I have had a similar experience.

Lately I have been doing interval swimming where I swim one lap freestyle as if my life depended on it, and then another lap doing the backstroke as fast as I can. I am going 100% the whole time.

I rest for a few minutes and as soon as I feel I've got my breath slightly back, I do it all over again.

Not sure what it is about getting the hard pumping that hard for short intervals, but I've lost weight in the process.

The thing about it is there is no way to become content. You see a lot of cardio people churning away at the same rate on the treadmill as they were doing last week.

But with my interval swimming, I am going 100%. You are always pushing full throttle; there's no allowance for contentment.

http://www.quitrunning.com/interval-training-swimmers.htm http://www.marksdailyapple.com/sprint-training/

read the whole article. They tried to get some of the women to do high intensity exercise, but they couldn't stick to the protocol.

> Jakicic and his colleagues originally designed their study to measure whether weight loss could really be achieved and maintained through moderate-intensity exercise... or whether it was preferable to engage in shorter bursts of more vigorous-intensity activity... The problem was that not enough of the women stuck with their assigned exercise categories for the researchers to gather enough meaningful data. Within a few months, most of the participants had resorted to exercising as much as they chose to. That left researchers with a slightly different data set than they had planned for

(FWIW I'm all about HIIT, but I don't know how you can get people into it once it becomes inconceivable to do high-intensity work.)