Infants are born with a very rudimentary ability to briefly lift the head when supported in sitting position. Over the course of the next 6-8 months learn to maintain the head erect and extend the trunk when when supported in sitting, then learn to sit with arm support and finally to sit independently, reach in all direction and transition to prone kneeling.
Theses functional developments are associated increasing ability to integrate information from the vestibular proprioceptive and visual systems that allow the infant to stabilize the trunk and head in space, and anticipate and adapt to perturbations created by moving the head and trunk, and reaching in all directions.
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A quick tour of the development of sitting
In supported sitting the neonate assumes a flexed position of the trunk. He is able to raise the very head briefly when supported around the upper torso.
By 4-6 weeks the infant has started to develop extension in the upper thoracic spine when supported around the upper torso in sitting.
When supported around the chest, the infant is able to lift the head briefly to look at an interesting object or face. (AIMS sitting sub-scale: sitting with support.)
When supported around the hips the infant's trunk slowly drops forward. However, the infant is able to maintain a degree of extension in the neck and upper thoracic spine.
At this age the infant is able to hold the head erect and in the midline when supported around the trunk.
When supported around the pelvis, the infant sits with the trunk inclined forwards and is able to maintain this position for longer periods of time before collapsing right forwards.
The head is extended, there is some upper thoracic extension and and the scapulae are retracted (Bly Third Month)
At this age infants start to prop themselves on their arms and can maintain this position briefly, before losing their balance and flexing forwards. (AIMS Sitting subscale: sitting with propped arms.)
When sitting on a lap and firmly supported around the hips, 4-5 months infants will actively reach for toys. At this age the infant is usually more focused on reaching a toy than on maintaining balance.
Sometime between the ages of 5-8 months infants learn to sit briefly without support. (AIMS sitting subscale: unsustained sitting without arm support it.)
However balance is poor and the infant is not able to reach for toys or turn head.
The newly sitting Infant is usually able to stabilize the trunk and maintain balance when provided with firm support around the hips and thighs.
By the age of 9 months most infants are able to sit erect with the shoulders aligned over the hips. (AIMS sitting subscale: sitting without arm support 1)
Balance is good and the infant is able move look around and reach for and manipulate toys.
At this age the infant is very active in sitting, reaching backwards, across the body, moving into and out kneeling.
7-9 months: infant rotates trunk to reach back for toys
(AIMS sitting subscale: reach with rotation in sitting)
8-11 month: moves from sitting to prone kneeling
(Aims sitting subscale: sitting to four point kneeling)
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Development of sitting