Development of Sitting

Original Editor - Pam Versfeld Top Contributors - Robin Tacchetti, Tarina van der Stockt and Jess Bell

Review of the biomechanics of infant sitting[edit | edit source]

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.

In infant 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
  • 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.[1]

Learning to sit has two components:[edit | edit source]

  1. Acquiring the strength and control needed to extend the spine and hold the head and trunk upright and steady.[2]
  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.[1]

Sitting requires experience[edit | edit source]

In the early months of a baby's life, carers provide maximum support for keeping the trunk erect and stable when carrying infants and also when holding them sitting on their laps.[3] The postural mechanisms that allow the infant to stabilise their head in space and keep their trunk erect and steady are initially learned from this repeated experience of being carried upright and sitting supported.

Sitting facing outwards on a carer’s lap has an impact on postural set, which is influenced by arousal, motivation and curiosity.[4]. It allows the infant to:

  • visually explore the environment
  • attend to interesting things and events.

As the infant gets older, carers decrease the amount of support they provide and expect the infant to work harder to stay erect,[3] and to use their upper limbs to provide additional support by holding onto available surfaces.


Learning to extend the neck and spine[edit | edit source]

  • 0-6 months - being picked up, carried and supported in sitting provides the repeated 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.[5][6][2]
  • 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.[5]

Postural orientation and balance[edit | edit source]

“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."[7]

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.[7]

Anticipatory postural adjustments[edit | edit source]

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.

  • Lifting, moving and vigorously shaking and banging rattles provide infants with repeated opportunities to elicit APAs that stabilise the neck and trunk.
  • As infants gain more experience of sitting independently, an important postural strategy emerges
    • the shift from a bottom up to a top down balance strategy
    • 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.
    • 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.[8]
  • 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.[9][8]

Importance of visual curiosity and desire to reach for toy[edit | edit source]

  • 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.
    • they 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 colourful 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.[10]

The natural progression of sitting development[edit | edit source]

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. It starts with the infant first learning to hold their head erect and steady when supported around the upper chest in an upright position.[6][2][11][12]


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.[13]


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.[14]


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[edit | edit source]

  • 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.[15]
  • Traditional view - 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 view - postural sway may be used by the CNS as an exploratory mechanism to ensure that continuous dynamic inputs are provided by multiple sensory systems.[15]
  • 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. Increasing the complexity of movements of the COP.
  • Postural sway patterns are adapted when performing a concurrent goal directed task. 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.[16] [17]

Sitting development is influenced by child rearing practices[edit | edit source]

A study found that proficiency of sitting in 5-month-old infants varies considerably within and between cultural groups:[18]

  • 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 how varied opportunities for practice and parental expectations influence the development of infant sitting.[18]

Rachwani and colleagues[19] 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.”[19]
  • 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.
  • 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. 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.[19]

Resources[edit | edit source]

References[edit | edit source]

  1. 1.0 1.1 Kyvelidou A, Stuberg WA, Harbourne RT, Deffeyes JE, Blanke D, Stergiou N. Development of upper body coordination during sitting in typically developing infants. Pediatric research. 2009 May;65(5):553-8.
  2. 2.0 2.1 2.2 Pin TW, Butler PB, Cheung HM, Shum SL. Relationship between segmental trunk control and gross motor development in typically developing infants aged from 4 to 12 months: a pilot study. BMC pediatrics. 2019 Dec;19(1):1-9.
  3. 3.0 3.1 Duncan K, Goodworth A, Da Costa CS, Wininger M, Saavedra S. Parent handling of typical infants varies segmentally across development of postural control. Experimental brain research. 2018 Mar;236(3):645-54.
  4. Franchak JM. Changing opportunities for learning in everyday life: Infant body position over the first year. Infancy. 2019 Mar;24(2):187-209.
  5. 5.0 5.1 Saavedra SL, van Donkelaar P, Woollacott MH. Learning about gravity: segmental assessment of upright control as infants develop independent sitting. Journal of Neurophysiology. 2012 Oct 15;108(8):2215-29.
  6. 6.0 6.1 Sangkarit N, Siritaratiwat W, Bennett S, Tapanya W. Factors Associating with the Segmental Postural Control during Sitting in Moderate-to-Late Preterm Infants via Longitudinal Study. Children. 2021 Sep 26;8(10):851.
  7. 7.0 7.1 Horak FB. Postural orientation and equilibrium: what do we need to know about neural control of balance to prevent falls?. Age and ageing. 2006 Sep 1;35(suppl_2):ii7-11.
  8. 8.0 8.1 Maki BE, McIlroy WE. The role of limb movements in maintaining upright stance: the “change-in-support” strategy. Physical therapy. 1997 May 1;77(5):488-507.
  9. Kyvelidou A, Stuberg WA, Harbourne RT, Deffeyes JE, Blanke D, Stergiou N. Development of upper body coordination during sitting in typically developing infants. Pediatric research. 2009 May;65(5):553-8.
  10. Bertenthal B, Von Hofsten C. Eye, head and trunk control: The foundation for manual development. Neuroscience & Biobehavioral Reviews. 1998 Mar 4;22(4):515-20.
  11. van Iersel PA, la Bastide-van Gemert S, Wu YC, Hadders-Algra M. Alberta Infant Motor Scale: Cross-cultural analysis of gross motor development in Dutch and Canadian infants and introduction of Dutch norms. Early Human Development. 2020 Dec 1;151:105239.
  12. Gontijo AP, de Melo Mambrini JV, Mancini MC. Cross-country validity of the Alberta Infant Motor Scale using a Brazilian sample. Brazilian Journal of Physical Therapy. 2021 Jul 1;25(4):444-9.
  13. Mlincek MM, Roemer EJ, Kraemer C, Iverson JM. Posture Matters: Object Manipulation During the Transition to Arms-Free Sitting in Infants at Elevated vs. Typical Likelihood for Autism Spectrum Disorder. Physical & Occupational Therapy In Pediatrics. 2022 Jan 9:1-5.
  14. Harbourne RT, Lobo MA, Karst GM, Galloway JC. Sit happens: Does sitting development perturb reaching development, or vice versa?. Infant Behavior and Development. 2013 Jun 1;36(3):438-50.
  15. 15.0 15.1 Carpenter MG, Murnaghan CD, Inglis JT. Shifting the balance: evidence of an exploratory role for postural sway. Neuroscience. 2010 Nov 24;171(1):196-204.
  16. Claxton LJ, Strasser JM, Leung EJ, Ryu JH, O'Brien KM. Sitting infants alter the magnitude and structure of postural sway when performing a manual goal‐directed task. Developmental psychobiology. 2014 Sep;56(6):1416-22.
  17. Arnold AJ, Liddy JJ, Harris RC, Claxton LJ. Task‐specific adaptations of postural sway in sitting infants. Developmental Psychobiology. 2020 Jan;62(1):99-106.
  18. 18.0 18.1 Karasik LB, Tamis-LeMonda CS, Adolph KE, Bornstein MH. Places and postures: A cross-cultural comparison of sitting in 5-month-olds. Journal of Cross-Cultural Psychology. 2015 Sep;46(8):1023-38.
  19. 19.0 19.1 19.2 Rachwani J, Soska KC, Adolph KE. Behavioral flexibility in learning to sit. Developmental psychobiology. 2017 Dec;59(8):937-48.