The Vikings were a fit aggressive race of people who were in the habit of raping, pillaging, plundering and conquering the coastal and river areas of Europe. They were in the habit of raping local women, and presumably their genes were spread throughout the local populations. Far from being a disadvantage for the local population, the genetic pool of the local population was presumably greatly enhanced by the invaders.

The Vikings were fit enough and strong enough to row their narrow boats thousands of miles, and still have the energy to conquer and rape the local population. They were presumably much fitter than the local population, and the injection of genes into the local gene pool made the locals stronger accordingly, though possibly also more aggressive.

Funny thing is, the Vikings were the original Boat People. And the modern Norwegians have never apologized for the actions of their ancestors, and in fact, don't even appear to be embarrassed about it.