dumbbell lying alternate extension

Lie flat on a bench with a dumbbell in each hand, palms facing each other. Extend your arms straight up over your chest, keeping a slight bend in your elbows. Lower one dumbbell down towards your head, bending at the elbow, while keeping the other arm extended. Pause for a moment at the bottom, then raise the dumbbell back up to the starting position. Repeat with the other arm, alternating sides for the desired number of repetitions.