Lying triceps extensions, also known as skull crushers and French extensions, are a strength exercise

Weight training

Weight training is a common type of strength training for developing the strength and size of skeletal muscles. It uses the weight force of gravity to oppose the force generated by muscle through concentric or eccentric contraction...

 used in many different forms of weight lifting. Lying triceps extensions are one of the most stimulating exercises to the entire triceps muscle group

Triceps brachii muscle

The triceps brachii muscle is the large muscle on the back of the upper limb of many vertebrates. It is the muscle principally responsible for extension of the elbow joint .-Terminology:...

 in the upper arm. It works the triceps from the elbow

Elbow

The human elbow is the region surrounding the elbow-joint—the ginglymus or hinge joint in the middle of the arm. Three bones form the elbow joint: the humerus of the upper arm, and the paired radius and ulna of the forearm....

 all the way to the latissimus dorsi

Latissimus dorsi muscle

The latissimus dorsi , meaning 'broadest muscle of the back' , is the larger, flat, dorso-lateral muscle on the trunk, posterior to the arm, and partly covered by the trapezius on its median dorsal region.The latissimus dorsi is responsible for extension,...

. Due to its full use of the Triceps muscle group, the lying triceps extensions are used by many as part of their training regimen.

Execution

1. Lie on a flat bench with feet on the ground and head hanging just off the top of the bench, so that the edge of the bench rests in the pit between neck and head.
2. Take the barbell
   Barbell
   A barbell is a piece of exercise equipment used in weight training, weightlifting and powerlifting. Barbells range in length from to although bars above 7' in length are used primarily by powerlifters and aren't commonplace...
   
   with an overhand grip (palms away from body) and hold it out above the head so that the arms are supporting the weight. Do not hold the arms straight over the face at 12 o'clock, but rather at an angle more like 10 o'clock, with feet at 3 o'clock. All of the weight should be on the triceps.
3. Now bend the arms at the elbow, bringing the bar down close to the top of the forehead.
4. Keep the elbows in the same position, do not let them sway outward.
5. Press back up to starting 10 o’clock position.

Try to avoid moving your elbows too much; try to keep them the same width apart during the whole movement.

Vertical French extension

In this variation, the exercise is performed while standing (or sitting on a device with a low backing—that allows the shoulders full range of movement). With respect to gravity, the weight is still lifted in the same manner. With respect to the body, the weight is instead held over the head rather than over the face.

As with all weight training movements, it is important to keep the weight under smooth control on both the way down and the way up. This exercise can be performed standing, sitting or lying on one's back. As with most weight training exercises, it is recommended to start off with a lighter weight and then to gradually increase the weight as the muscles get stronger.

Other

Another variation is done with dumbbells, lying on the ground, touching the dumbbell to the forehead and then fully extending the arm.