulus strength increases, faster motor units are recruited.
The contractile properties of muscle fibres are related to their intrinsic speed, or the maximum unloaded strain rate at which they can shorten. The shortening velocity at which the maximum mechanical power is generated is typically between 0.25 and 0.36. Generating mechanical power at a high efficiency is best achieved using faster muscle fibres for faster contractions.
Muscle fibres can also adapt to changing demands by changing size or fibre type composition. This plasticity serves as the physiologic basis for numerous physical therapy interventions designed to increase a patient's force development or endurance.
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Muscle fibre recruitment and muscle contraction
Muscle fibres respond to the mechanics of muscle contraction. The primary purpose of muscle fibres is to control physical forces moving through the body. Muscle-shortening actions can generate a force to move a resistance, for example, when moving from a seated to a standing position, the quadriceps and gluteus maximus shorten to help the body stand up against gravity. Muscle-lengthening actions can be applied to control and decelerate a force, for example, the quadriceps and glutes lengthen to control the motion of the body as it returns to a seated position.
During locomotion, the skeletal muscles are excited by impulses from the α-motorneurons that originate in the spinal cord. The excitability of the α-motorneurons is related to the size of their cell bodies. 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.
The contractile properties of muscle fibres are related to the intrinsic speed at which they can shorten. The shortening velocity at which the maximum mechanical power is generated and the velocity at which the maximum mechanical efficiency is achieved. Therefore, generating mechanical power at a high efficiency is best achieved using faster muscle for faster contractions. This preferential recruitment of the faster fibres for the faster tasks indicates that in some circumstances, motor unit recruitment during locomotion can match the contractile properties of the muscle fibres to the mechanical demands of the contraction.
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Muscle fibre types
Muscle fibres are responsible for controlling the physical forces moving through the body. Muscle-shortening actions can generate a force to move a resistance, for example, when moving from a seated to a standing position, the quadriceps and gluteus maximus shorten to help the body stand up against gravity. Muscle-lengthening actions can be applied to control and decelerate a force, for example, the quadriceps and glutes lengthen to control the motion of the body as it returns to a seated position.
There are three types of muscle fibres: slow oxidative (SO), fast oxidative (FO), and fast glycolytic (FG). Most skeletal muscles in the human body contain all three types, although in varying proportions. Muscle fibres can adapt to changing demands by changing size or fibre type composition. This plasticity serves as the physiologic basis for numerous physical therapy interventions designed to increase a patient's force development or endurance. Changes in fibre type composition may also be partially responsible for some of the impairments and disabilities seen in patients who are deconditioned due to prolonged inactivity, limb immobilization, or muscle denervation.
During locomotion, the skeletal muscles are excited by impulses from the α-motorneurons that originate in the spinal cord. The excitability of the α-motorneurons is related to the size of their cell bodies. 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. The size principle has been a cornerstone in our understanding of how different muscle fibres are used within a muscle.
The contractile properties of muscle fibres are related to the intrinsic speed, also known as the maximum unloaded strain rate, at which they can shorten. The shortening velocity at which the maximum mechanical power is generated and the velocity at which the maximum mechanical efficiency is achieved. Therefore, generating mechanical power at a high efficiency is best achieved using faster muscle for faster contractions. However, preferential activation of fast fibres for fast tasks would contravene predictions made by the size principle.
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Muscle fibre and locomotion
Muscle fibres are responsible for controlling physical forces moving through the body. Muscle-shortening actions can generate a force to move a resistance, for example, when moving from a seated to a standing position, the quadriceps and gluteus maximus shorten to help the body stand up against gravity. Muscle-lengthening actions can be applied to control and decelerate a force; for example, the quadriceps and glutes lengthen to control the motion of the body as it returns to a seated position.
During locomotion, the skeletal muscles are excited by impulses from the α-motorneurons that originate in the spinal cord. The excitability of the α-motorneurons is related to the size of their cell bodies. 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.
The three types of muscle fibres are slow oxidative (SO), fast oxidative (FO) and fast glycolytic (FG). Most skeletal muscles in a human contain all three types, although in varying proportions. In addition, muscle fibres can adapt to changing demands by changing size or fibre type composition. This plasticity serves as the physiologic basis for numerous physical therapy interventions designed to increase a patient's force development or endurance.
Contractile properties of muscle fibres are related to the intrinsic speed at which they can shorten. The shortening velocity at which the maximum mechanical power is generated and the velocity at which the maximum mechanical efficiency is achieved. Therefore, generating mechanical power at a high efficiency is best achieved using faster muscle for faster contractions. However, preferential activation of fast fibres for fast tasks would contravene predictions made by the size principle.
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Muscle fibre and exercise
Muscle fibres are responsible for controlling physical forces moving through the body. Muscle-shortening actions can generate a force to move a resistance, for example, when moving from a seated to a standing position, the quadriceps and gluteus maximus shorten to help the body stand up against gravity. Muscle-lengthening actions can be applied to control and decelerate a force, for example, the quadriceps and glutes lengthen to control the motion of the body as it returns to a seated position.
There are three types of muscle fibres: slow oxidative (SO), fast oxidative (FO) and fast glycolytic (FG). Most skeletal muscles in the human body contain all three types, although in varying proportions. Muscle fibres can adapt to changing demands by changing size or fibre type composition. This plasticity serves as the physiologic basis for numerous physical therapy interventions designed to increase a patient's force development or endurance. Changes in fibre type composition may also be partially responsible for some of the impairments and disabilities seen in patients who are deconditioned because of prolonged inactivity, limb immobilization, or muscle denervation.
During locomotion, the skeletal muscles are excited by impulses from the α-motorneurons that originate in the spinal cord. The excitability of the α-motorneurons is related to the size of their cell bodies. 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.
The contractile properties of muscle fibres are related to the intrinsic speed at which they can shorten. The shortening velocity at which the maximum mechanical power is generated and the velocity at which the maximum mechanical efficiency is achieved. Therefore, generating mechanical power at a high efficiency is best achieved using faster muscle for faster contractions. However, preferential activation of fast fibres for fast tasks would contravene predictions made by the size principle.
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Frequently asked questions
Slow oxidative (SO), fast oxidative (FO) and fast glycolytic (FG).
To control physical forces moving through the body.
They can generate a force to move a resistance, for example, when moving from a seated to a standing position, the quadriceps and gluteus maximus shorten to help the body stand up against gravity.
They can be applied to control and decelerate a force, for example, the quadriceps and glutes lengthen to control the motion of the body as it returns to a seated position.
Muscle fibre recruitment can respond to the mechanics of muscle contraction. During locomotion, the skeletal muscles are excited by impulses from the α-motorneurons that originate in the spinal cord.
