Development of Sitting

Original Editor - Pam Versfeld
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Brief review of the biomechanics of sitting

Maintaining an upright sitting posture requires the ability to align the body segments so that the vertical projection of the centre of mass (COM), known as the line of gravity (LOG) falls within the base of support.

In infants, the COM when sitting lies within the chest.

When infants sit erect on a flat surface, their trunk is tilted forwards slightly so that the LOG runs just anterior to the flexion/extension axis of the hips.

This creates a flexion moment around the hips which is counteracted by the hip extensor muscles. Because the base of support (BOS) extends forwards over the lower limbs, the trunk can be tilted far forwards without the infant losing their balance.

However, in erect sitting, the LOG falls close to the posterior border of the BOS. This tilts the trunk backwards, moves the COP beyond the posterior edge of the BOS, which makes it difficult to maintain balance.

Infants learn over time to flex their neck and trunk to stop the LOG from moving past the posterior border of the BOS.

In sitting, the base of support in the side-to-side direction is relatively narrow. Any action that shifts weight onto one buttock moves the LOG to the edge of the BOS.

This is an unstable position, but stability can be restored by: realigning the position of the shoulder girdle relative to the pelvis; or by adapting a change-in-support strategy to increase the BOS in a sideways direction.

Learning to sit has two components:

1. Acquiring the strength and control needed to extend the spine and hold the head and trunk upright and steady.
2. Maintaining balance by adapting the alignment of the head and trunk relative to the BOS, and responding effectively to forces that perturb the head and trunk.

Sitting does not just happen – it requires experience

The postural mechanisms that allow the infant to stabilise their head in space and keep their trunk erect and steady are initially learned from repeated experience of being carried upright and sitting with support over many months.

In the early months, carers provide maximum support for keeping the trunk erect and stable when carrying infants and also when holding them sitting on their laps.

Sitting facing outwards on a carer’s lap allows the infant to visually explore the environment and attend to interesting things and events. This has an impact on postural set, which is influenced by arousal, motivation and curiosity.

Over time, carers decrease the amount of support they provide and expect the infant to work harder to stay erect. Infants also start to use their upper limbs to provide additional support by holding onto available surfaces.

Learning to extend the neck and spine

Over the first six months, repeated experience of being picked up, carried and supported in sitting provides the experience needed for strengthening the spinal muscles and learning to hold the head erect and extend the spine against gravity.

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 lumbar spine extension.

This progressive development of extension of the spine can be seen when infants are supported in sitting at different levels: the axilla, thoracic spine and pelvis.

In typically developing infants, carers adapt the support they provide when lifting and carrying the infant, as well as when the infant sits on the carer’s lap.

As seen in these pictures, a small difference in the level of support can make a big difference to the infant's ability to sustain head and trunk extension.

Postural orientation and balance

“Postural control is no longer considered simply a summation of static reflexes but, rather, a complex skill based on the interaction of dynamic sensorimotor processes [...] Sensory information from somatosensory, vestibular and visual systems is integrated, and the relative weights placed on each of these inputs depend on the goals of the movement task and the environmental context."

Balance, also known as postural equilibrium, involves the coordination of movement strategies to stabilise the centre of body mass during both self-initiated and externally triggered disturbances of stability.

The specific response strategy selected depends not only on the characteristics of the external postural displacement, but also on the individual's expectations, goals and prior experience.

Firstly anticipatory postural adjustments

Anticipatory postural adjustments (APA) occur prior to voluntary limb movement, and serve to maintain postural stability by initiating muscle contraction to compensate for destabilising forces associated with moving a limb.

An infant's experience with lifting, moving and vigorously shaking and banging rattles provide them with repeated opportunities to elicit APAs that stabilise the neck and trunk.

Another important postural strategy that emerges as infants gain more experience of sitting independently is the shift from a bottom up to a top down balance strategy whereby the infant learns to stabilise their head in space and adjust the alignment of their trunk segments to stay upright.

To do this, the alignment of the upper and lower thoracic segments and the pelvis are adjusted to keep the head upright and stable.

However, infants will still topple over sideways with the head and trunk moving as a unit when the lateral displacement of their COM is too far or too rapid.

With experience of independent sitting, infants also learn to use change-in-support strategies, involving grasping or stepping movements of the limbs to increase the BOS.

When learning to balance in sitting, the most common change–in-support strategy used is reaching out to the side and taking weight on the hand when losing balance to the side. This action increases the BOS in a lateral direction, stops the falling COM and prevents a fall to the side.

Importance of visual curiosity and desire to reach for toy

In very young infants, lifting the head and keeping it erect requires sustained effort, especially for an infant who has weakness of the neck and trunk extensors.

However, infants are usually willing to put in the effort needed into holding the head erect if there is something really interesting to look at such as a colorful object or an animated face that invites social interaction.

From about four months, typically developing infants take a great interest in reaching for, holding and manipulating toys. These actions require anticipatory and reactive muscle activity to stabilise the head and trunk and are important in providing infants with the experience needed for learning to sit independently with good balance.

The natural progression of sitting development

The development of sitting involves increasing control over the neck and trunk segments that are involved in sitting erect and maintaining balance in the head, upper torso, lower torso and pelvis.

The development of sitting starts with the infant first learning to hold their head erect and steady when supported around the upper chest in an upright position.

Over time and with practice, infants learn to stabilise their head and trunk when supported around the waist, and later with support around the pelvis.

At this stage, typically developing infants are often more interested in reaching for and grabbing an interesting toy than maintaining balance.

This increasing level of control of the trunk on the pelvis is promoted by frequent opportunities to sit with support during every-day routines and handling.

Once the infant can stabilise their trunk on their pelvis, and sit briefly without arm or external support, typically developing infants start to pay attention to maintaining their balance as they reach out to connect with people and objects in their environment.

With experience, infants learn to reach in all direction, pick up and move objects, and transition into and out of prone kneeling.

The role of postural sway

Humans and other species are unable to sit or stand perfectly still; their bodies continuously sway. Postural sway (or variability) inevitably emerges from the control of a system with many degrees of freedom.

Postural sway has been traditionally viewed as a non-functional byproduct of a noisy motor system, leading to the concept that minimal sway was indicative of a healthy and stable postural system.

Contrary to these traditional views, Mark Carpenter and colleagues posit that postural sway may be used by the CNS as an exploratory mechanism to ensure that continuous dynamic inputs are provided by multiple sensory systems.

Postural sway provides a continuous flow of sensory information from different sources (touch, proprioception, vestibular and visual) needed for estimating the position of the body as a whole in an ongoing manner.

Besides providing sensory feedback, these ongoing trunk movements in lateral and anterior-posterior directions allow the infant to explore the boundaries of the BOS and how far the COM can be moved without toppling over.

The extent of postural sway both in the anterior-posterior and side-to-side direction decreases with sitting experience. However, the complexity of movements of the COP increases.

Postural sway patterns are adapted when performing a concurrent goal directed task.

Laura Claxton and colleagues studied the postural sway patterns of independently sitting infants while either holding or not holding a toy. Infants exhibited less postural sway when holding the toy. This reduction in sway allowed infants to look at and stabilise the toy in their hand.

Sitting development is influenced by child rearing practices

Karasik and colleagues observed 5-month-olds and their mothers from six cultural groups around the world during one hour at home while they engaged in natural daily activities. They examined group differences in infants’ sitting proficiency, everyday opportunities to practise sitting, the surfaces on which sitting took place, and mothers’ proximity to sitting infants. Infants had opportunities to practise sitting in varied contexts—including on the ground, in infant chairs, and on raised surfaces.

Proficiency of sitting in these 5-month-old infants varied considerably within and between cultural groups:

• 64% of the sample sat only with support from mother or furniture
• 36% sat independently.

Some infants sat unsupported for 20+ minutes, in some cases so securely that mothers moved beyond arms’ reach of their infants even while infants sat on raised surfaces.

These observations of infant sitting across cultures illustrate the how varied opportunities for practice and parental expectations influence the development of infant sitting.

Rachwani and colleagues describe the experience of pre-sitting infants as follows:

“Put another way, when pre-sitting infants practice sitting over and over on the floor, the couch, their caregiver’s knee, and so on, all the while engaged in various tasks of looking and interacting with objects and people, these experiences across a variety of surfaces and tasks, ensure the acquisition of behavioral flexibility.”

Experience with varied body-environment relations may teach infants to find appropriate solutions for the current situation while also shoring up constituents common to all sitting postures.

For example, sitting on the floor and on the couch involve different demands on balance control. Shaking a rattle and turning to watch an event involve different constraints on balance. However, all of these situations strengthen an infant’s torso and increase coordination to counteract destabilising forces, thus allowing the infant to adapt posture for a novel situation such as sitting on slopes.

Resources

• Development Skills Training for the 1-2 month old

• Family Centred Intervention and Early Diagnosis

• Newborn Perceptual Motor Behaviour