Holding a dumbbell, sit on a flat bench or chair and rest the dumbbell on its side on one knee.
Grasp the base of the bar of the dumbbell with both hands, one hand over the other.
Kick the dumbbell up with your knee and rest the other end of the dumbbell on your shoulder.
Adjust your grip so that your hands make a heart shape under the plate.
Lift the dumbbell around your shoulder and hold it behind your back, with your upper arms vertical and elbows flexed.
Execution
Keeping your elbows close to your head and your upper arms vertical, exhale as you raise the dumbbell by extending your elbows.
Inhale as you lower the dumbbell to the starting position by flexing your elbows.
Repeat.
Comments and tips
When you have finished, bring the dumbbell around your back and rest the lower end on your shoulder. Grab the bar securely with both hands, one on top of the other, raise your knee, and gently bring the other end of the dumbbell down onto your knee.
When performing the seated dumbbell overhead triceps extension, keep your elbows close to your head. This will keep the emphasis on your triceps brachii. If you have to open up your elbows (move them away from your head) to raise the dumbbell, it means that the dumbbell is too heavy.
Your upper arms must be kept vertical. If your shoulder flexibility does not permit this, sit with your hips forward and rest your back on a chair.
Try not to raise the dumbbell directly over your head, as this can be dangerous. Tilt your head forward a little, and raise and lower the dumbbell behind your back.
The point of sitting down is to reduce your ability to cheat by using body momentum to raise the dumbbell.
