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Muscle tone is the measure of a muscle's resistance to stretching while in a passive resting state. 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. Muscle tone is controlled by neuronal impulses and influenced by receptors found in the muscle and tendons. Disorders of muscle tone can arise from dysfunction in these pathways and manifest as hypertonia or hypotonia.
| Characteristics | Values |
|---|---|
| Definition | Muscle tone is a measure of a muscle's resistance to stretching while in a passive resting state. |
| Muscle Tone in Action | When someone stretches your arm out straight, your muscle strength is the ability to contract and resist that force actively. |
| Muscle Tone in Resting State | Even when at rest, muscle fibres are at least partially contracted, possessing a small degree of tension. |
| Muscle Tone and Strength | Muscle tone and muscle strength are different. Muscle strength is defined as a muscle's ability to contract and create force in response to resistance. |
| Muscle Tone and Reflexes | Muscle tone is important for generating reflexes, maintaining posture and balance, and controlling the proper function of other organ systems. |
| Muscle Tone and Physical Disorders | Physical disorders can result in abnormally low (hypotonia) or high (hypertonia) muscle tone. |
| Types of Hypertonia | Spasticity and rigidity are the two types of hypertonia. |
| Types of Hypotonia | Dystonia and paratonia are the two types of hypotonia. |
| Muscle Tone and Flexibility | Low tone increases flexibility and high tone decreases it, but there are many exceptions. |
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What You'll Learn
Muscle tone is the resistance to passive stretch
The main regulator of muscle tone is the muscle spindle, a small sensory unit closely associated with the muscle. It contains nuclear bag fibres and nuclear chain fibres, which are similar to muscle fibres in that they contain actin and myosin myofilaments that allow them to stretch with the muscle. When the muscle is stretched, the muscle spindle is activated, triggering impulses to the spinal cord that generate an immediate reflex. The muscle spindle also operates a feedback loop by directly triggering motor neurons linked to their associated muscles. If the tone decreases and the muscle stretches the spindle, an impulse results in a muscle contraction to counteract the stretch.
Muscle tone is influenced by neuronal impulses and receptors found in the muscle and tendons. These receptors, called golgi tendon organs, assess the level of stretch within the tendon. While the sensitivity of the golgi tendon organ is less than that of the spindle, they are thought to exist to prevent damage rather than control muscle tone. Smooth and cardiac muscles do not have specialised muscle spindles.
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 hypertonia (high muscle tone) or hypotonia (low muscle tone). Hypertonia can be further classified as spasticity, which is velocity-dependent resistance to passive stretch, or rigidity, which is velocity-independent resistance. Spasticity can present as the clasp-knife response, with increased resistance at the beginning or end of a movement. Rigidity can be of the leadpipe type, with resistance throughout the passive movement, or the cogwheel type, with jerky resistance. Hypotonia, on the other hand, can present as muscle flaccidity, with decreased resistance to passive movement.
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Disorders of muscle tone
Muscle tone refers to the resistance to passive stretching of a joint while in a resting state. Disorders of muscle tone can arise from dysfunction in the neural circuits in the brain, spinal cord, and muscle spindle, manifesting as hypertonia or hypotonia. Hypotonia, or poor muscle tone, is often detected at birth or during infancy and is usually associated with underlying neurological conditions or genetic disorders. It can cause developmental delays, speech difficulties, and coordination issues in children. Physical and occupational therapy can help improve motor control and overall body strength in individuals with hypotonia.
Hypertonia, on the other hand, is characterized by abnormally high muscle tone. It can present as spasticity or rigidity and is commonly seen in upper motor neuron diseases, such as lesions in the pyramidal and extrapyramidal tracts. Spasticity is velocity-dependent resistance to passive stretch, while rigidity is velocity-independent. Two other disorders of altered tone, dystonia and paratonia, are associated with network dysfunction between the basal ganglia, cerebellum, thalamus, and cortex. Dystonia causes a lack of surround inhibition, co-contraction of antagonistic muscles, and overflow of activity into unintended muscles. Paratonia is often associated with dementia.
The treatment for muscle tone disorders depends on the specific diagnosis and can include physical therapy, occupational therapy, and speech therapy. Early intervention is crucial for improving long-term outcomes, especially in children with hypotonia, as it can help with developmental milestones and speech development. In some cases, supportive devices may be necessary to improve postural stability and safety.
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Muscle tone and 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 controlled by neuronal impulses and influenced by receptors found in the muscle and tendons. This results in reflexes in the spinal cord, such as the knee-jerk reaction, and other key functions such as posture maintenance and proper digestive system function.
The muscle spindle is a small sensory unit that is closely associated with and lies parallel to a muscle. When stretched, muscle spindles become activated, triggering impulses to the spinal cord that can generate an immediate reflex. Spindles can also trigger impulses to the cerebral cortex, providing information about the degree of stretch within the muscle. To maintain tone, spindles also operate a feedback loop by directly triggering motor neurons linked to their associated muscles. If the tone decreases and the muscle stretches the spindle, an impulse results in a muscle contraction.
Muscle tone is important for generating reflexes, maintaining posture and balance, and controlling the proper function of other organ systems. Resting muscle tone varies along a bell-shaped curve. Low tone is perceived as "lax, flabby, floppy, mushy, dead weight", and high tone is perceived as "tight, light, strong". Muscles with high tone are not necessarily strong, and muscles with low tone are not necessarily weak. In general, low tone increases flexibility and decreases strength, and high tone decreases flexibility and increases strength, but there are many exceptions.
Muscle strength is defined as the muscle's ability to contract and create force in response to resistance. It is what your muscles do when they aren't at rest. When you purposefully move a muscle, your brain sends signals to your muscle fibres to contract. The power of your contraction is dependent on your muscle mass, which can be changed by weight training and exercise.
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Muscle tone in children
Muscle tone is the term for the resting length of muscles in the body, or the length of the muscle before a contraction. It is a measure of a muscle's resistance to stretching while in a passive resting state. The resistance to passive stretch is a complex network encompassing neural circuits in the brain, spinal cord, and muscle spindle. The main regulator of muscle tone is the muscle spindle, a small sensory unit that is closely associated with and lies parallel to a muscle.
Low muscle tone, or hypotonia, in children, is typically identified within the first five years of life. A child with low muscle tone may have floppy or loose limbs and may seem easily fatigued, preferring to lie down rather than sit upright. They may also attain their motor milestones later than average. Low muscle tone can be caused by joint hypermobility, which is the most common condition causing low muscle tone in children. Joint hypermobility means that the structures holding the joint together are more pliable than normal and provide less support for the joints. It can also be caused by muscle weakness, which is often associated with joint laxity. Infants and toddlers with low muscle tone may be late in learning to stand, cruise, and walk. They may also have a cautious nature, avoiding challenging tasks and showing low motivation to move.
Clinically significant high muscle tone, or hypertonia, in children is most commonly caused by in-utero stroke, brain bleeds, or cerebral palsy. High muscle tone in children is typically identified within the first 18 months of life. A child with high muscle tone may have delayed gross and fine motor skill development and may have difficulty "relaxing" their muscles. They may maintain a fisted hand beyond six months of age or present with very stiff legs that appear to move like scissors when standing or attempting movement.
Children with low muscle tone can benefit from physical and occupational therapy to strengthen muscle groups and decrease the impact of low muscle tone on overall functioning, including coordination and attainment of motor skills. Similarly, children with high muscle tone can benefit from physical and occupational therapy to stretch affected muscles and strengthen opposing muscles, thereby improving overall functioning.
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Muscle tone and physical therapy
Muscle tone is a commonly used term in physical therapy, especially for children diagnosed with Down syndrome, cerebral palsy, or autism. Muscle tone is used to describe how much tension or partial contraction muscles have at rest. It is the amount of tension present when muscles are relaxed. Normal muscle tone allows us to contract and relax our muscles appropriately. Those with normal tone are able to use just enough energy to perform tasks at the proper speed.
There are three types of muscle tone: normal, high, and low. Physical disorders can result in abnormally low (hypotonia) or high (hypertonia) muscle tone. Hypotonia, or floppy baby syndrome, is not the same as muscle weakness, although the two can often go hand in hand. Those with low muscle tone tend to have a "floppy" or "mushy" feel to their muscles when at rest. They have difficulty with graded muscle control and require more energy and effort to move their muscles. They have lower tension in their muscles, and these muscles tend to be in an elongated position when resting.
High muscle tone, on the other hand, means that the muscles are contracted more than typical and are getting more input than is necessary, so they don't have the opportunity to fully relax. Those with high tone will usually not be flexible in activities such as dance and yoga. Physical therapy can help with both high and low muscle tone. For both types, there is an underlying muscle weakness that needs to be addressed. When thinking of kids with high tone, one may not think of muscle weakness. However, if asked to do skills that require fewer muscles, they tend to have difficulty, indicating underlying muscle weakness.
In physical therapy, each goal is specific to each individual child. For kids with low tone, physical therapists typically focus on core strength and endurance. Some activities that can help are sitting on a pillow while watching TV or playing a game, or playing games where the child reaches for the opposite side of their body and back, which helps with rotational core strength. For kids with high tone, therapists usually focus on rotational movements and skills that require the opposite muscles of where they are "tight."
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Frequently asked questions
Muscle tone is a measure of a muscle's resistance to stretching while in a passive resting state. It is the muscle's response to an outside force, such as a stretch or change in direction.
Muscle tone is defined as the tension in a muscle at rest. Muscle strength is defined as the muscle's ability to contract and create force in response to resistance.
Low muscle tone is perceived as "lax, flabby, floppy, mushy, dead weight" and high muscle tone is perceived as "tight, light, strong". Low tone increases flexibility and decreases strength, and high tone decreases flexibility and increases strength.
