Infants are born with a very rudimentary ability to briefly lift the head when supported in a sitting position. Over the course of the next 6-8 months they 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.
These functional developments are associated with an 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.
Learning to sit has two aspects
1 The first aspect is the ability to maintain the head and trunk erect when supported in an upright position. Infants first learn to keep the head upright and steady when support is provided at the level of the axilla. Over time they progressively gain control over first the thoracic spine and then the lumbar spine.
This progressive development of extension of the spine can be seen when infants are supported in sitting at different levels.
In typically developing infants care givers adapt the support they provide the infant when lifting and carrying the infants, as well as when the infant sits on the caregivers lap.A small difference in the level of support can make a big difference to the infant's ability to sustain trunk extension.
2 The second aspect of learning to sit is learning to balance the trunk on the pelvis. Infants who are just learning to sit tend to move the trunk as a single unit balanced on a stable base formed by the buttocks and thighs. Balance is maintained as long as the center of mass remains fairly close to the center of the base of support. If the center of mass is displaced too far, the infant topples over with the head and trunk moving as a unit.
|
Saavedra, S. L., van Donkelaar, P., & Woollacott, M. H. (2012). Learning about gravity: segmental assessment of upright control as infants develop independent sitting. Journal of Neurophysiology, 108(8), 2215–2229. http://doi.org/10.1152/jn.01193.2011 |
A quick tour of the development of sitting
The neonate
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.
Sitting 4-6 weeks
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.
10-12 weeks
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 forwards.
The head is extended, there is some upper thoracic extension and and the scapulae are retracted (Bly Third Month)
4-5 months
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.
5-8 months: sitting without arm support
Sometime between the ages of 5-8 months infants learn to sit briefly without support. (AIMS sitting subscale: unsustained sitting without arm support.)
However balance is poor and the infant is not able to reach for toys or turn the 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.
Sitting without support (7-9 months)
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.
Active sitting 8-10 months
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)
