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The tonic and phasic muscle theory was popularized by Czech neurologist and physiotherapist Dr. Vladimir Janda in the 1960s. Tonic muscles, also known as postural muscles, are those that are always active and resistant to fatigue, helping to sustain posture and oppose gravity. Phasic muscles, on the other hand, are activated on demand and are prone to fatigue, being primarily responsible for movement. While tonic muscles are considered background muscles, phasic muscles are designed for specific tasks and can be trained to improve overall body function.
| Characteristics | Values |
|---|---|
| Evolutionary development | Phasic muscles were responsible for movement |
| Tendency | To weaken and lengthen |
| Activation | On demand |
| Response to overuse, underuse or trauma | Lengthen and weaken |
| Response to stimulus and conditioning | Activation when insisted upon |
| Type of muscle fibres | Fast-twitch, type II muscle fibres |
| Metabolism | Anaerobic |
| Fatigue | Easily fatigued |
| Endurance | Low endurance |
| Relationship with other muscles | Work in relation to postural muscles |
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What You'll Learn
- Phasic muscles are available on demand but are not responsive to gravity
- They are fast-twitch muscles, designed for brief activation
- Phasic muscles tire quickly and are easily fatigued
- They are action muscles, generating a pattern of weakness and inhibition
- Phasic muscles are used for movement, gestures, and actions
Phasic muscles are available on demand but are not responsive to gravity
Phasic muscles are those that are activated on demand, but they do not respond to gravity. They are responsible for movement and are easily fatigued due to their high density of fast-twitch muscle fibres. These fibres rely on anaerobic metabolism, which means they can tire quickly as they depend on the body's glycogen stores. Phasic muscles include the abdominals, gluteals, deep neck flexors, and rhomboids.
In contrast, tonic muscles can be considered "background" muscles, as they act with little to no stimulus and are responsible for holding us upright. Tonic muscles tend to shorten in response to overuse, underuse, or trauma.
The two types of muscles have a reciprocal relationship, with phasic muscles tending to lengthen and weaken in response to overuse, underuse, or trauma, while tonic muscles shorten and tighten. This relationship can be understood as a natural, physiologically balanced imbalance between two systems.
The differing responses of tonic and phasic muscles to improper stimuli can lead to musculoskeletal imbalances and joint instability, particularly when these muscles are located on opposing sides of the agonist-antagonist relationship. For example, an office worker who sits at a desk all day may try to sit up straight but finds themselves slouching due to their overused tonic muscles pulling their underused phasic muscles into an excessively lengthened position.
It is important to understand the unique characteristics of phasic and tonic muscles to develop effective exercise programs that strengthen and stretch the appropriate muscle groups.
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They are fast-twitch muscles, designed for brief activation
Phasic muscles are designed for movement and are activated on demand. They are fast-twitch muscles, designed for brief activation. They are easily fatigued and do not have the endurance of postural muscles. This is because they rely on anaerobic metabolism to fuel their contraction, which means they rely on a metabolic process that does not use oxygen, such as the body's glycogen stores. They are fed by sugar and starch.
Fast-twitch muscle fibres are red to pink in colour due to their oxygen stores. They are also referred to as "fast oxidate, type IIA". They are the last type of muscle fibre to be recruited for a task. In contrast, slow-twitch muscle fibres are rich in oxygen and are therefore red in colour. They have a high threshold for fatigue.
Phasic muscles are not meant for stabilisation and are farther away from the joints. They are also insufficiently active and may exhibit a kind of false paralysis. They are not necessarily injured, but they can be weakened by a sedentary lifestyle. They seldom act alone, typically acting in relation to postural muscles.
Phasic muscles include the abdominals, the gluteals, the deep neck flexors, and the rhomboids. They can be strengthened through thoughtful exercise choices, such as Pilates, which emphasises proper alignment and technique to ensure faulty movement patterns are avoided.
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Phasic muscles tire quickly and are easily fatigued
Phasic muscles are designed for brief bursts of activation and are responsible for movement. They are available on demand but are not responsive to the pull of gravity. They are farther away from the joints and have less integration into the brain. Examples of phasic muscles include the abdominals, gluteals, deep neck flexors, and rhomboids.
Phasic muscles contain a high density of fast-twitch muscle fibers. They rely primarily on anaerobic metabolism—a process that does not use oxygen—to fuel their contraction. As a result, phasic muscles can easily be fatigued. During low to moderate-intensity activities, the body uses oxygen as fuel, which is why we become fatigued during such activities when done over a prolonged period.
In contrast, postural muscles are imbued mostly with slow-twitch muscle fibers, which rely on aerobic or oxygen-based metabolism and have a high threshold for fatigue. Low to moderate-intensity activities such as walking or jogging rely heavily on the aerobic nature of these slow-twitch muscle fibers.
Imbalances between postural and phasic muscles can lead to musculoskeletal imbalance and joint instability. For example, an office worker who sits at a desk all day may find themselves constantly slouching due to their overused tonic muscles pulling their underused phasic muscles into an excessively lengthened position.
To bring balance, one would need to focus on strengthening the weakened phasic muscles and stretching the tightened postural muscles.
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They are action muscles, generating a pattern of weakness and inhibition
Phasic muscles are designed for movement and are activated on demand. They are fast-twitch muscles that rely on anaerobic metabolism for contraction. This means that they can easily fatigue and are not suitable for stabilisation. Phasic muscles are not responsive to gravity and are farther away from the joints.
Phasic muscles are action muscles, generating a pattern of weakness and inhibition. They tend to weaken and lengthen in response to overuse, underuse, or trauma. This can result in a poor movement pattern. For example, the phasic abdominal muscles tend towards weakness, and conventional training often tries to strengthen them in isolation, neglecting their relationship with shortened postural muscles.
The pattern of weakness and inhibition in phasic muscles can lead to musculoskeletal imbalance and joint instability, particularly when these muscles are located on opposing sides of the agonist-antagonist relationship. This imbalance can manifest as upper crossed syndrome in the upper body and lower crossed syndrome in the lower body.
To address this imbalance, one must focus on strengthening the weakened phasic muscles and stretching the tightened postural muscles. However, it is important to note that strengthening phasic muscles in isolation may further weaken them. Therefore, exercises must be chosen thoughtfully and performed correctly to create ideal muscle length and tension throughout the body.
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Phasic muscles are used for movement, gestures, and actions
Phasic muscles are designed for movement, gestures, and actions. They are activated on demand and are not responsive to gravity. These muscles are available for use when we insist that they be used, but they are not meant for stabilisation. They are easily fatigued and do not have the endurance that postural muscles have.
Phasic muscles contain a high density of fast-twitch muscle fibres. They rely primarily on anaerobic metabolism to fuel their contraction and, for that reason, can easily be fatigued. Anaerobic means without oxygen. So whenever these muscle fibres are recruited, they're relying primarily on a metabolic process that does not use oxygen, such as the body's glycogen stores.
The fast-twitch muscle fibres in phasic muscles are further divided into two types: fast oxidate, type IIA, and fast glycolytic, type IIB. The type IIA fibres are red to pink in colour due to their oxygen stores, while the type IIB fibres are white or pale due to their low oxygen stores. These two types of fibres have different fuel sources, with type IIA using aerobic metabolism and type IIB using anaerobic metabolism.
Phasic muscles include the abdominals, gluteals, deep neck flexors, and rhomboids. They seldom act alone but typically work in relation to postural muscles. For example, the phasic abdominal muscles are paired with the postural erector spinae. Similarly, the phasic biceps brachii are further away from the joints and are used for our actions and movements.
The tendency of phasic muscles to be short or tight is not fully understood, but they tend to exhibit a pattern of weakness and inhibition. They are insufficiently active and may even show a kind of false paralysis. This can lead to imbalances between postural and phasic muscles, resulting in conditions like upper crossed syndrome in the upper body and lower crossed syndrome in the lower body.
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Frequently asked questions
Phasic muscles are those designed for movement and activated on demand. They are fast-twitch muscles that rely on anaerobic metabolism and fatigue quickly.
Phasic muscles include the abdominals, gluteals, biceps, deep neck flexors, and rhomboids.
Tonic muscles are responsible for holding the body upright and act with little to no stimulus. They tend to shorten with overuse, while phasic muscles lengthen and weaken.
The postural/phasic relationship is a natural, balanced imbalance between two systems. Postural muscles are active to oppose gravity and keep us upright, while phasic muscles are available on demand for movement.
To strengthen phasic muscles, specific exercises like Pilates can be performed to effectively engage and challenge them. However, it's important to choose exercises thoughtfully to avoid improper stimuli, which can lead to imbalances.
