From the first weeks infants start figuring out how to effectively use all available support surfaces to support their intentional and goal directed actions.
In this video you see Will (4 months 2 weeks) sitting with support on my lap. As is common with infants this age his interest in exploring the toy he has in his hand is the focus of his attention.
His ability to hold his head steady when he is upright is well developed, as long as he is provided with adequate support for his trunk. But to achieve this goal he needs to find ways to support his trunk.
In this episode of lap sitting, I start off by supporting Will around the front of his chest, and he happily leans against my hand which provides him with the stability he needs to lift his hands to reach for a proffered toy.
When I move my hands down to his pelvis and legs, he leans forwards and props himself on his elbows as he bring the toy to his mouth.
To reach to the right, he leans against my arm.
You will also see Will keeping his head in line with his trunk as he tipped sideways when I shift my position on the floor.
And when I pick him up to move him to the floor, he keeps his attention on rattle in his hand and he is able to keep his head erect as I tip him back. I also trunt him to support his head during this maneuver and offer support for his upper trunk but not for his head.