Dominic Stone
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, muscle tone may reflect a state of preparedness for movement, and it is controlled by neuronal impulses and influenced by receptors in the muscle and tendons. Normal muscle tone is important for generating reflexes, maintaining posture and balance, and controlling the proper function of other organ systems. It is also important to distinguish between normal muscle tone and physical disorders that can result in abnormally low (hypotonia) or high (hypertonia) muscle tone.
| Characteristics | Values |
|---|---|
| Definition | Muscle tone is traditionally defined as 'the tension in the relaxed muscle' or 'the resistance, felt by the examiner during passive stretching of a joint when the muscles are at rest'. |
| Muscle Tone vs Muscle Strength | Muscle tone is not a reflection of muscle strength. Those with low tone may not have decreased strength. |
| Perception | People are usually unaware of their muscle tone in their daily activities. |
| Measurement | Muscle tone is measured by the degree of resistance to passive stretch. |
| Disorders | Physical disorders can result in abnormally low (hypotonia) or high (hypertonia) muscle tone. |
| Exceptions | Muscles with high tone are not necessarily strong and muscles with low tone are not necessarily weak. |
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What You'll Learn
Muscle tone is not a reflection of muscle strength
Muscle tone is the continuous and passive partial contraction of muscles, or the muscle's resistance to passive stretch during a resting state. It is the state of your muscles when at rest, and is defined as the tension in a muscle at rest. It is important to note that muscle tone is not a reflection of muscle strength. Those with low tone may not have decreased strength, and those with high muscle tone do not necessarily have adequate muscle strength.
Muscle tone is traditionally defined as 'the tension in the relaxed muscle' or 'the resistance, felt by the examiner during passive stretching of a joint when the muscles are at rest'. Studies with electromyographic (EMG) assessment often equate muscle tone with baseline EMG levels in a relaxed state. However, this definition has some ambiguities, such as the subjective variation during clinical examination and inter-rater variability of the assessment.
Muscle strength, on the other hand, is defined as the muscle's ability to contract and create force in response to resistance. It is what your muscles do when they are not at rest. When you purposefully move a muscle, your brain sends signals to your muscle fibres to contract. The power of your contraction depends on your muscle mass, which can be changed by weight training and exercise.
A child with low tone, or hypotonia, has muscles that are slower to react to a stretch and are unable to sustain a prolonged muscle contraction. If their arm is stretched quickly, there may be minimal to no response from their biceps muscle. However, it is important to note that those with low tone do not necessarily have decreased strength. They may require more energy and effort to move their muscles, but they can still have adequate muscle strength.
A child with high tone, or hypertonia, has muscles that are in an "over-reactive" state to stretch and are in a state of high tension. If their arm is stretched, their biceps muscle may react even quicker and maintain a prolonged contraction. Children with hypertonia are at risk of a loss of range of motion and orthopedic concerns. However, it is important to note that those with high tone do not necessarily have increased muscle strength. They may have difficulty relaxing their muscles, but they can still have adequate muscle strength.
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Muscle tone is controlled by neuronal impulses
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". However, this definition is ambiguous and subjective, and muscle tone also has a passive or viscoelastic component independent of neural activity.
The "Lock with Two Keys" framework by Gurfinkel et al. suggests that both tonic and phasic commands are involved 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.
The static stretch reflex response is the physiological basis for maintaining muscle tone. When a muscle is passively stretched, there is a threshold where a stronger stretch elicits a stronger reflex contraction. This response is important in clinical elicitation, such as in deep tendon reflexes.
While muscle tone is controlled by neuronal impulses, it is important to note that muscle tone and strength are not directly correlated. Individuals with low muscle tone may not have decreased strength, and those with high muscle tone may not have adequate muscle strength. Resting muscle tone varies along a bell-shaped curve, with low tone perceived as "floppy" and high tone perceived as "tight".
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Physical disorders and muscle tone
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'. However, this definition is ambiguous, and muscle tone is generally assessed by examining several areas of the body. Physical disorders can cause abnormal muscle tone, which can be either abnormally low (hypotonia) or high (hypertonia). Hypotonia is characterised by muscle flaccidity, with floppy limbs, decreased stretch reflex responses, and decreased resistance to passive movement. It is associated with lower motor neuron diseases like poliomyelitis. On the other hand, hypertonia is associated with upper motor neuron diseases, such as lesions in pyramidal and extrapyramidal tracts. Hypertonia can manifest as spasticity or rigidity. Spasticity is velocity-dependent resistance to passive stretch, while rigidity is velocity-independent.
Spasticity can vary in severity, ranging from a mild feeling of muscle tightness to severe, uncontrollable stiffness and spasms. It can cause muscle deformities, abnormal posture, contractures, and sleep disruption. Spasticity is also a symptom of certain neurological conditions, disrupting movement patterns and affecting speech and gait. It is often treated with oral medications, botulinum toxin injections, and physical therapy.
Rigidity is commonly associated with basal ganglia injuries and is seen in Parkinson's disease. Unlike spasticity, rigidity affects all muscles surrounding a joint equally, and the increased muscle tone remains constant throughout the range of movement.
Dystonia and paratonia are also disorders of altered muscle tone, arising from network disruption in the basal ganglia and thalamocortical circuits. Paratonia is associated with dementia and is described as increased muscle tone in response to passive movement, proportional to the strength of the stimulus applied.
Rehabilitation strategies such as constraint-induced movement therapy (CIMT) and functional electrical stimulation (FES) leverage neuroplasticity to promote recovery from muscle tone disorders. CIMT encourages the use of the affected limb, improving neural pathways and motor function. FES uses electrical currents to stimulate muscle contractions, aiding in maintaining muscle mass and improving motor control.
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Muscle tone and flexibility
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. It is important to assess several areas 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. Hypotonia can present as muscle flaccidity, with floppy limbs and decreased stretch reflex responses, while hypertonia can manifest as spasticity or rigidity.
Muscle tone is not a direct reflection of muscle strength. Individuals with low muscle tone may not necessarily have decreased strength, and those with high muscle tone may not have adequate muscle strength. Low muscle tone is often associated with increased flexibility and decreased strength, while high muscle tone tends to decrease flexibility and increase strength. However, there are exceptions to these general trends.
The maintenance of muscle tone involves partial contraction of a muscle, which is essential for generating reflexes, maintaining posture and balance, and controlling the proper function of other organ systems. Muscle tone is regulated by the muscle spindle, a small sensory unit associated with muscle tissue. When stretched, muscle spindles activate and trigger impulses to the spinal cord, generating immediate reflexes. Spindles also operate a feedback loop by directly triggering motor neurons linked to their associated muscles.
Flexibility, in the context of muscle tone, refers to the range of motion or mobility of a joint. While muscle tone can influence flexibility, it is important to note that joint laxity, or looseness, also significantly contributes to overall flexibility. For example, a person with high muscle tone may have poor flexibility in most areas but still be able to achieve certain flexible movements due to joint laxity.
To improve muscle tone and flexibility, physical therapists can provide strengthening exercises and compensatory strategies, especially for individuals with low muscle tone who may struggle with explosive movements. Additionally, flexibility training, such as stretching exercises, can help enhance overall flexibility and improve the range of motion at specific joints.
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Muscle tone and movement
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. 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. Disorders of muscle tone can arise from dysfunction in neural circuits in the brain, spinal cord, and muscle spindle.
Muscle tone is not a reflection of muscle strength. Those with low tone may not have decreased strength, and those with high muscle tone do not necessarily have adequate muscle strength. Low tone increases flexibility and decreases strength, while high tone decreases flexibility and increases strength. However, there are many exceptions to this generalization. For example, a person with low muscle tone will likely not be able to engage in explosive movements such as sprinting or high jumping, while a person with high muscle tone will likely not be flexible in activities such as dance or yoga.
The maintenance of partial muscle contraction, or muscle tone, is important for generating reflexes, maintaining posture and balance, and controlling the proper function of other organ systems. Muscle tone is controlled by neuronal impulses and influenced by receptors in the muscle and tendons, which generate reflexes in the spinal cord. Muscle tone is also an active contributor to movement and postural tasks, reflecting a state of preparedness for movement. The main regulator of muscle tone is the muscle spindle, a small sensory unit associated with muscle tissue that measures muscle stretch.
Distinct stretch receptors called Golgi tendon organs also assess the level of stretch within the tendon. The sensitivity of the Golgi tendon organ is less than that of the spindle, so it is thought to prevent damage rather than control muscle tone. Smooth and cardiac muscles do not have specialized muscle spindles, but muscle tone is not limited to skeletal muscles and is also a property of these muscle types.
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Frequently asked questions
Normal muscle tone is the maintenance of partial contraction of a muscle, important for generating reflexes, maintaining posture and balance, and controlling the proper function of other organ systems. It 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'.
Normal muscle tone is controlled by neuronal impulses and influenced by receptors found in the muscle and tendons. It is also important to assess several areas before deciding if a person has normal muscle tone. Generally, low tone is perceived as "lax, flabby, floppy, mushy, dead weight", and high tone is perceived as "tight, light, strong". However, it is important to note that muscle tone is not a reflection of muscle strength.
Normal muscle tone is controlled by the sensory muscle spindle, which measures muscle stretch. The main regulator of muscle tone, the muscle spindle, is a small sensory unit that is closely associated with and lies parallel to a muscle. Connecting to the endomysium of a muscle fibre, muscle spindles are composed of nuclear bag fibres and nuclear chain fibres.
