[That Foreign Guy]

What shemp said pretty much, except I like the elliptical for HIIT. Like the rowing machine, it can get the whole body involved, and I found that pushing with the arms forced my legs to rotate faster than they would otherwise.

Whatever method you use though (eliptical, rowing machine, squats, sprints, whole body masturbation, whatever) and whatever work intervals you choose (20s on 10 off, 30 /30, even up to 2 mins / 1 mins) the key to High Intensity Interval Training is to make the work intervals MAX effort. Get that right and the rest is mere details.

Thing is, the body hates max efforts because it means we're using up precious survival resources. It would rather make you think you're doing max effort by sending tired signals. Your job is to ignore these and force a real effort.

Anyone who's been in a tight sporting contest knows what I mean, when you're just dead on your feet but then suddenly find a bunch more energy because you need it. That's what you're looking for with HIIT (it's also one of the reasons I mostly do fast paced sports instead of classic intervals, because I find it easier to find that extra gear when there's winning and losing involved).