The Bulgarian split squat is a subclass of the split squat exercise. They share the same joint movements and exercise technique, but the Bulgarian version is a more challenging movement. The body position during the Bulgarian version shifts your weight forward, increasing the balance challenge and the range of motion.
The Bulgarian and standard split squat work the same muscles. The front leg is the working leg; the back leg is for balance and should not have much weight on it -- the majority of your weight should be shifted to the front leg. The quadriceps of the front leg are the primary movers, with the glute and inner thigh muscles assisting. The hamstrings and calf muscles act as stabilizers.
Although your body position is different, the exercise technique is the same for both versions of the split squat. For the standard version, both feet are on the floor. For the Bulgarian version, elevate your rear foot on a step or bench. With most of your weight over your front foot, bend both knees and drop into a lunge. Keep your front knee over your ankle; do not allow it to push past your toes. When your front thigh is parallel to the floor, reverse the movement and push through your front heel to straighten your front knee.
Because your rear foot is elevated, the Bulgarian split squat increases the exercise range of motion -- you can lower past the point where your front thigh is parallel to the floor. The Bulgarian split squat also naturally shifts more of your weight forward to the front leg. These factors increase the difficulty of the Bulgarian squat, making it a more challenging movement than a regular split squat.
Although an increased range of motion can increase the effectiveness and intensity of an exercise, it can also increase your risk of injury. Start with body weight only and limit your range of movement to the point where your thigh is parallel to the floor. Once you become more adept at the exercise, gradually increase the weight and range of motion. The elevation of your back leg creates a strong stretch in the groin and hip flexors of that leg. If the stretch is too intense, lower the height of the bench or step.