Dumbbell one arm triceps extension (on bench)

Sit on a bench with your back straight and feet flat on the ground. Hold a dumbbell in one hand and place your other hand on the bench for support. Raise the dumbbell overhead, keeping your upper arm close to your head and your elbow pointing forward. Lower the dumbbell behind your head by bending your elbow, keeping your upper arm stationary. Extend your arm back up to the starting position, fully straightening your elbow. Repeat for the desired number of repetitions, then switch arms.