The triceps muscle makes up the bulk of the back of the upper arm. Its primary function is to extend (straighten) the elbow. The triceps has three distinct parts, called “heads”—long head, lateral head, and medial head. The long head has an attachment site on the shoulder blade, so in addition to elbow extension, this part of the triceps plays a minor role in extending the shoulder (moving the arm backward behind the body). Because any exercise that promotes extension of the elbow will work the triceps, skull crushers (also called lying triceps extensions) and triceps kickbacks (where the exerciser stands and extends the arm behind the body) are both effective triceps exercises.
In an ACE study on the best triceps exercises, researchers found that some exercises stimulated the triceps muscle more intensely than others The triceps kickback exercise was found to work the entire muscle slightly more than the skull crusher movement. Intuitively, this makes sense when you consider that the kickback involves shoulder extension (one of the roles of the long head of the triceps muscle) and the skull crusher does not.
The bottom line is that both exercises will promote work in the triceps muscles, but the kickback may be a better choice for three reasons: (1) increased stimulation of the entire triceps muscle (as explained above), (2) shoulder flexibility issues, and (3) the need for a spotter. In my own practice of training clients, I evaluate a person’s form and technique and use that information to help me decide which triceps exercise will be most beneficial for him or her. I have found that for many people shoulder flexibility is a problem, meaning that they cannot properly hold their shoulders in the correct alignment in order to perform the skull crusher exercise. Furthermore, the skull crusher exercise earned its name because it is quite easy to hit yourself in the head with the weights while you are performing the downward phase of the movement! For this reason, I recommend having a fitness professional or a trained a spotter on hand when performing the skull crusher, so this exercise may not be practical for many people who work out on their own. If you want more information on a variety of triceps exercises (and their demonstrations), I encourage you to visit the free ACE Online Exercise Library.