The 5-6 month old infant has learned to roll from supine to prone and is more willing to spend time in prone.
At the beginning of this period the increased strength of the thoracic and lumbar extensor muscles makes it easier to lift the head and shoulders up off the SS and the infant is starting to take weight on the forearms as well as the hands with the elbows extended.
As before, pushing up onto his forearms and hands is supported by extension of the trunk which now includes extension of the thoracic and the lumbar spine.
This allows the infant to lift one hand to reach for toys, but lifting the hand is not associated with transfer of weight onto the other arm. Instead the infant keeps the head and shoulder girdle steady by increasing spinal extension.
Exploring balance, weight shift moving in prone
With the torso lifted up off the support surface and some weight being taken on the upper extremities, the infant's COP has shifted towards the pelvis.
In order to maintain stability and balance as they explore looking around and reaching for toys, the infant explores how best realign the head, limbs and trunk to support these actions.
The video frames below show how Will (5 months) explores different options for shifting his weight over the pelvis and UE's to maintain his balance. As he does this his COP shifts laterally and in a cephalo-caudal direction.
With experience and practice extension of the thoracic and lumbar spine is associated with increasing extension and adduction of the hips, which decreases the BOS in the lateral direction. This series of frames nicely illustrates how Will (6m 2w) works at maintaining balance as he passes a toy from one hand to the other.
Extended arm support and prone pivot
With experience the infant starts to push down on the SS and extend the elbows. They also start to shift between extended arm support and lifting the head and trunk up off the SS (a position sometimes prone pivot). This shift from extended UE support to prone pivot is possible because the extended arm support is associated with active thoracic and lumbar extension. At 5 months infants can maintain the prone pivot position briefly.
Pivoting in prone
Infants who actively explore taking weight on the UE and lateral weight shift to free one hand, will usually also learn to move around in prone and swivel in place. This series of frames shows how Will pivots in place through an arc of 80 degrees
To do this pushes up into elbow support,.shifts weight onto the left hand and lowers shoulder girdle in a new position. He then moves his hands to just in front of his body, and repeats the sequence of push up, tip over and move hands several times.
It is important to remember that the infant's abilities in prone lying are strongly influenced by the support surface: Pivoting in place is easier on a smooth slippery floor than on a carpet.
Bilateral bounce forwards and slide back
At this age infants also discover how to push up into prone kneeling and then extend the knees and "drop" the head and shoulders forwards.
With practice this push-up-flop-forward action becomes more controlled.
When infants are motivated to move forwards they will often push up into extended UE support, but this action on a slippery floor, may result in the infant moving backwards away from the desired toy.
Some, but not all infants learn to commando crawl, using a symmetrical or a reciprocal pattern.
Pushing up into prone kneeling
With experience the 5-6 month infant gains greater control of keeping the trunk in the midline as well UE strength them which allows then to push down on the hands, engage the trunk flexors to lift the pelvis up off the SS. Once the pelvis is lifted, the infant flexes the hip and knee of one LE and moves the knee forwards under the hip.
Maintaining this position takes practice and it usually takes time and early efforts at crawling are varied. Infants will often spend time rocking backwards and forwards and exploring lifting one hand or knee off the support surface.
The frames below show Will at 7 months exploring different options for maintaining balance in prone kneeling, as he attempts to lift one hand to play with a toy.
Transition to sitting
Once the infant is able to move from prone to prone kneeling, they start to experiment with lowering the buttocks to one side into sitting with support on one UE.
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