When they sit, your baby uses information from what they see, along with their balance system and receptors in their muscles to maintain their posture.
Your baby will need lots of practice sitting to develop control of their posture and to find a stable position. People often think a stable sitting position is one with the body upright, but many adults and children actually lean slightly forward to maintain stability when sitting on flat ground.[1]
How your baby gets their balance
Sitting also involves controlling ‘postural sway’. Every time your baby moves their head or arms, and each time they take a breath, the forces acting on their body will change. To maintain their balance, your baby needs to move a little to compensate for these changing forces. At first, the movements your baby makes might be greater than needed to compensate for these changes. So, when they try to correct their posture, they move too far and lose their balance.[1]
With practice, your baby will learn to use the information from their senses to adjust their position just the right amount to maintain their posture without losing their balance.
Ways to help them practise
You can help them develop these skills by giving them time sitting to play with interesting toys or objects close to them. They will practise adjusting their posture as they look at and reach for the objects around them.
Reference:
[1] Rachwani, J., Soska, K.C. & Adolph, K.E. (2017). ‘Behavioral flexibility in learning to sit’. Developmental Psychobiology, 59, 937-948.