Who gets a diagnosis of low muscle tone?
Infants who appear floppy at birth, who are born pre-term and who fail to reach their milestone on time, are often given a diagnosis of low muscle tone. Toddlers who are late learning to walk, who trip and fall a lot, appear to be weak and have poor sitting posture may also be considered low toned.
In most instances the underlying reason for a diagnosis of low muscle tone is joint hypermobility (laxity) and muscle weakness, and can be improved by exercises to strengthen the muscles and train balance and coordination.
What is muscle tone?
Muscle tone refers to the amount of give and elasticity in the muscle when it is stretched or lengthened. For the most part this is determined by two factors:
- The structure of the muscle, including the connective tissue sleeves that hold the muscle together, and the size of the giant elastic molecule titin within the muscle cell. (See below).
- Fluctuations in the state of alertness and readiness for action. This is known as active, or responsive, muscle tone and is important for maintaining good balance and stability.
Active/responsive muscle tone
Toes is very engaged in playing with the marble run. He is alert, sets erect and keeps his head and trunk steady as he reaches up to put a marble on the run.
The active tone in his trunk and neck muscles has increased to help him maintain a stable and upright posture.
Muscle tone, elasticity and muscle strength
In order the pick up the ball Thabo must bend forwards at the hips and bend the knees in a controlled manner. These actions are controlled by the amount of give (elasticity) in the hamstring and quadriceps muscles.
Strong muscles have more elasticity (ie better muscle tone) and provide a more controlled movement.
Weak muscles have less muscle tone and are more easily stretched, and provide less support for the bending movement of the hips and the knees.
Why strong muscles have better tone
The elasticity and give in a muscle is determined by two factors: the size of the titin molecule found in the muscle cell and the stiffness of the connective tissue that holds the muscle fibers in place.
Titin - a giant elastic molecule
When a lengthening/stretching force is applied to a muscle, the muscle gives a little and becomes longer. When the stretching force is removed, because the muscle is elastic it returns to its resting length.
Titin, a giant elastic molecule that forms part of the muscle cell, is responsible for the muscle's elastic properties. Importantly, the size of titin molecule is influenced by loading. The more the muscle is loaded, and the stronger it is, the larger the titin molecule. This is why strong muscles have better tone.
The connective tissue component of a muscle
Each muscle fiber is held in place by a connective tissue sleeve, groups of muscle fibers and the muscle as a whole are also enclosed by a connective tissue sleeve. The muscle tendon is formed by dense connective tissue.
Typically the connective tissue found in muscles and tendons has a small amount of give. In people with joint hypermobility, the connective tissue is less stiff and has more give. This affects the "tone" of the muscle - when palpated the muscle feels softer than usual.
Importantly, the strength and stiffness of the muscle and tendon connective tissue can be improved by strengthening the muscle.
What is active muscle tone?
Active muscle tone refers to a muscle's readiness for action, and this is determined by the amount of contraction in a muscle's intrafusal fibres, which in turn sets the sensitivity of the stretch receptors in the muscle.
It is very important to understand that setting the sensitivity of the stretch receptor is part of motor planning and coordination, and is specific to the task.
Here you see Toes preparing to take a step. To to ensure that he keeps his balance as he steps, the brain increases the active tone in his trunk and leg muscles to keep him steady.
Active muscle tone, especially in the postural muscles is also affected by mood and motivation. When we are motivated and ready to move, our postural muscles are activated to support our actions. We sit up and take notice.
An alert and interested infant or toddler is ready for action and this automatically increases muscle tone in preparation for movement.
Muscle tone at birth
Infants born at term have been lying curled up in the womb for several weeks before birth. As a result the muscles of the arms and legs are stiff, and cannot be fully extended.
Fetuses are also very active in the womb; they do a lot of kicking, stretch out and turning. All this activity strengthens their muscles, which gives them the strength to kick, move their arms and lift the head briefly straight after birth.
Pre-term infants' muscles are weak and lax
Because preterm infants have missed the period of lying curled up in the womb, they do not develop the same degree of muscle stiffness and strength as a full term infant. As a result their muscles appear to be floppy and weak in the first weeks after birth.
They tend to lie with their arms and legs extended and are less active than full term infants. More about development of preterm infants
However, with early nesting to support the arms and legs in some flexion, and encouragement to be active, kick vigorously and reach towards faces and interesting objects that are within reach, preterm infants will develop good muscle tone and strength.
Neonatal hypotonia (low muscle tone)
There are many possible reasons for marked hypotonia and weakness in newborn infants. In all cases the pediatrician will do a full clinical examination of the infant, and run number of tests, to determine the cause of the hypotonia.
Joint hypermobility is the term used for joints that are able to bend further than usual. Other terms for joint hypermobility are joint laxity or double jointed.
Joints and muscles are held together by connective tissue. In individuals with hypermobile joints the connective tissue is more pliable (ie less stiff) then usual. This means that the joints and muscles can be stretched more easily - in other words they have less "tone".
Infants with joint hypermobility sit with their legs wide part and the back flexed. This leads to some stiffness in the hip muscles which in turn affects crawling and standing.
Infants with low muscle tone often refuse to spend time on their tummies. This is especially true if the infant has a sensitive/cautious temperament and reacts strongly to situations that require effort.
Tummy time is important for developing arm, trunk and hip muscle strength, so it is really important to find ways to encourage your infant to spend time on his or her tummy. Read more
Infants will often tolerate tummy time if their chests rest on a small step.
In this position they will start to push up on their hands.
Infants who have very flexible hips and sit with their legs wide apart often find getting up onto all fours difficult, especially if they also have hypermobile elbows.
For these infants bottom shuffling is often the easier option, especially if they have some tightness in the hip muscles
Toddlers with a diagnosis of low muscle tone
Joint hypermobility is the most common underlying reason for the movement difficulties experienced by toddlers who have a diagnosis of low muscle tone.
Toddlers with low muscle tone associated with joint hypermobility often have some tightness in the hip muscles which affects their ability to sit erect. Instead they may sit with the legs wide apart and a flexed spine.
The toddler may also have weakness in the leg muscles which may lead to tripping and falling. If the leg muscles are weak, the toddler will not like walking distances.
Hypermobility in the hand joints may affect fine motor function.
Infants and toddlers with low muscle tone may be late learning to stand, cruise and walk. The developmental delay is often associated with having a very cautious nature, avoiding challenging tasks and a low motivation to move. The other underlying factor is muscle weakness, often associated with some joint laxity.
Read more: Why some children are late learning to walk
How to improve low muscle tone
The two most important ways to improve low muscle tone in infants and toddlers:
- Improve muscle strength
- Improve alertness, motivation to move, persistence and willingness to take on new challenges also improves muscle tone.
Activities that require effort and repetition help to strengthen muscles
Infants and toddlers become stronger when they repeatedly engage in activities that load the muscles adequately for an adaptive response. In other words an exercise should require a measure of effort and be repeated several times.
You know when an infant is working hard because they grunt with effort - I call it going for the grunt.
This sit uo and fall over game is good for strengthening the neck and tummy muscles. Roan is working hard to sit up so she can reach the ball. Read more: Functional task training toddlers for strength and coordination
How attention, and motivation improve muscle tone
Dynamic muscle tone is a reflection of the brain's readiness for action. A child who is motivated to engage in an activity, works hard to figure out how to do somethitrying to achieve goals is naturally more alert and ready for action. This increase in alertness primes the muscles for action via the gamma motor neurones. Increased gamma activity stiffens the muscle stretch receptors and makes them more sensitive to stretch.
When a child is unmotivated and reluctant to participate in an activity, the brain does not prime the muscles for action.
SfA Developmental Gym for Infants and Toddlers
Activities and exercises
► Strengthen muscles and improve flexibility
► Improve hands tasks (fine motor abilities)
► Train movement activities: sitting, standing, cruising, crawling, clambering, walking, steps, ball skills and more