The number of sets, repetitions, and speed of the repetitions are all by design. Here is a breakdown of each of the 4 sets:
Set #1/20 Reps: The common presumption with a set of 20 repetitions is that it’s just a warm-up set. This is not completely untrue, but there’s more to it than merely warming up the muscle. One of the biggest reasons that individuals have a hard time building muscle is they haven’t figured out mind/muscle connection. In any given movement, or exercise, our brain and body naturally want to recruit as many muscle groups as possible to perform the given task. This is natural, but with resistance training, the trick is learning how to target a particular muscle as much as possible. To do this goes against our instinct so it requires focus to learn to target a specific muscle. Most people have difficulty learning this because they are told that you always have to pick up the heaviest resistance you can find if you want to build muscle. The irony is that the heavier you go, the more likely you are to recruit other muscles. The point of doing the first set with 20 reps is going with a resistance level that you can do with strict form so that you can focus on good quality contractions each and every rep. This is why I call Set #1 an “Activation Set”, where you are forcing your body to activate all of the motor units in the muscle, for a peak muscle contraction, as opposed to recruiting assistor muscles, to complete the movement. If you’ve selected a resistance that’s still challenging (don’t sandbag it) you will really start to feel that muscle burn after 12-14 reps. Embrace that burn and etch that feeling into your mind. Learning to really feel a muscle is the first critical step in learning Mind/Muscle connection. Now besides warming up the muscle and developing that connection with the muscle, you have the additional benefit of pre-fatiguing the muscle, priming it for your remaining 3 sets.
Sets 2 & 3: In both Set #2 and Set #3 the focus is on controlled rep speed, both in the concentric, isometric and eccentric contractions. In these two sets you will go with a heavier resistance than you used in Set #1, but make sure that you can still get the same quality contractions that you did in your first set. It’s best to aim for a resistance that feels really difficult at 5-6 reps and then the last 4-5 are extra challenging. If you got to 10-reps and feel like you could do a few more then the weight is too light.
Set 4: Just like the first set, your final set serves multiple purposes. The primary focus it to build power. One of the advantages of bands, over free-weights, is that it makes it easy to train more explosively, which activates those fast-twitch muscle fibers. These are the type of muscle fibers that make you run faster, jump higher, and lift heavier. When training with free-weights there’s a significant downside to training with faster rep speeds. Once you get that weight moving fast, momentum carries the weight and robs you of resistance. When using bands you are not able to create momentum, so no matter how explosively you do the movement you get maximum resistance all the way through the range of movement. In Set #4 we take advantage of this opportunity by doing 15-reps explosively. Use the same resistance that you did in your first set, but now we are going to do them quickly. The one adjustment we want to make is to shorten our Range of Motion (ROM) slightly, to make sure that we have constant tension throughout the movement. In other words, we don’t ever want the band going slack. Even though we are doing 15 reps, our total Time Under Tension (TUT) is shorter.