One-Arm Dumbbell Extensions

Sit on the end of a flat bench and grab the dumbbell with your right hand.
 
Lift the dumbbell directly above your right shoulder, with your palm facing forward. Bend your arm so the dumbbell is as low as possible behind you and near the midline of your body.
 
Straighten your right arm; your right upper arm 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 upwad.
 
After completing your reps for one side, reverse sides and repeat.