An infant or child will get a diagnosis of low muscle tone when:
For a more technical discussion please see webmanual.skillsforaction.com/low-muscle-tone
It is important to note that all these difficulties are in fact related to a mixture of coordination problems, muscle tightness, weakness and poor endurance, generalised joint hypermobility, attention difficulties, a very cautious nature and avoidance of tasks that require effort and persistence.
A careful assessment of all these issues will highlight which are contributing to the child's difficulties so that they can be addressed.
The most common cause of these difficulties is joint hypermobility associated with a cautious temperament.
In individuals with hypermobile joints (loose or lax joints) the connective tissue structures that hold the body together have more give (are more easily stretched) than usual. This means that the muscles must work harder than usual to hold the keep a good posture and move.
Joint hypermobility is probably the main (and often unrecognized) reason a child get a a diagnosis of low muscle tone.
Children with a cautious temperament are often quite anxious which leads to avoidance of tasks that require effort and a tendency to give up very quickly. This in turn leads to low levels of activity, weakness and poor endurance.
Read more here about the link between joint hypermobility and low tone
Recognizing joint hypermobility
Fingers that bend back to 90 degrees
Wrist can be bent so that thumb touches the forearm
Knees and elbows bend backwards
Sometimes joint hypermobility is missed because the child appears to have tight muscles. It is important to understand tight muscles is a common feature of joint hypermobility, especially in the muscles crossing the back and sides of the hips. It is also important to understand that this muscle tightness if often an important underlying cause of some of the difficulties experienced by children with a diagnosis of low muscle tone.
|Tight back and hip muscles affect sitting posture in children with joint hypermobility|
The term muscle tone refers to two aspects of muscles structure and function.
The term low muscle tone is used when the muscles' stiffness is less than usual (ie it is low) and the readiness for action is also low (muscles respond slowly).
How is a muscle's firmness (tone) tested?
The clinician makes a judgment about a muscles firmness and inherent stiffness by:
The firmness of a muscle is determined by its inherent stiffness and this in turn depends on the stiffness in the connective tissue sheaths that encase each muscle fiber, hold the bundles of muscle fibers together and connects the muscle to the tendons.
In children with hypermobile joints the connective tissue is less stiff than usual. (It has more give and is more easily stretched).
This tendency for less stiff connective tissue which results in hypermobile (lax) muscles and joints is genetically determined. About 10-15% of people are hypermobile.
The other very important structure that that determines a muscle's firmness and resistance to being stretched is the giant molecule titin.
Titin is a very large elastic molecule found within the muscle cell. The size and strength of the titin molecules is determined by the amount of work a muscle is made to do on a regular basis. Strong muscles have larger and stiffer titin molecules - and this makes the muscle firmer and stiffer. The muscle has better tone.
Generally speaking, strong muscles are firmer and have better tone.
The term muscle tone is also used to describe the a muscle's readiness for action. This is set by the child's alertness, general level of arousal and readiness for action.
The brain primes the muscles for action by sending signals to the muscles’ sensory receptors. In effect the sensory receptors become more alert and provide more effective sensory feedback and respond more quickly. This allows for the rapid adjustments needed for maintaining balance and postural stability.
The general stiffness of a child's muscles - which is what provides the muscles' tone, depends on two factors:
The stiffness in the connective tissue - this is genetically determines (it is in the DNA) and cannot be changed. (See Joint Hypermobility Syndrome)
The size of the titin molecules in the muscle cells. Stronger muscles have larger titin molecules. Stronger muscles have better tone.
Muscle and joint laxity cannot be "cured". But, and this is important, the movement difficulties associated with muscle and joint laxity can be improved by training
Task specific training has been shown to be the most effective approach to increase muscle strength, flexibility, endurance, coordination and attention.
Task specific training means that you work on the aspects of posture and motor control that need improving.
This is because motor learning and fitness training are task specific: if you want to learn to ride a bicycle you need to get on a bicycle. And if you want to win a bicycle race you have to train on a bicycle. Swimming and running are simply no good.
Joint hypermobility syndrome – The connective tissue is more compliant than usual, leading to joint and muscle laxity and weak muscles
A cautious nature – The child avoids tasks and activities that are difficult or require physical effort, which affects engagement and readiness for action.
Muscle weakness – as a result of lack adequate opportunities for engagement in activities that load the muscles in a way that leads to increased muscle strength
Developmental coordination disorder where a child has difficulty learning new skills through experience and exploring ways to perform new and challenging tasks, leading to avoidance and lack of experience.
Autism - is often associated with both joint hypermobility and DCD, along with anxiety and avoidance of challenging tasks.
Down syndrome where joint hypermobility and muscle laxity is a defining feature.