Lower back, glutes, shoulders, lower back, upper back
Proper Exercise Technique
Pick up a barbell off the ground using power from your legs (not your back). Space your feet slightly narrower than shoulder-width apart and hold the bar with an overhand grip just outside of your legs. Keeping a slight bend in your knees, bend forward at the waist and lower the weight toward the ground. As you lower the weight your lower back should remain arched as this will keep the tension on your hamstrings throughout the entire exercise. Lower the weight down until you feel a full stretch in your hamstrings. The distance that you are able to lower the bar will vary from person to person and will depend on body structure and flexibility. Once you feel a full stretch in your hamstrings, pull the weight back up into the standing position but stop just short of being fully erect. By not allowing your body to stand up completely straight you will keep the stress on your hamstrings at all times. Continue the motion until you have reached muscular failure.
Tip: It is extremely important that you do not round out your lower back at any point during this exercise but instead maintain a slight arch at all times. In order to keep your back flat, really focus on sticking your butt out and keeping the bar close to your body. You must also remember to keep a slight bend in your knees as you perform your reps as this will keep the tension on the hamstrings and will minimize your risk of injury.