Rear Delt Flies are an excellent exercise for targeting the posterior deltoids and upper back muscles. To perform this exercise, stand with your feet shoulder-width apart and hold a dumbbell in each hand. Bend your knees slightly and hinge forward at the hips, keeping your back straight and your core engaged. Let your arms hang down in front of you with a slight bend in the elbows. Exhale as you lift the weights out to the sides, squeezing your shoulder blades together at the top of the movement. Inhale as you slowly lower the weights back to the starting position. Focus on controlled movements and avoid using momentum to lift the weights.