Is it normal for an Eight month infant female to not yet control her body and fall over if no support is given? She can sit for few seconds(10-20) but it seems she does not like sitting much and just falls over purposely. And hence cant leave her in the sitting position without supervision. She gets stiff and prefers to stand with full support rather then sit with the partial support.
Here's what I know for sure: every child progresses through physical development differently. Our daughter didn't walk until 15 months; some kids are walking unassisted at 9 months. Some kids skip cruising altogether and go from crawling to walking. There are many factors in development, and most of them are not "problems", just differences.
To help you feel less worried, I recommend you call/visit your pediatrician, and tell them your concern. They're used to these sorts of questions, and see hundreds of kids each year. It'll help.