I've read a few things about interval training, the most recently in Body for Life. Seems to me, as a layperson, that if you cannot sustain your "9" for 90 seconds, it's not really a "9" but higher. Why don't you bring it down a bit. Or do one minute intervals.
In Body for life, his recommendation goes like this, with changes every minute (numbers are for perceived exertion based on 10 point scale): 5,5,6,7,8,9,6,7,8,9,6,7,8,9,6,7,8,9,10,5. I did this a few times on a machine (walking is my preferred exercise), and felt challanged and worked-out after just the 20 minutes. This method is also what my trainer recommended to me (and he didn't read the book).
Hope this helps.
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