The initial position for the Skull Crusher. Personal Photo
The Skull Crusher is a triceps exercise that targets the entire length of the muscle.
Start by laying flat on a bench. Your feet may be raised up as in the picture or flat on the ground. Shorter individuals tend to raise their feet because their backs aren't flat when their feet are on the ground.
Hold the weight in both hands with an overhand grip directly above the chest. Hand are traditionally placed shoulder width apart, but they can be placed closer or further apart to alleviate strain on the shoulders and wrists.
The down position for the Skull Crusher. Personal Photo
Bending only at the elbows, lower the weight toward the forehead. Since this is an exercise to work the triceps, there is no need to bend or move the shoulders.
Note that the bar should not touch your head, that is simply the direction the bar is going. Do the motion slowly with the appropriate weight to avoid injury.
The triceps control extension of the elbow so you should feel those muscles working as you straighten your arms back up.
A good weight for women to start with is 15-20 pounds for this lift. Men can range anywhere from 20-50 pounds for this lift.