The primary concept of the program is to use one big compound lift per body part, limiting the session to three key exercises per session. The exercises must be able to be safely and efficiently loaded to a four-to-six rep maximum, while recruiting as many muscle groups as possible. Many barbell exercises meet this criteria. However, the case could also be made, for example, to use the trap bar deadlift (as seen in the sample workout above) or the leg press. Certain machine exercises, like a machine chest press, T-bar row, or Smith machine squat, likely cannot be safely performed with the necessary load. Use your judgment but, when in doubt, try to maintain the focus on barbell lifts. There are plenty of variations of basic movement patterns (squats, deadlifts, and presses) and you likely don't "need" to stray too far off-course.