dumbbell lying one arm supinated triceps extension

Lie flat on a bench with your back and head supported, and your feet flat on the ground. Hold a dumbbell in one hand with an underhand grip, and extend your arm straight up above your shoulder. Keeping your upper arm stationary, slowly lower the dumbbell behind your head by bending your elbow. Pause for a moment at the bottom, then extend your arm back up to the starting position. Repeat for the desired number of repetitions, then switch arms.