A bit of excercise

I read somewhere that now babies start to crawl and walk much later than a few years ago. They say the reason for this delay is that now babies do not lay on their tummies (because of the risk of cot death) and thus do not exercise certain muscles and certain movements. In this article it was said that, if you want, you can turn the baby on her tummy while you are watching her so she can exercise this muscles. It strengthens, for example, their neck muscles helping them keep their head earlier. I turn ours every now and then, maybe once a day, on his tummy for a few minutes (or less if he complaints).