I have read about the importance of exercises in small bouts, especially when it comes to strength training. Exercising 30 seconds and then resting for 60-90 seconds. In a lot of the same articles it says never to exercise for more than 20 minutes at a time. What I'm confused about is does it mean 20 minutes all together as in 20 minutes of 30 sec, 60-90sec for 20 minutes? OR does it mean a cumuilative 30 secs of 20 minutes and the rest of 60-90sec just makes it longer?