I was about to link a related news article I recently read, but realised it was the same study (done by my alma mater). One thing it also found was that they had a better quality of life, and those training together ended up being fond of the time spent together with new friends.

The 4x4 protocol btw is a staple in Norwegian training, at all levels. 4x4 and "fartlek" and you are soon an elite athlete is almost the mantra.