Have there been studies done re specific fitness losses?

I have seen it stated on here (and also experienced somewhat myself thru personal experience) that run fitness is more easily lost and more difficult to restore than bike or swim. I have noticed that I can swim one good session per week and maintain a good conditioning for swimming... and I rarely am able to get a second bike session in during the week in general and yet my bike fitness holds steady too. But if I drop below three runs a week I immediately notice a difference. Why is this? Is it only because of the higher impact and intensity of running? or is it more complicated than that?

Edited by Caroleena 2014-09-30 9:04 PM