Sit upright on a bench, with your feet firmly on the ground and holding one dumbbell vertically between both hands. Bring the dumbbell directly behind your head, keeping your elbows pointed forwards and close together. This is the start position. Keeping your elbows pointed forwards and upper arms still, extend your forearms up overhead until your arms have reached full extension. Squeeze your triceps as you do this and exhale. Reverse the movement with control to return to the start position. Repeat for the required number of reps (or time).