I recently heard of this study done in Japan some time ago with Japanese speed skaters. They took a large group of elite speed skaters, and divided them into two groups with two different training regimes. One group would do their workout while holding a steady constant pace. The other group, would go for 8 seconds hard, and 10 seconds easy. It was found that the athletes with the 8 seconds hard, and 10 seconds easy, actually had a better aerobic base, and performed better in longer events. I guess this is what p90x does in their workouts from what I've heard. Any thoughts?