Learning to activate your glutes is important so that you can strengthen them. Strong glute muscles are extremely important as these muscles can have a major impact on your overall body strength; your glutes support your core, help to support a range of exercises and compound movements, as well as help avoid muscle imbalances which can lead to decreased mobility.
This phase continues to employ a four-day split, but bodyparts are paired differently—namely, chest and back are trained on the same day (Day 1), as are biceps and triceps (Day 4). This is little more than a means of changing things up, giving your muscles a slightly different stimulus to spark new muscle growth. Each workout includes drop sets to increase intensity, but for only one set per bodypart, so as to avoid overtraining and muscle catabolism.
Another common reason I see glutes that aren’t working properly is due to injury. Often an injury happens that changes the mechanics and motor programming of a person’s body. This can lead to some muscle groups becoming overactive, while others become underactive (think: compensation). This can alter things for a long time without the person even knowing it.
Prolonged sitting and activities like running or cycling can lead to tight hip flexor muscles and a variety of skeletal imbalances. Think: if you only cycle for exercise, certain muscles in your legs will get stronger (in a lot of cases you overwork these muscles) yet your core and outer hip muscles might get weaker from lack of engagement. So what? Well, these muscle imbalances often lead to skeletal imbalances and injuries down the line. If you have particularly tight hip flexors, your body will start to create an anterior pull on the pelvis (anterior pelvic tilt). You can identify an anterior pelvic tilt if your belly protrudes slightly in the front while your butt sticks out in the back (what some people refer to as “duck butt”).
Note that this recommendation is for total weekly volume, which means it would need to be divided up based on how many times you’re training each muscle group per week. So, for example, someone training everything twice per week would do 30-70 reps for each bigger muscle group in each of those workouts, and 15-35 reps for each smaller muscle group in each of those workouts.