What the?? A skull crusher is a weight lifting exercise that works the triceps muscles - called "skull crusher" because of the way the weight is lowered toward you head (ie if you let go of the weight while doing the exercise - uh ohhh on the skull). But flexible - it can be performed almost anywhere with a variety of different weight types.
To perform the skull crusher exercise, pick up weights with an overhand grip. Lie flat on your back on a weight lifting bench or other flat surface. Raise both arms straight up above your chest. Your arms should be perpendicular to your body. This is your starting position. Bend both elbows and carefully lower the weight toward your head.
Keep your upper arm perpendicular to your body as you do this. Only your forearms should move as you bend your elbows. When your arms are bent at a 90 degree angle, raise the weight again to the starting position.
Dumbbells are safest since they allow you to drop the weight if it gets too heavy. Barbells force your wrists to remain in a stationary position and an EZ curl bar does the same but lets your wrists turn at a more comfortable angle.
The skull crusher exercise can be done while lying down on any flat surface, but a padded weight bench is the most comfortable. You should try and use a relatively small size weight for the skull crusher exercise.
The triceps are a small muscle group and do not require heavy weights. In fact, heavy weights will prevent you from using good form and wont work your triceps properly. If you have trouble lowering the weight, it is too heavy (remember the skull crusher thing?) Go nuts - we all need a good set of triceps...