Hypermobile infants and toddlers have joints that bend further than usual, often have some muscle weakness which leads to a diagnosis of low muscle tone, and often have a very cautious nature.

These differences in their joints, muscles and temperament affect the baby's development in the following ways:

• May be late learning to sit, sit with a very rounded back or W-sit
• Skip crawling and bottom shuffle instead
• Hate tummy time
• Have tight hip muscles which affects crawling, walking and balance
• Avoid activities that are new or require effort and may have sensory issues lanin sit, sit with a very 

What is joint hypermobility?

Babies with joint hypermobility have joints that are able to bend further than usual and a trunk and limbs that appear to be floppy and weak. 

The increased range of  movement at the joints (sometimes called joint laxity or being double jointed) is due to differences in the connective tissue that forms the joint capsule and ligaments.

The joint capsule and ligaments hold together the bones that form the joint.  The joint capsule and ligaments are formed by a strong but pliable material called connective tissue. Connective tissue can be stretched a little way, just enough to allow the joint to move through the typical full range of movement.  

In joint hypermobility the connective tissue has more give and can be stretched further than usual. This means that the joints are less firmly held together and can be moved further than usual. 

Why does joint hypermobility cause developmental delay?

The increased pliability in the connective tissue also affects the muscles, which may appear to be floppy and are often weak. This in turn affects how the infant moves and develops and may mean that the infant is late achieving the major developmental milestones. 

Read more: Causes and diagnosis of joint hypermobility

Low muscle tone and joint hypermobility

Infants who appear to be floppy and have hypermobile joints are often given a diagnosis of low muscle tone.  The term muscle tone refers to the muscles' readiness for action. This readiness for action depends on the sensitivity of the stretch receptors in the muscles. If the sensitivity to stretch is very low the muscles are slow to respond and they appear to be weak and floppy. 

In infants with joint hypermobility the connective tissue that holds the muscles together and connects the muscles to the bones via the tendons is very pliable. This affects the sensitivity of the stretch receptors and the muscle's readiness for action. 

In most instances the low muscle tone is linked to joint hypermobility. However, if a baby is floppy and appears to be weak a full assessment by a pediatrician is important to rule out other causes. 

Read more   Low muscle tone  

Different developmental pathways for hypermobile and "low tone" infants 

How joint hypermobility affects a baby's posture at birth

At birth typically developing full term infants lie with their arms and legs flexed. In fact the the muscles of the hips and knees are tight and cannot be completely stretched out. 

The slight tightness (stiffness) in the muscles of the hips and knees helps the newborn infant to lift the arms and legs up when kicking and reaching. 

Newborn hypermobile  (and pre-term) babies lie with their legs and arms more extended and flat on the cot mattress. The usual tightness of the hip and knee muscles is absent and the the hips and knees can be fully extended. 

The laxity in the muscles means that it requires more effort to lift up the arms to reach for toys and kick the legs.

Hypermobile babies are less active

Typically developing babies are very active which is important for strengthening the muscles

Within weeks active infants have  learned to keep their trunks steady when they kick, lift their legs up  and reach for toys. They also start  to push down on their feet and lift up their buttocks. All this movement strengthens the muscles and gets them ready for sitting.  

Hypermobile, low tone and premature babies may be less active and fail to develop good muscle strength

If the infant is hypermobile and also cautious she may not kick vigorously and fail to develop the muscle strength needed to support the hypermobile joints and get them ready for rolling and sitting up.  

Hip stiffness affects the baby's ability to get up onto the knees 

The  stiffness in the hip muscles affects the infants ability to get up into kneeling or onto all fours. The legs are held wide apart when they are on their tummies and this makes it difficult to get up onto the hands and knees in readiness for crawling. 

Hypermobile infants lock their elbows 

Hypermobile infants lock their elbows into hyperextension when they push up on their arms. As a result they do not develop the arm strength needed for getting up onto all fours and crawling. 

Hypermobile (low tone) infants often bottom shuffle, and skip the crawling stage

Hypermobile and low tone  infants sometimes do not crawl. Instead they may scoot on their bottoms to get around. 

This little fellow manages to hop along on his bottom with great speed.  

Read more: Why some infants bottom shuffle and why crawling matters.

Standing, cruising and walking may be delayed

Hypermobile infants stand with their knees locked back in hyperextension. They may also  start standing with their feet wide apart and turned out. This makes it difficult to shift the weight onto one foot to take a step.

If their hip and knee muscles are weak, they have difficulty bending down to retrieve objects from the floor, and take longer to sit down with good control. 

They often fall back with their knees straight  when they want to sit down again.

Walking is often delayed in babies with hypermobile joints and low muscle tone

Hypermobile and low tone infants tend to walk a few months later than usual, sometimes as late as 18-20 months.

What can I do to help my baby?