One-Dumbbell Triceps Extensions

Sit on the end of a flat bench and grab the dumbbell and lift it directly above your head with both hands.
 
Make sure your palms are flat on the inside of one of the ends of the dumbbell, your thumbs are circled around the bar of the dumbbell, and the bar is perpendicular to the floor.
 
Bend your arms so the dumbbell is as low as possible directly behind you, being sure to keep your upper arms close to your head.
 
Straighten your arms; your upper arms and torso should remain motionless through the entire movement.
 
Maintain the natural curve of your spine. Do not arch your back.
 
The entire motion should be done in a smooth, controlled manner that lasts about 3 seconds, with the downward movement no faster than the upward.