How much difference can three minutes of exercise a week make? it turns out quite a lot. How can that be? From this article, Can three minutes of exercise a week help make you fit?, comes this quote:
"part of the explanation is (probably) that HIT uses far more of our muscle tissue than classic aerobic exercise.
When you do HIT, you are using not just the leg muscles, but also the upper body including arms and shoulders, so that 80% of the body's muscle cells are activated, compared to 20-40% for walking or moderate intensity jogging or cycling.
Active exercise also seems to be needed to break down the body's stores of glucose, deposited in your muscles as a substance called glycogen. Smash up these glycogen stores and you create room for more glucose to be sucked out of the blood and stored."