If you're doing cardiovascular exercise for the purpose of burning fat... and you're doing the same "mode" of cardio for a long period of time... you're NOT getting the results you should out of your cardio!
Many people run, or bike, or take spinning classes, or a hundred other sources of heart-rate-increasing exercise for the purpose of fat loss. But for SOME reason... the effectiveness of this cardio starts to diminish... The workout that helped you lose 2lb per week, is now doing nothing!
Jeff Ward and Lisa Ward (Brightman)