Improvement of _VO2 max; by cardiac output and oxygen
extraction adaptation during intermittent versus continuous
You really need to get the whole article but the researchers compared interval training with the same period of continuous training, so if someone did 20 mins of continuous training they had someone else do 4 sets of 1 hard minute and followed by 4 easier ones. They were looking at how the training affected the various systems - either making the heart itself more efficient, or making the muscles more efficient at getting oxygen out of the blood.
They found that interval training improved both the heart and the muscles' efficiency. Anyway, an interesting thing that the researchers do not really make much of is that the interval training improved vo2 max 300% more than the continuous training! (a 34% increase for the intervals compared with an 11% for the continuous training). That is a lot more efficient for the same length of time.