Human functionality occurs in the shape of an X across the body from one arm down through the core to the foot, and the same on the other side. A big criss-cross, all driven by the power and synergy generated from the core. In P90X2, it's almost as if the X now stands for that aspect of human kinesiology. That's what X2 Core is all about. The core is our body's foundation: the point where all movement begins. By working our cores using instability, we force our bodies to fire both their prime mover and stabilizer muscles in harmony. As you master this routine, all your movement patterns will improve. The cornerstone of P90X2: Master this core workout and see all your movement patterns improve.