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Muscle tone, or tonus, refers to the tension in a relaxed muscle, or the resistance felt during passive stretching when the muscles are at rest. Muscle tone is controlled by neuronal impulses and influenced by receptors in the muscle and tendons, which generate reflexes in the spinal cord. The degree of muscle tone varies from person to person, with some exhibiting high tone and others low tone. High muscle tone does not necessarily equate to strength, but rather decreased flexibility, while low muscle tone is associated with increased flexibility but decreased strength. Disorders of muscle tone, such as hypertonia or hypotonia, can arise from dysfunction in neural circuits involving the brain, spinal cord, and muscle spindle.
| Characteristics | Values |
|---|---|
| Definition | Muscle tone is traditionally defined as 'the tension in a relaxed muscle' or 'the resistance felt by the examiner during passive stretching of a joint when the muscles are at rest.' |
| Muscle Tone at Rest | Even when at rest, muscle fibres are at least partially contracted, possessing a small degree of tension. |
| Muscle Tone and Movement | Muscle tone may reflect a state of preparedness to move. |
| Muscle Tone and Strength | Muscles with high tone are not necessarily strong and muscles with low tone are not necessarily weak. |
| Muscle Tone and Flexibility | Low tone increases flexibility, while high tone decreases it. |
| Muscle Tone and Posture | Muscle tone helps the body move freely against gravity and is needed for overall posture. |
| Types of Muscle Tone | Muscle tone can be classified as 'postural' and 'phasic' types. |
| Disorders | Disorders of muscle tone can arise from dysfunction in neural circuits in the brain, spinal cord, and muscle spindle, manifesting as hypertonia or hypotonia. |
| Treatment | Muscle relaxants or quinine can help with muscle cramps. |
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What You'll Learn
Muscle tone is the tension in relaxed muscles
Muscle tone, or tonus, is the technical term for the tension in a relaxed muscle. Even when our muscles are at rest, they are at least partially contracted, possessing a small degree of tension. This tension is necessary for our bodies to move freely against gravity and maintain our posture. The amount of tension a muscle has at rest is also known as its tone.
The muscle tone is controlled by neuronal impulses and influenced by receptors found in the muscle and tendons. When a muscle is stretched, muscle spindles—small sensory units that lie parallel to a muscle—are activated, triggering impulses to the spinal cord that can generate an immediate reflex. These reflexes include the obvious knee jerk reaction, but also include key functions such as posture maintenance and proper digestive system function.
Muscle tone can be classified as 'postural' and 'phasic' types. Postural tone is seen in axial muscles where gravity is the most important factor. It results from a steady stretch on the muscles and tendons and manifests as prolonged muscle contraction. Phasic tone, on the other hand, is commonly assessed clinically in the extremities as a rapid and short-duration response. It results from the rapid stretching of a tendon and attached muscle.
Muscle tone may also reflect a state of preparedness to move. According to Bernstein's hierarchical model of movement construction, muscle tone is an adaptive function of the neuromotor apparatus that responds to commands from upper levels of movement construction. This definition makes muscle tone an active contributor to movement and postural tasks.
It is important to note that muscles with high tone are not necessarily strong, and muscles with low tone are not necessarily weak. Generally, low tone increases flexibility and decreases strength, while high tone decreases flexibility and increases strength. However, there are many exceptions to this general rule.
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Muscle tone is necessary for posture and movement
Muscle tone, also known as residual muscle tension or tonus, is the continuous and passive partial contraction of muscles, or the muscle's resistance to passive stretch during a resting state. It is necessary for posture and movement.
In skeletal muscles, muscle tone helps to maintain a normal posture. It is regulated by the activity of motor neurons and can be affected by factors such as age, disease, and nerve damage. The body maintains a balance between the tone of flexor and extensor muscle groups, and the loss of this balance can lead to muscle cramps. Generally, low muscle tone increases flexibility and decreases strength, while high muscle tone decreases flexibility and increases strength. However, there are exceptions to this generalization, as muscle strength and flexibility are also influenced by joint laxity.
Muscle tone can be classified as postural and phasic types. Postural tone is observed in axial muscles where gravity is the most significant factor. It results from a sustained stretch of the muscles and tendons, leading to prolonged muscle contraction. Phasic tone, on the other hand, is commonly assessed in the extremities and is characterized by rapid and short-duration responses. It arises from the rapid stretching of a tendon and its attached muscle, specifically the muscle spindle.
The importance of muscle tone in posture and movement is highlighted by Bernstein's hierarchical model of movement construction. According to Bernstein, muscle tone reflects a state of preparedness for movement. It is an adaptive function that allows the body to respond to commands from upper levels of movement construction, fine-tuning the excitability of sensory and motor cells for active postural or movement control. This definition underscores the active contribution of muscle tone to movement and postural tasks. Additionally, muscle tone is closely related to excitability, which has been implicated in chronic pain.
Disorders of muscle tone, such as hypertonia and hypotonia, can affect movement coordination and motor performance. For example, newborns exhibit a flexed posture that prevents locomotion until the extensor tone becomes predominant. Abnormalities in muscle tone can also be a sign of disease or disability, particularly in infancy, and are assessed in early neurological evaluations. Thus, maintaining healthy muscle tone is crucial for posture and movement, and disruptions in muscle tone can have significant implications for overall physical function.
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Muscle tone is controlled by neuronal impulses
Muscle tone is the tension in a relaxed muscle or the resistance felt during passive stretching. It is traditionally thought of as distinct from voluntary movement, but it is now understood that muscle tone and movement share overlapping circuitry. Neurons in the brainstem participate in both movement and tone regulation, and descending monoaminergic gain control pathways affect muscle recruitment during voluntary actions. Vestibulospinal projections that influence gait coordination also impact muscle tone.
The Gurfinkel et al. “Lock with Two Keys” framework provides a conceptual basis for the involvement of both tonic and phasic commands in movement. Tonic commands set excitability levels to prepare lower circuits for action, while phasic commands initiate and coordinate the movement. Evidence supporting the role of tonic commands in movement control includes findings that increasing the excitability of spinal circuits can invoke gait rhythms.
Muscle tone is regulated by spinal and supraspinal mechanisms. Spinal control depends on the interaction between the muscle spindle and spinal cord along with the interneurons, while supraspinal control is regulated by facilitatory and inhibitory long tracts and the cerebellum.
Disorders of muscle tone can arise from dysfunction in these pathways and manifest as hypertonia or hypotonia. The loss of supraspinal control mechanisms gives rise to hypertonia, resulting in spasticity or rigidity. Dystonia and paratonia also manifest as abnormalities of muscle tone, but they arise from network dysfunction between the basal ganglia and the thalamo-cerebello-cortical connections.
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Muscle tone abnormalities include hypertonia and hypotonia
Muscle tone is traditionally defined as 'the tension in a relaxed muscle' or 'the resistance, felt by the examiner during passive stretching of a joint when the muscles are at rest'. It is the resistance of muscles to stretch, and it helps posture and allows a person to stand up, sit up and function effectively. Correct muscle tone is essential for a person to be able to stand up, sit up and function properly.
Muscle tone can be classified as 'postural' and 'phasic' types. Postural tone is seen in axial muscles where gravity is the most important inciting factor. It results from a steady stretch on the muscles and tendons and manifests as prolonged muscle contraction. Phasic tone, on the other hand, is what is commonly assessed clinically in the extremities as a rapid and short-duration response. It results from the rapid stretching of a tendon and attached muscle and more precisely, the muscle spindle.
Disorders of muscle tone can arise from dysfunction in these pathways and manifest as hypertonia or hypotonia. Hypertonia is defined as an abnormal increase in muscle tone caused by upper motor neuron pathology in the brain or spinal cord. It is characterised by an abnormal increase in tension within the muscles of the body, causing a reduced ability for a muscle to stretch, which can lead to contractures. Hypertonia can be caused by injury or disruption to the nerve pathways in the central nervous system that are responsible for switching on or off muscle activity. It may also be caused by the loss of supraspinal control mechanisms, resulting in spasticity or rigidity.
Hypotonia, on the other hand, is characterised by a decrease in muscle tone. It can occur due to incomplete/partial myelopathy involving the lateral funiculus, resulting in weakness, hypotonia, and a loss of superficial reflexes. Cerebellar lesions can also commonly produce hypotonia due to the cerebellum's role in regulating muscle tone.
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Muscle tone can be classified as postural and phasic types
Muscle tone refers to the state of muscle activity or tension present at rest or during contraction. It is important to assess several factors before deciding if a person has high, low, or normal muscle tone. Physical disorders can result in abnormally low (hypotonia) or high (hypertonia) muscle tone. Generally, low tone increases flexibility and decreases strength, while high tone decreases flexibility and increases strength. However, there are exceptions to this generalization.
On the other hand, phasic muscles are more languid and only activate when strictly necessary, quickly returning to a relaxed state. They lengthen and weaken in response to over-use, under-use, or trauma. Phasic muscles can be thought of as the opposite of tonic muscles.
It is important to note that the classification of muscles as tonic or phasic is not absolute, and some muscles exhibit a combination of characteristics. Tonic and phasic muscle theory was developed by Dr. Vladimir Janda, a Czech neurologist and physiotherapist, in the late 1960s using electromyography (EMG).
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Frequently asked questions
Muscle tonus is the tension in a relaxed muscle, or the resistance felt when a relaxed muscle is stretched. Even when at rest, muscle fibres are partially contracted, and this is controlled by neuronal impulses.
Muscles with high tonus are not necessarily strong, and muscles with low tonus are not necessarily weak. However, generally, low tonus increases flexibility and decreases strength, and high tonus does the opposite. A person with low tonus will struggle with explosive movements, and a person with high tonus will struggle with flexibility.
Muscle tonus is assessed by the feeling of resistance offered by a person's body segment when they are asked to relax, and the clinician slowly moves the limb. A person with low tonus may be very good at relaxing their muscles.
Muscle tonus can be classified as postural and phasic. Postural tonus is seen in axial muscles where gravity is the most important factor, and it results in prolonged muscle contraction. Phasic tonus is commonly assessed in the extremities and results in a rapid and short-duration response.
