I think the prescription of "Tummy Time" has come about due to babies now needing to sleep on backs due to SIDS and also due to some parenting regimes (there was one on that UK show where you interact very little with the baby) where babies were just in their cots, bassinets, floor etc on their backs all the time and held for minimal periods. Really I think it means just not on their back all the time (preventing head flattening as well as giving muscle strengthening) - and all the baby wearing, holding on chest, across knees for easing wind, along the forearm when really little are all "Tummy Time". Of course like most things the "Tummy Time" have been commercialized as well with lots of toys and gadgets promoting that they are for Tummy Time.