Have any of you ever had a child who 'walks' on their knees?

I have a child who is 16 months old. He was late to crawl and, for a while, didn't like to be on his feet at all. He then started crawling and now pulls himself to stand and cruises round the furniture. But he makes no effort to walk other than that. If you try giving him a push along walker, or you try holding his hands to walk, he sits down.

Last week he started knee walking. He moves really quickly, but on his knees. He's sitting up as he does it, so not sat on his legs, and zooms round on his knees. I don't remember ever having seen a child do it before.