My husband is basically 100% muscle (I'm not complaining hehe) and our kids are quite strong and fit. He does special muscle stretches when they are babies.
However I was gobsmacked when my 4th child stood up about 3 hours after being born! Obviously I held his arms as he couldt balance, but he was standign and putting all his weight on his feet. Which explains the terrible stomach ache I had carrying him lol.
He does resemble his father physically and is all muscle. I have never heard of a baby standing up the first day they were born - has anyone else had this?