The routine 15 sets of 4 reps (15 x 4) is done when you’ve experimented with routines like 3 x 8, 6 x 6, 8 x 8, 4 x 12 – it is used for “muscle confusion.”
It’s a routine that’s “down the road,” and not a beginner’s routine. This is a full-body program where every body part is worked on one day, and, as the name indicates, you pick an exercise and do 15 sets of just 4 reps.
Doing this routine you don’t just change the exercise, you change the sets and reps significantly, too. That’s why it works. 8 x 8 is a great routine, but the body will adapt to it eventually, so you need routines like 15 x 4 from time to time to confuse the muscle to stimulate growth.
Never allow the muscle to become accustomed to anything – sets, reps, or exercises. If you do not use muscle confusion, the muscle will no longer respond and you will no longer have growth.
For 15 x 4, the same rest rules and intensity rules apply as for the other routines. The goal is to work toward 15-second rests between sets, and if you really want to increase the intensity, try 10 seconds between sets. That’s what I do, and that’s really intense.
One thing I would like to make sure everyone understands is that you don’t expect to do this overnight. You don’t expect a 70-year-old man to go and shovel a whole driveway full of snow if he hasn’t done it in 20 years. You can give him a heart attack. You must adapt to this type of exercise, and that’s why Vince called this “progressive resistance.”