Phase 2 The Two-Day Split (Weeks 4-6)
After three weeks of whole-body training, your muscles need a new challenge. For the next three weeks, follow a two-day training split repeated twice a week, for a total of four weekly workouts. A two-day training split means splitting up your entire body into two separate workouts, training half in one workout and the other half in a second. In this particular two-day split, train all torso muscle groups (chest, back, shoulders and abs) in Workout 1 and all limb muscles (biceps, triceps, legs and calves) in Workout 2.
A two-day training split allows for more exercises per muscle group. It also allows for training each muscle group with greater intensity. Upping the volume and intensity both produce progress. Training fewer muscle groups means more effort applied to each one in turn. This allows for going heavier and making sure each set goes to muscle failure.
Weeks 4-6 Workouts
Do each of the following workouts twice per week. For example, do workout 1 on Monday and then again on Thursday and do workout 2 on Tuesday and then again on Friday.
Phase 2 Rundown
The exercises you'll be using will be the major mass builders you started in Phase 1, along with an added exercise or two for that same muscle group. For most muscle groups, this will allow you to do one multi-joint mass builder and one single-joint or isolation exercise to build both overall size and shape. You'll also be able to work a new muscle group in this phase, the trapezius, the kite-shaped muscle at the base of your neck.
You'll still doing three sets per exercise. However, since you're now doing two or three exercises per muscle group, total sets jump from 3 per muscle group to 6 (or 9, for legs). This increase in the amount of work done for each muscle group is important for continued progress.
In this phase you'll drop reps down to 8-10 on the first exercise for each muscle group. This allows you to train a bit heavier than in the previous phase. Result: more strength, more muscle mass. On the second (and third) exercise, keep the reps higher at 10-12. Since these exercises will likely be new to you, the higher reps will help you to perfect them. For calves, increase the reps to 15-20.
You'll be going heavier this phase, at least on the first exercise for each muscle group. Find a weight that limits you to 8-10 reps on the first exercise; and then a weight that allows you to complete 10-12 reps on the second and third exercises. Again, once you can do more than the rep range listed for each exercise, add 5-10 pounds or whatever weight brings your reps into the listed rep range.
During this phase, rest 2-3 minutes between sets. That way you can stick with heavier weight and compete more reps for maximizing strength and size gains.