Victor Forge
Muscle tone is a term used to describe a skeletal muscle's inherent resistance to passive movement. It is important to differentiate muscle tone from muscle strength, which is a muscle's maximum voluntary resistance to movement. Normal muscle tone serves three important functions: maintaining posture, storing and releasing energy, and dampening jerky movements to allow for more fluid-like motions. Muscle tone can be classified as postural and phasic types, with postural tone resulting from a steady stretch on the muscles and tendons and manifesting as prolonged muscle contraction, and phasic tone being commonly assessed clinically in the extremities as a rapid and short-duration response. Disturbances in the pathways regulating muscle tone can result in abnormal conditions such as hypertonia or hypotonia, which are characterised by increased or decreased muscle tone, respectively.
| Characteristics | Values |
|---|---|
| Definition | Muscle tone is a skeletal muscle's inherent resistance to passive movement. |
| Function | Muscle tone assists in maintaining posture, or the resistance of the muscle to the forces of gravity. |
| Types | Muscle tone can be classified as postural and phasic types. |
| Disorders | Physical disorders can result in abnormally low (hypotonia) or high (hypertonia) muscle tone. |
| Perception | Resting muscle tone varies along a bell-shaped curve. Low tone is perceived as "lax, flabby, floppy, mushy, dead weight", while high tone is perceived as "tight, light, strong". |
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What You'll Learn
Muscle tone is a skeletal muscle's resistance to passive movement
Muscle tone is 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". This definition has some ambiguities, such as the vague phrase "felt by the examiner", which introduces subjectivity to the assessment.
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 and manifests as a prolonged muscle contraction. In contrast, phasic tone is commonly assessed clinically in the extremities as a rapid and short-duration response. It arises from the rapid stretching of a tendon and its attached muscle, specifically the muscle spindle.
Skeletal muscle is found throughout the body and is responsible for various functions, including producing movement, maintaining body posture and position, regulating body temperature, storing nutrients, and stabilising joints. Skeletal muscle comprises about 40% of human body weight and consists of 50-75% of all body proteins. It is composed of bundles of muscle fibres called myofibers, which contain several myofibrils. These muscle fibres are multinucleated cells ranging from 10 to 100 micrometers in diameter and several centimetres in length.
The degree of resistance in muscle tone depends on the speed of movement. Facilitatory responses are observed in the early stages, where the patient actively assists passive movements. With advancing pathology, resistance increases with increasing movement. Physical disorders can result in abnormally low (hypotonia) or high (hypertonia) muscle tone. Hypotonia is characterised by muscle flaccidity, decreased stretch reflex responses, and reduced resistance to passive movement. On the other hand, hypertonia can manifest as spasticity or rigidity. Spasticity is velocity-dependent resistance to passive stretch, while rigidity is velocity-independent.
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Disorders of muscle tone
Muscle tone refers to the resistance to passive stretching of a joint. Disorders of muscle tone can arise from dysfunction in the neural circuits in the brain, spinal cord, and muscle spindle. These disorders can manifest as either hypertonia (increased resistance) or hypotonia (decreased resistance).
Hypotonia
Hypotonia, or poor muscle tone, is often detected at birth or during infancy. It is a state of low muscle tone, which can result in reduced muscle strength and impaired active movement. Central hypotonia originates from the central nervous system, while peripheral hypotonia is related to problems within the spinal cord, peripheral nerves, or skeletal muscles. Children with hypotonia may experience developmental delays, learning disabilities, and speech difficulties. They may also exhibit coping mechanisms, such as locking their knees while walking, and tend to observe others' physical activities for a long time before imitating them.
Hypertonia
Hypertonia, on the other hand, is characterised by increased muscle tone. It can present clinically as spasticity or rigidity. Spasticity is velocity-dependent resistance to passive stretch, while rigidity is velocity-independent resistance. Both types of hypertonia are associated with dysfunction in the supraspinal pathways and the interaction between the spinal cord and muscle spindle.
Other Disorders
In addition to hypertonia and hypotonia, dystonia and paratonia are also disorders of muscle tone. These disorders arise from network dysfunction between the basal ganglia and the thalamo-cerebello-cortical connections. Paratonia is specifically associated with dementia.
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Muscle tone and movement
Muscle tone refers to the continuous and passive partial contraction of muscles or their resistance to passive stretch during a resting state. It is regulated by motor neuron activity and can be influenced by factors like age, disease, and nerve damage. The body automatically increases muscle tension in response to sudden pulls or stretches, helping maintain balance and guard against danger. This reflexive adjustment highlights muscle tone's role in adapting to movement and posture requirements.
Muscle tone can be categorised into postural and phasic types. Postural tone, influenced by gravity, results from sustained stretching of muscles and tendons, leading to prolonged muscle contraction. It is commonly observed in axial muscles. On the other hand, phasic tone is characterised by rapid and short-duration responses to tendon and muscle stretching and is frequently assessed in the extremities.
The clinical definition of muscle tone, as provided by Carpenter et al., describes it as "the constant muscular activity that is necessary as a background to actual movement to maintain the basic attitude of the body, particularly against the force of gravity." This definition underscores the role of muscle tone in providing a foundation for motor control, enabling the safe and thermodynamically efficient execution of both static and dynamic tasks.
Variations in muscle tone can be classified as high tone (hypertonia) or low tone (hypotonia). High tone is characterised by increased resistance to passive movement, often caused by overactive neural signals. It can lead to stiffness and reduced flexibility, impacting voluntary movements. Low tone, on the other hand, is marked by diminished muscle tone and a lack of firmness, resulting in weakness and impaired postural control.
Disorders of muscle tone, such as hypertonia and hypotonia, can significantly affect an individual's motor functions. These disorders are often associated with neurological conditions such as spinal cord injuries and strokes. Rehabilitation strategies like constraint-induced movement therapy (CIMT) and functional electrical stimulation (FES) leverage the brain's neuroplasticity to promote recovery and enhance motor control.
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Muscle tone and posture
Muscle tone refers to the resistance to passive stretch and is a complex network of neural circuits in the brain, spinal cord, and muscle spindle. Disturbances in muscle tone can affect movement performance and posture.
Posture control has been studied since ancient Greece, and it has been recognised that posture is maintained by tonic muscle contractions that act against gravity to stabilise the body. Postural tone is critical in the body's axis, where the four limbs and head are joined into a single functioning unit during complex motor tasks and static postures. The control of postural tone is not simple and requires specialised neural circuitry, with individual morphology and specific low-level muscle activity also playing a role.
Postural tone can be further classified into 'postural' and 'phasic' types. Postural tone is associated with antigravity support and is seen in axial muscles, resulting from a steady stretch on the muscles and tendons, manifesting as prolonged muscle contraction. It is regulated by spinal and supraspinal mechanisms, with spinal control depending on the interaction between the muscle spindle, spinal cord, and interneurons. Phasic tone, on the other hand, is commonly assessed clinically in the extremities as a rapid and short-duration response, resulting from the rapid stretching of a tendon and attached muscle.
Bernstein proposed that muscle tone reflects a state of preparedness for movement, contributing actively to movement and postural tasks. This view is supported by Carpenter et al., who defined muscle tone as "the constant muscular activity that is necessary as a background to actual movement in order to maintain the basic attitude of the body, particularly against the force of gravity".
In summary, muscle tone plays a crucial role in posture control and movement performance. Postural tone, in particular, helps stabilise the body against gravity, with its control requiring specialised neural circuitry. Both postural and phasic tones contribute to overall muscle tone, which is essential for maintaining the body's equilibrium and facilitating movement.
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High and low muscle tone
Muscle tone refers to the amount of tension or resistance in a muscle at rest. It is the continuous and passive partial contraction of the muscles, which helps to maintain posture and declines during REM sleep. Muscle tone is regulated by the activity of motor neurons and can be affected by various factors, including age, disease, and nerve damage.
Low muscle tone, or hypotonia, is a reduced amount of tension in the muscles, which can decrease muscle strength and impact functional tasks. Individuals with low muscle tone generally have floppy or loose limbs and increased flexibility due to constantly relaxed muscles. Low muscle tone is often associated with ataxic cerebral palsy, caused by damage to the cerebellum, which regulates balance, coordination, and fine motor skills.
High muscle tone, or hypertonia, is an increased amount of tension in the muscles, which can lead to stiffness, decreased flexibility, and rigid movements. Individuals with high muscle tone may experience tight muscles and reduced range of motion, which can pull the body into abnormal postures. Hypertonia is commonly associated with upper motor neuron diseases, such as lesions in the pyramidal tract, and can be a result of damage to the central nervous system, including spinal cord injuries or strokes.
Both low and high muscle tone can impact an individual's ability to perform daily activities and may require specialised physiotherapy treatments to improve strength, flexibility, and range of motion. Treatments may include electrical stimulation, hydrotherapy, deep tissue massage, and targeted exercises to stimulate motor nerves and improve muscle function.
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Frequently asked questions
Muscle tone is the continuous and passive partial contraction of muscles, or the muscle's resistance to passive stretch during a resting state.
Muscle tone serves three important functions. Firstly, it helps to maintain posture and balance. Secondly, muscles can store energy and release it later, which is important for activities like walking. Finally, muscles help dampen jerky movements and allow for more fluid-like movements.
Muscles with high tone are perceived as "tight, light, strong", whereas low tone muscles are perceived as "lax, flabby, floppy, mushy, dead weight". High tone generally decreases flexibility and increases strength, while low tone increases flexibility and decreases strength.
Muscle tone is regulated by the activity of motor neurons and can be affected by factors such as age, disease, and nerve damage.
Abnormal muscle tone can result in either abnormally low muscle tone (hypotonia) or high muscle tone (hypertonia). Hypotonia can be seen in lower motor neuron diseases like poliomyelitis, while hypertonia can be seen in upper motor neuron diseases like lesions in pyramidal and extrapyramidal tracts.
