My girls aren't quite 6 months, but I was curious when your LO sat up on their own? My girls sit assisted and like it; if we let go, they sink down over their legs (face plant).
One girl does stomach crunches all day long. Only getting her head/neck off the floor....sometimes her shoulders a little. I'm not sure if there is something I should be doing to help her gain strength or if I should just let things happen when they do.
I'm honestly not that worried at this point especially as I know many babies don't sit up still at 6 months, but if there is something I should be doing to help, I want to be doing it!
By the way, my girls were born at almost 38 weeks so they weren't that early, but maybe there is still a chance some milestones would be slightly delayed.
Re: Trying to sit up....
Mine is just now starting to sit for longer periods unassisted but I have to put her in a sitting position. I can't remember when my son was able to get himself to a seated position, but it was a LONG time after being able to stay sitting. He really liked crawling and tummy time and had no interest in sitting still. If I put him in a seated position he would usually get down on his tummy and crawl away.
When babies do learn to get into a seated position they do it from their tummies first initially (like a push up and then scoot back to seated). The ability to sit up with a crunch doesn't come until at least 2 years old usually. My son does a developmental assessment every year and at the 2 year old assessment they had him lay down on his back and asked him to sit up. He rolled over and sat up, they said that was normal. He may physically be able to do it but he doesn't.
A.J. sat unassisted a week or two after his 6 month birthday. But he is nosy and headstrong. he doesn't get in a sitting postion by himself we have to do it. he does the army crawl over the house and lifts his belly off the ground like he is making a bridge, A.J. was full term.
Right before 6 months he could sit unassisted, if I put him there. At first for a couple minutes, and then right around 6 months he began sitting unassisted for as long as he wanted to.
He didn't start sitting himself up until around 8 months. At that time he'd roll to his side and then push himself to seated from there.