Skull crushers, also known as lying triceps extensions, are an exercise that works your triceps, the muscle on the back of your upper arm. The triceps is a muscle with three heads, and skull crushers work all three heads of the triceps, but the medial head is the main driver. This exercise can be performed with dumbbells, barbells, EZ bar, and the cable machine, and can be done on a bench or on the floor. Skull crushers are an isolation lift that work your triceps in a way that’s unique from pressing movements, allowing you to bulk up the long head of your triceps. They can help improve complex movements such as the bench press and add mass to the triceps. Skull crushers are a sturdy triceps isolation exercise that allow you to lift quite a lot of weight through a deep range of motion, making them one of the best exercises for building bigger triceps.