I can't see how . . . 5x5 means 3 excercises for each body part. HST looks only at the contractile load upon the muscle (say pectoralis) as a stimulus to encourage growth, which is a biochemical process. So, clearly you only need one pectoralis stimulus. To avoid too much isolation work, and increase efficiency, they use compound movements (bench, squat, dip) as much as possible. 5x5 is basically just a catchy name for arnold's old split routines. No different. A mass exercise, and some "shapers" (although it's unclear scientifically how you can "shape" a muscle that either contracts or doesn't). HST is compound movements 3x week with unique periodization. They can't mix. I can't see how . . . I guess you could do the rep periodization, but that wouldn't make it HST, and I would guess that the specific rep periods (the exact number) would be considered by HST folks to be the least essential part of the system. In fact, I think I heard Brian say it could just as easily be 12,8,4 reps and be HST; that was arbitrary.