At six months our son was just starting to lift his head. At sixteen months his arms were still too week to support crawling, despite being in occupational therapy (OT) for over a year and physical therapy (PT) for three months. We did tummy time. We tried every trick in the book recommended to us. With the help of OT and PT he’s not falling further behind, but he’s certainly not catching up either.
He struggles with what’s called Hypotonia, or low muscle tone. He’s much more flexible than you would expect a child to be. He can put his feet up by his ears without even straining and he’s well over a year old. This flexibility causes him to need additional strength to keep his muscles in place, and he can’t make progress on physical milestones by himself because he lacks the strength to lift the weight of his body and head without significant assistance. Thankfully, with help he does get stronger. However, if you left him to his own devices he doesn’t work on his physical milestones like you would expect. In fact, he doesn’t work on them at all.