I love having friends and family who have little ones who are growing and moving through their milestones. And I especially love when they say I can use their pictures and videos.
When I saw this video of this little one sitting up, I thought it was great. He is a fairly new sitter and you can see him working on exploring his movement. He shows rotation, and reaching outside his base of support, and coming back up to the middle, and propping for some extra support. As you can hear in the video, he found a new limit. He was able to reach a little further and return to the middle without falling down. But it was all the times of reaching and falling down that helped him to gain the skills to know his limits and to know what muscles he needed to turn on to keep himself from falling over.
As he feels more confident with his sitting he begins to release his degrees of freedom. So when he rotated to reach for something he was relaxing his trunk because he didn’t need to hold it rigid to stay upright. He is able to begin picking and choosing the muscles he needs to be successful, rather than turning them all on and having limited movement.
If you get a chance watch some little ones as they begin to master a new movement and you will see them slowly relax and be able to have fluid movement rather than tightening up all their muscles to try to maintain control.