Fully developed triceps add size and shape to your upper arms. The impressive horseshoe shape of the muscle looks great in a t-shirt and counterbalances a flexed biceps. One of the most popular triceps exercises is the skull crusher, also known as the lying triceps extension . In this article, we go over the unilateral version of the conventional exercise — the single-arm skull crusher . You’ll learn how to do the single-arm skull crusher with the perfect form, the benefits, and the variations and alternatives of the exercise.