Quadriceps
Calves, Glutes, Hamstrings, Lower Back
Place box/bench behind you in squat rack.
Set bar in rack below shoulder height. Place bar on upper back.
Lift off rack, step back, position feet shoulder-width in front of box.
Chest up, back straight. Initiate squat by pushing hips back.
Lower until glutes gently touch box. Keep shins relatively vertical.
Pause briefly on box (relax hip flexors, don't bounce).
Drive through heels, chest leading up, return to start.
Repeat.