Hack squats target the quadriceps muscles, or quads, on the front of your thigh more than barbell squats do. The quads are responsible for straightening your knee. Placing your feet toward the back of the foot plate will further emphasize the quadriceps contraction.
Hack squats also work the gluteus maximus, the largest muscle of your buttocks. The gluteus maximus extends, or straightens, the hip. Placing your feet closer to the front of the foot plate will place more emphasis on the gluteus maximus.