I thought it had been proven in a pervious post on this same topic, that the Septim Emperors were not Dragonborn. They were normal humans.
The Septim Dynasty were in fact Dragonborn. Only those of the Dragonblood can wear the Amulet of Kings and can light the Dragonfires in the Temple of the one.
The reason the Champion of Cyrodiil can't wear the Amulet of Kings is because he doesn't have the Dragonblood in his veins like the Dragonborn do.
also, the Akaviri Dragonguard IE: The Blades confirmed the Septims as Dragonborn....
Not just anyone could smash the Amulet of Kings and become an Avatar for Akatosh...no one must have the Blood of Akatosh running through their veins...they must be Dragonborn....Martin Septim proved he was Dragonborn when he became Akatosh's Avatar to allow Akatosh to manifest himself in Tameriel to do battle with Mehrunes Dagon and cast him back into Oblivion.
Martin Septim poessed the same ability to devour Dragon souls and learn shouts that The Dovahkiin in skyrim has....the difference being there were no dragons in Tameriel at the time of Martin Septim, and well Martin never got the chance to see the Greybeards......if he did he would have been able to use the Th'um just like any other Dragonborn.
The fact that Martin could wear the Amulet of Kings(just like all the Septims before him) proves beyond any doubt that the septims...only a Dragonborn can wear the Amulet of Kings.....So yes the Septims were indeed Dragonborn.......