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Tonic muscles are those that act with little or no stimulus and are responsible for holding the body upright. Tonic muscles contract slowly, have great endurance, and are prone to overactivity. They are slow-twitch dominant, composed of at least 51% slow-twitch muscle fibres, and are highly resistant to fatigue. Tonic muscles are also known as postural muscles, as they help us oppose gravity and keep us on our feet. Tonic and phasic muscles work together in a cycle of contracting and relaxing to bring about movement at a joint. Phasic muscles, on the other hand, are activated when we want them to be and are responsible for movement.
| Characteristics | Values |
|---|---|
| Type of muscle fibres | Slow-twitch dominant, composed of at least 51% slow-twitch muscle fibres |
| Activation | Tonic muscles act with little or no stimulus |
| Function | Tonic muscles are responsible for holding you upright |
| Contraction speed | Tonic muscles contract slowly |
| Endurance | Tonic muscles have great endurance and are highly resistant to fatigue |
| Prone to | Tonic muscles are prone to overactivity and tightening |
| Examples | Pectoralis minor, upper trapezius, iliopsoas, levator scapulae, biceps |
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What You'll Learn
Tonic muscles are slow-twitch dominant
Tonic muscles are those that can be thought of as "background" muscles. They act with little or no stimulus and are responsible for holding the body upright. Tonic muscles contract slowly and have great endurance. They are also prone to overactivity and tend to tighten when they are overactive.
Slow-twitch fibres develop greater numbers of mitochondria and capillaries, making them better for prolonged work. In larger animals, different muscle groups will require different fibre type proportions within muscles for different purposes. Turtles, for example, have complementary muscles within the neck that show a potential inverse trend of fibre type percentages.
In contrast, phasic muscles are composed of at least 51% fast-twitch muscle fibres. These are powerful muscles, but they fatigue more easily than tonic muscles. They are primarily responsible for movement. Fast-twitch fibres rely on a well-developed, anaerobic, short-term, glycolytic system for energy transfer and can contract and develop tension at 2–3 times the rate of slow-twitch fibres. They are much better at generating short bursts of strength or speed.
It is important to understand the differences between tonic and phasic muscles, as they can affect posture and movement. Tonic muscles can shorten and tighten in response to over-use, under-use, or trauma, while phasic muscles lengthen and weaken in response to these stimuli. This can lead to musculoskeletal imbalances and joint instability.
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Tonic muscles are prone to overactivity
Tonic muscles are "background" muscles that require little to no stimulus to activate. They are responsible for holding us upright and have a high resistance to fatigue. Tonic muscles contract slowly and have great endurance. Tonic muscles are also prone to overactivity and will tighten when they are overactive.
In contrast, phasic muscles are activated voluntarily and are powerful, but fatigue more easily. Phasic muscles contract quickly and have poor endurance. Tonic and phasic muscles have a natural, physiologically balanced imbalance, with tonic muscles being physically dominant and pulling the bone out of alignment.
The differing responses of tonic and phasic muscles to improper stimuli can lead to musculoskeletal imbalance and joint instability. Tonic muscles are prone to shortening and tightening in response to overuse, underuse, or trauma. This can result in faulty joint kinematics, where forces generated by movement or loading cannot be adequately dissipated, leading to premature degradation of the joint.
To prevent this, strategic flexibility exercises can be employed to relax tonic muscles, while strength work can be used to stimulate phasic muscles. Pilates, for example, focuses on proper alignment and technique to avoid faulty movement patterns, ensuring that tonic muscles are not inadvertently overused and phasic muscles are effectively engaged.
Understanding the characteristics of tonic and phasic muscles is essential for trainers and therapists to develop effective exercise programs and address individual client needs, such as improving posture and reducing joint pain.
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Tonic muscles are responsible for holding you upright
Tonic muscles are those that act with little to no stimulus. They are responsible for holding you upright and can be thought of as "background" muscles. Tonic muscles contract slowly and have great endurance. They are also prone to overactivity and tend to tighten when they are overactive.
Tonic muscles are slow-twitch dominant, composed of at least 51% slow-twitch muscle fibres. This makes them highly resistant to fatigue and gives them a greater propensity for work. Tonic muscles are also more prone to tightness and shortening, especially in a deconditioned or improperly trained body. This can lead to musculoskeletal imbalances and joint instability when the tonic and phasic muscles are located on opposing sides of the body.
Phasic muscles, on the other hand, are activated when we demand that they act. They contract quickly due to their fast-twitch muscle fibre composition and have poor endurance. These muscles are primarily responsible for movement and are more prone to weakness and lengthening.
The understanding of tonic and phasic muscles is essential for strategic planning to reduce joint and back pain, improve posture, and prevent faulty movement patterns. Tonic muscles, such as the iliopsoas, play a crucial role in maintaining proper alignment and stability, while phasic muscles, like the gluteals, are responsible for generating powerful movements.
The relationship between tonic and phasic muscles is a "natural, physiologically balanced imbalance." Tonic muscles are constantly active, helping us to oppose gravity and maintain our posture. This constant activity contributes to their higher resting tonus and shorter-than-normal resting length. By understanding this relationship, trainers can develop effective exercise programs that address individual client needs and goals.
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Tonic muscles are shortened and tightened by over-use, under-use, or trauma
Tonic muscles are a type of muscle group that can be thought of as "background" muscles. They act with little to no stimulus and are responsible for holding the body upright. Tonic muscles are slow to contract and have high endurance. They are also prone to overactivity and tend to tighten when overused.
Tonic muscles can be shortened and tightened by over-use, under-use, or trauma. For example, an office worker who spends long hours sitting at a desk may try to sit up straight but constantly finds themselves slouching. This is partly because their overused tonic muscles (pectoralis minor and upper trapezius) are pulling their underused phasic muscles (rhomboids and mid/lower trapezius) into an excessively lengthened position.
In addition, improper training or conditioning can lead to tonic muscles being shortened and tightened. For instance, a gym routine that focuses on chest presses, shoulder presses, and bicep curls can further shorten and tighten the pectoralis minor, upper trapezius, and biceps, while ignoring the rhomboids, middle and lower trapezius, serratus, and triceps. This can result in musculoskeletal imbalance and joint instability.
Tonic muscles are also susceptible to faulty loading, which can take the form of under-use, misuse, or disuse. For athletes, the form of faulty loading they are typically concerned with is overuse. Strategic flexibility exercises can be employed to relax tonic muscles and prevent postural compensation and distortions.
It is important to understand the differences between tonic and phasic muscles to develop effective exercise plans and prevent injuries. Phasic muscles, in contrast to tonic muscles, are activated on demand and are composed of at least 51% fast-twitch muscle fibers. They are powerful but fatigue more easily. Understanding the characteristics of tonic and phasic muscles can lead to a deeper understanding of strategic planning and help reduce joint and back pain associated with postural compensations.
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Tonic muscles are highly resistant to fatigue
Tonic and phasic muscles work together in a cycle of contracting and relaxing to bring about movement at a joint. Tonic muscles are slow-twitch dominant, composed of at least 51% slow-twitch muscle fibres. They contract slowly and have great endurance. Tonic muscles can be thought of as "background" muscles. They act with little or no stimulus and are responsible for holding us upright.
In a deconditioned or improperly trained body, it is easy for tonic muscles to shorten and tighten in response to overuse, underuse, or trauma. This can lead to musculoskeletal imbalance and joint instability. Faulty loading, which refers to underuse, misuse, or disuse, can also cause issues. For example, an office worker who sits at a desk all day may find themselves constantly slouching, as their overused tonic muscles pull their underused phasic muscles into an excessively lengthened position.
To prevent these issues, it is important to understand the differences between tonic and phasic muscles and how they affect posture. This understanding can lead to better strategic planning, which can reduce joint and back pain. Exercises such as Pilates can help to ensure that faulty movement patterns are avoided, tonic muscles are not overused, and phasic muscles are effectively engaged and challenged.
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Frequently asked questions
Tonic muscles are slow-twitch dominant, composed of at least 51% slow-twitch muscle fibres. They are highly resistant to fatigue and have a greater propensity for work. Tonic muscles can be thought of as "background" muscles, acting with little to no stimulus.
Tonic muscles contract slowly and have great endurance. They are prone to overactivity and tend to tighten when they are overactive. Phasic muscles, on the other hand, contract quickly and have poor endurance. They are available on demand but are not responsive to the pull of gravity.
The iliopsoas is an example of a tonic muscle group. Most tonic muscles are flexors, and many have agonist-antagonist relationships with phasic muscles.
