The role of variability and exploration of possibilities for action
During the 4-6 month period the infant's sense of agency continues to increase along with frequent initiation of goal directed action including turning to look towards interesting sounds and reaching for, and playing with toys.
At the same time the infant actively explores and takes pleasure in her ability move in different directions, assume new and interesting postures and roll from and into prone. This drive to explore the limits of her abilities is closely linked to emerging and more complex ability to initiate postural adjustments that help to maintain ability and balance.
The infant's ability to lift the head and upper torso up off the support surface and extend the spine improves markedly over this period.
At 4 months the infant is able to lift the head and upper torso up when this action is supported by pushing up on the UEs. The neck and spinal extensors work to maintain neck and trunk extension. The hip extensors may also be activated.
With practice and increasing trunk extensor muscle strength the infant learns to lift up the upper torso, usually with the shoulders in abduction lateral rotation.
By the end of the 4-6 month period infants are able to assume the prone pivot position with the head and shoulders and thighs lifted up off the support surface.
Control of postural stability and adaptive postural sway
A primary goal for the postural system during this period is to learn to control the extent of the lateral postural sway while at the same time learning to make subtle adaptive changes to the sway path to support weight shift onto one arm with release of the other arm for reaching.
Turning the head to look to one side is associated with weight shift over the UEs and changes in the alignment of the trunk as the postural system explores ways to hold the head and trunk steady.
Development of UE support and weight bearing
At the beginning of this period infants have usually learned to extend the neck and upper thoracic spine lifting the head and upper torso up off the support surface and at the same time pushing down on the support surface with one or both forearms or hands.
Infants will assume a variety of UE positions depending on the exploratory or intentional goal directed actions that are being pursued.
Infants also start to push up onto extended arms. In this position the hips may be extended or flexed.
Weight shift to free one arm for reaching
At 4 months infants are starting to explore ways to shift their weight onto one UE freeing the other to reach for toys. Lifting one hand is associated with some lateral shift of the shoulder girdle and increased trunk extension but with limited adaptive trunk and pelvic rotation and side flexion.
When weight bearing on the arms is associated by trunk extension, the infant is able to rotate the head with minimal adjustments in weight distribution over the UEs.
When the infant does shift weight laterally over one shoulder with some ipsilateral backwards rotation of the shoulder girdle at the same time, there may be little adaptive response in the contralateral shoulder muscles so that the the infant falls onto the shoulder.
By the end of the 4-6 month period infants will usually have learned to adapt the alignment of the trunk to allow support on one UE when reaching out with the contralateral hand.
The infant learns to rotate the shoulder girdle backwards on the side of the reaching UE, at the same time weight is shifted laterally over the weight bearing shoulder, as well as the trunk and pelvis. This weight shift is associated with side flexion of the trunk with elongation on the weight bearing side and flexion of the contralateral hip and knee.
These adaptations in alignment of the head trunk and limbs allow the COM to be balanced over a stable base of support.
Exploring weight shift for reaching
In the series of frames below you see Will exploring different options for weight bearing over the pelvis as he shifts his weight onto the left UE so as to free the right UE for reaching for a toy.
Will 5 months prone
Will 6 months 2 weeks