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Muscle tone is traditionally defined as the tension in a relaxed muscle or the resistance felt during passive stretching of a joint when the muscles are at rest. However, the mechanisms of muscle tone and its changes are not well understood, and the clinical assessment of muscle tone is often subjective. Muscle tone can vary from person to person, with some exhibiting high tone, perceived as tight, light, strong, while others may have low tone, described as lax, flabby, floppy, mushy, dead weight. Disorders or abnormalities in muscle tone can lead to physical issues, and it is important to assess an individual's muscle tone to determine if they have high, low, or normal tone.
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What You'll Learn
Muscle tone is the tension in a relaxed muscle
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". This definition has some ambiguities, such as the unclear meaning of "resistance to passive stretch", and the subjectivity of "felt by the examiner". Studies with electromyographic (EMG) assessments often associate muscle tone with the baseline EMG level in a relaxed state. However, muscle tone also has a passive or viscoelastic component that is independent of neural activity and cannot be detected by EMG.
The concept of muscle tone being due to constant 'background' neural activity, with high tone (hypertonus) indicating increased excitatory activity and low tone (hypotonus) indicating the opposite, has been disputed. Tone is now thought to be due to a combination of factors, such as reflex contraction and mechanical-elastic properties like stiffness and physical inertia. The assessment of muscle tone is based on findings in multiple areas, including strength and bulk, with hypertonic children exhibiting joint extension and consistently clenched fists, normal children showing a flexed posture at rest, and hypotonic children appearing 'flat' against the resting surface.
Abnormal muscle tone can arise from dysfunction in neural circuits in the brain, spinal cord, and muscle spindle, manifesting as hypertonia or hypotonia. Hypertonia, or high muscle tone, can be further classified as spasticity or rigidity. Spasticity is velocity-dependent resistance to passive stretch, while rigidity is velocity-independent resistance. Hypotonia, or low muscle tone, can present as muscle flaccidity, with decreased stretch reflex responses and decreased resistance to passive movement.
In contrast to the traditional definition, Bernstein proposed that muscle tone reflects a state of preparedness for movement, suggesting that it may not be possible to estimate muscle tone when a person is asked to relax and remain still. Carpenter et al. defined 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". This definition highlights the active contribution of muscle tone to movement and postural tasks.
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Disorders can cause abnormal muscle tone
Muscle tone is traditionally defined as "the tension in a relaxed muscle" or "the resistance felt by an examiner during passive stretching of a joint when the muscles are at rest". Normal muscle tone is controlled by the central nervous system and is essential for posture, movement, and functional tasks such as sitting up or standing.
Disorders of muscle tone can arise from disruptions in the neural circuits in the brain, spinal cord, and muscle spindle, resulting in abnormal muscle tone. These disorders can be classified as either hypertonia (high muscle tone) or hypotonia (low muscle tone). Atonia, or a complete lack of muscle tone, is another condition that can affect mobility and daily functioning.
Hypertonia
Hypertonia is characterised by an abnormal increase in muscle tone, resulting in reduced muscle flexibility and range of movement. It is often caused by injury or damage to the central nervous system or spinal cord, disrupting nerve pathways that control muscle tone. This can lead to spasticity, rigidity, or dystonia, affecting posture and movement. Cerebral palsy, stroke, severe head injury, and spinal cord injuries are all associated with hypertonia. Physiotherapy is an effective treatment for hypertonia, helping to improve muscle function and reduce stiffness through stretching and movement exercises.
Hypotonia
Hypotonia, or low muscle tone, is characterised by decreased muscle tension and can be associated with lower motor neuron diseases such as poliomyelitis. It can present as muscle flaccidity, with decreased stretch reflex responses and reduced resistance to passive movement. Hypotonia can affect mobility and daily functioning, impacting a person's ability to engage in explosive movements required in activities like sprinting or high jumping.
Muscular Dystrophy
Muscular dystrophy is a group of diseases that cause progressive muscle weakness and loss of muscle mass due to abnormal genes interfering with protein production necessary for healthy muscles. Various types of muscular dystrophy affect different muscle groups and can present at different ages, with some forms progressing slowly and causing mild disability, while others cause rapid impairment. Myotonic muscular dystrophy is characterised by an inability to relax muscles after contractions, often affecting facial and neck muscles first. Facioscapulohumeral muscular dystrophy causes muscle weakness beginning in the face, hips, and shoulders, while limb-girdle muscular dystrophy affects the hip and shoulder muscles, impacting a person's gait and balance. While there is no cure for muscular dystrophy, medications and therapy can help manage symptoms and slow the disease's progression.
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Muscle tone is related to movement and posture control
Muscle tone is traditionally defined as the tension in a relaxed muscle or the resistance felt during the passive stretching of a joint when the muscles are at rest. However, this definition is ambiguous and subjective to the examiner's interpretation. Muscle tone is also difficult to evaluate, and experience is required to accurately assess it. It is traditionally associated with the amount of neural activity, with high tone indicating increased activity and low tone indicating the opposite. This concept has been disputed, and it is now believed that muscle tone arises from a combination of factors, including reflex contraction and mechanical-elastic properties.
Bernstein's hierarchical model of movement construction supports this idea, with muscle tone being an adaptive function that responds adequately to commands from higher levels of movement construction. Additionally, Carpenter et al. define tone as the constant muscular activity necessary as a background to actual movement, helping to maintain the body's basic attitude, especially against the force of gravity.
Disorders of muscle tone can lead to physical abnormalities. For example, hypertonia, or high muscle tone, can result in spasticity or rigidity, while hypotonia, or low muscle tone, can lead to muscle flaccidity and decreased stretch reflex responses. These disorders can affect an individual's movement and posture, with high muscle tone often associated with increased strength and low muscle tone associated with increased flexibility.
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Muscle tone can be evaluated by checking for asymmetries
Muscle tone is the tension in a relaxed muscle or the resistance felt during passive stretching when the muscles are at rest. It is a complex and dynamic state, resulting from hierarchical and reciprocal anatomical connectivity. The body maintains the balance between the tone of flexor and extensor muscle groups.
To check for asymmetries, physiotherapists commonly use digital palpation to assess muscle tone, which is felt as tension, tightness, or subjective stiffness. However, these assessments are subjective and may vary between practitioners. More objective methods, such as electromyographic (EMG) assessment, can be used to measure baseline EMG levels in a relaxed state, although this cannot detect the passive or viscoelastic component of muscle tone, which is independent of neural activity.
Apps and machines, such as those offered by EGYM, can also be used to measure muscular imbalances and provide targeted exercise regimens to correct them. These tools can help identify which muscles need to be stretched or strengthened to restore balance. For example, shortened muscles should be treated with low-intensity exercises or easy stretching, while weakened muscles should be supplemented with targeted strength exercises.
By evaluating muscle tone through the detection of asymmetries, individuals can identify and address imbalances, preventing injuries and improving overall physical performance and quality of life.
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Muscle tone is difficult to define and measure
Muscle tone is a complex physiological concept that is challenging to define, assess, and measure accurately. While it is traditionally defined as the tension in a relaxed muscle or the resistance to passive stretch, this definition has been questioned and refined over time.
The traditional definition of muscle tone as resistance to passive stretch is ambiguous and subject to interpretation. The phrase resistance to passive stretch is not clearly defined, and the inclusion of "felt by the examiner" introduces subjectivity during clinical examinations, leading to inter-rater variability. Muscle tone is also influenced by various factors, such as neural activity, reflex contraction, and mechanical-elastic properties, making it difficult to attribute it solely to passive stretch.
Additionally, muscle tone may reflect a state of preparedness for movement. Bernstein proposed that muscle tone is an adaptive function of the neuromotor apparatus, responding to commands from upper levels of movement construction. This definition positions muscle tone as an active contributor to movement and postural tasks, rather than a static state of relaxation. Carpenter et al. supported this idea by defining tone as the constant muscular activity necessary to maintain the body's basic attitude, particularly against gravity.
Assessing muscle tone is a multifaceted process that requires experience. Clinicians must evaluate multiple areas, including strength, bulk, and asymmetries between muscle groups. They also need to consider the range of motion and the resistance encountered during passive movement of joints. However, the most common methods for measuring muscle tone are subjective, and objective measurements, such as electromyographic (EMG) assessments, capture only the active or contractile component of muscle tone, missing the passive or viscoelastic component.
Furthermore, muscle tone varies along a bell-shaped curve, with individuals exhibiting low, normal, or high muscle tone. Low muscle tone is associated with increased flexibility and decreased strength, while high muscle tone is linked to decreased flexibility and increased strength. However, there are exceptions to these general trends, and the specific causes of reduced or increased resistance during examinations can be multifaceted, making it challenging to provide a definitive interpretation of low or high muscle tone.
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Frequently asked questions
Muscle tone is the tension in a relaxed muscle or the resistance felt during the passive stretching of a joint when the muscles are at rest.
High muscle tone is perceived as "tight, light, strong" and low muscle tone is perceived as "lax, flabby, floppy, mushy, dead weight". However, high muscle tone does not necessarily indicate stronger muscles.
Muscle tone reflects a state of preparedness to move and is an active contributor to movement and postural tasks. It is the body's way of fine-tuning the excitability of sensory and motor cells for active postural or movement control.
Muscle tone can be tested by passively flexing and extending the joints of the upper and lower extremities. Normal muscle tone is indicated by mild resistance on a range of motion.
Abnormal muscle tone can manifest as hypertonia or hypotonia, which are associated with various physical disorders. Hypertonia is often seen in upper motor neuron diseases, while hypotonia is observed in lower motor neuron diseases.
