Chances are, when you hit the gym to train weights, you're typically working in a rep range of 8-12, depending on your goals. If strength is what you're after, you're pushing the bar for singles, doubles, and possibly up to 5 reps per set. But when was the last time you did a set of 30? Maybe never?
This workout is designed to raise your heart rate and boost your metabolism, so you torch calories faster and for hours to come. It's also going to train your muscles to keep working despite fatigue so that when you do perform those heavier sets, you'll know how to push through the rough reps and smash your PRs.
To make this workout harder, try to get more reps in for the allotted time period of each exercise. You ideally want to hit 15-20 reps, or higher as you progress, doing about 20-30 reps per minute. When you feel you can't do any more reps in that time, bump up the intervals from 60 to 90 seconds. There is always a way to advance this routine.