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Muscle tone is just one of many descriptors you may use when observing posture and movement.For many years, the terms hypertonia (or high muscle tone) and spasticity were used interchangeably. You will still often find this in today’s literature. In Pediatric, there is currently a strong movement to differentiate between muscle tone and spasticity. The physiological mechanisms and motor expression of each differ. Hence, by describing each separately in our assessments, we can provide a richer, more accurate, picture of a child’s movement.This mini-module will address muscle tone and the variations you may see in children.
Clinicians show a good ability to identify tone muscle tone that differs from normal, however, it is difficult to find one consistent definition in the literature. Definitions vary widely depending on the author’s discipline…for example biomechanics, physiology, clinical. Some components that are frequently found in descriptions of muscle tone are listed on this slide.First, note that muscle tone is a description of the state of the muscle at rest. In other words, tone does not refer to the muscle during a purposeful, active contraction.Secondly, there is an acknowledgement that the muscle is in a state of contraction, low level though it may be.Lastly, this “resting state” or partial contraction demonstrates as resistance when the muscle is passively stretched.
Tone can be used to describe a single muscle, a group of muscles, a single or multiple limbs or trunk, or the entire body. For example, in pediatrics you will may hear that a child with spastic quadriplegia has high tone extremities and a low tone trunk….for a child with spastic diplegia may be described as having high tone lower extremities with no atypical tone in the upper extremities.“Tone” is a general term that requires a qualifier when it is observed to be atypical. In addition to qualifiers like high or low, you may see descriptions such as mildly low tone or extremely high tone.Clinically, muscle tone is difficult to quantify and tends to be subjective. Clinical measures of muscle tone will be discussed later in this mini-module.
Recently, there has been an increase in research investigations of muscle tone. Contrary to the previous belief that muscle tone was only a manifestation of a stretch reflex, current research supports the concept that muscle tone is a product of both intrinsic and Central nervous system components. Intrinsic properties may include: stiffness of muscle fiber crossbridges impacting muscle viscoelasticity, alterations in fiber size/number/ and/or length, and changes in surrounding soft tissues structures. When reporting atypical levels of muscle tone, we need to keep in mind that muscle tone is impacted by both the peripheral properties as well as by the central nervous system.Another characteristic of muscle tone important for clinicians to recognize is it’s dynamic quality. Normal or typical resting muscle tone varies from position to position, moment to moment, and day to day. All muscle tone varies within a range. If you are sitting and watching TV, your resting muscle tone will be lower than it would be if you were standing on snow skiis getting ready to skii down a black diamond skii run for the very first time. Still, your lowest low would not be in the range we consider pathologically low and your highest tone would not be in the range we consider pathologically high.Muscle tone in children with identified high or low tone will also fluctuate depending on their emotions, their health, arousal, and other similar situations. As with normal tone, abnormal tone states do not generally fluctuate into the normal range or into extreme ranges like flaccidity and rigidity.Atypical levels of muscle tone can be difficult for families to understand. I will often describe it like the idle of a car. When sitting at a stop light with the motor running, the motor may be idling faster than optimal, optimal, or slower than optimal but, in all instances, the idle isn’t high enough to move the car forward.
There are rarely distinct breaks between different types of resting muscle tone so we can conceptualize muscle tone on a continuum from little or no firing of fibers on the left side, through normal range in the middle, to a high level of firing of fibers on the right side. All resting muscle tone will be somewhere along this continuum.
On the furthest left side of the continnum you find the least amount of firing of muscle fibers and the least amount of instrinsic stability. This end of the continnum represents muscles that are flaccid. These muscles are unable to contract enough for the contraction to be manually palpated or electronically detected. The etiology of flaccidity can originate from the central nervous system or it can be peripheral due to injury or pathology. Flaccidity can be temporary or permanent.
Flaccidity with a CSN etiology is seen less often in children than in adults. In chlldren, flaccidity with a CNS etiology is most often seen related to brain tumors and post TBI. Peripheral etiologies include brachial plexus injury, Spinal cord injuries and other traumatic injuries.
Moving toward the right on the continuum you can see a range for low muscle tone or hypotonicity. Low muscle tone is comprised on a lower resting level (or idle) than typical muscle tone. It is also frequently accompanied by joint and soft tissue laxity.
Children with low muscle tone tend to move less frequently against gravity compared to children with normal tone. Parents will often describe their child as particularly “content” or even lazy. Lazy is perhaps an unfair label as it is simply more difficult for those children to move. Children with low muscle tone develop motor milestones at a slower rate than children with typical tone and the milestones look a little different. Compared to children with normal tone, children with low tone tend to use a larger base of support in supine, prone, sitting, standing, and walking. Due to the constant wide base of support, their trunk postural system develops poorly and makes development of higher level balance skills difficult. This is particularly evident in 1 foot standing, stair climbing, jumping, skipping, running, and sports.
As children with low muscle tone begin to move off the supporting surface, they prefer to move to end joint ranges where they can use joint stability rather than muscle tone to maintain the position. The child with low tone in the bottom picture is using neck hyperextension rather than holding his head in a mid position and his shoulders are elevated and internally rotated for stability. For those children with joint laxity, stability is found by hyperextending elbows, wrists, fingers. When the child supports weight on his or her feet, they will frequently lock their knees into hyperextension.
Children with low muscle tone have difficulty controlling movement in mid ranges and holding postures off the supporting surface. They are reluctant to leave their large base of support and you will often note a delay between the request or stimulus for movement and the child’s initiation of movement. Then, they will move very quickly, almost flinging themselves through space until they reach their new large base of support. If given their choice, they will maintain their wide base of support and keep their moving limbs as close to the supporting surface as possible.Often parents and healthcare professions are not particularly worried about children with moderate to milder levels of low muscle tone and will not request intervention services…they tend to take a “he’ll catch up and outgrow this” perspective. As the child nears school age or enters elementary school, the family frequently becomes concerned about clumsiness, inability to keep up with peers in physical activities, and playground safety.
Muscle tone 11
MUSCLE TONE CONTINUUM 2012 Jan McElroy PT, PhD, PCS, C/NDT
WHAT IS “MUSCLE TONE” INSKELETAL MUSCLES?Consistencies in muscle tone definitions:1. muscle activity at rest (Campbell)2. a state of partial contraction present in a muscle in it’s passive state (medical dictionary)3. resistance to muscle stretch (passive) 1, 2
“TONE” Can describe a single muscle, a group of muscles, a single or multiple limbs or trunk, or the entire body A general term Measures tend to be estimates, very subjective
Characteristics of Tone Both intrinsic and reflex components 2, 3 Is dynamic and changes within it’s usual range depending on emotions, illness/health, arousal etc. “Tone” can be compared to the level of the “idle” of a car
The ContinuumNo firing High level of Normal Range firing fibers of fibers
The ContinuumNo firing • CNS etiology of fibers = Flaccid • Peripheral etiology