dumbbell lying one arm deltoid rear

Lie face down on a flat bench with a dumbbell in one hand, palm facing inwards. Extend your arm straight down towards the floor, keeping it close to your body. Raise your arm up and back, squeezing your shoulder blade towards your spine. Pause for a moment at the top, then slowly lower your arm back down to the starting position. Repeat for the desired number of repetitions, then switch arms.