What exercise will strengthen my middle deltoids?

An exercise you can use to strengthen your middle deltoids is the seated dumbbell lateral raise: 2-arm.  To perform this exercise, begin by sitting on a bench with your feet pointed straight ahead and placed flat on the ground.  Draw-in your belly button to tighten the abs and stabilize the core.  Hold a dumbbell in each hand, with your arms hanging at your sides.  Next, raise both your arms up and away from the sides of your body with your palms facing down until you reach shoulder level.  Be careful not to shrug your shoulders or arch your lower back. Hold the top position briefly and then return your arms to the sides of your body.  To increase muscular endurance perform 12-20 repetitions for 1-3 sets, and to increase strength perform 8-12 repetitions for 2-4 sets.