The barbell seated alternate leg raise is an exercise that targets the abdominal muscles, particularly the rectus abdominis or six-pack muscles. To perform this exercise, you will need a barbell and a bench. Start by sitting on the bench with your feet flat on the floor and the barbell across your lap. Hold the barbell with an overhand grip, with your hands shoulder-width apart. Slowly lift one leg up until it is parallel to the floor, then lower it back down. Repeat with the other leg. Continue alternating legs for the desired number of repetitions. This exercise is a great way to strengthen and tone your abdominal muscles.