The long head of the triceps crosses both the shoulder and elbow joints and is responsible for joint actions at both. Overhead triceps exercises increase the overall length change in the long head throughout the movement while kick-backs also force the long head to perform shoulder extension during the movement. Use variations like kickbacks and overhead extensions to target the long head of the triceps.
Reference: Boehler, B. (2011). Electromyographic analysis of the triceps brachii muscle during a variety of triceps exercises (Doctoral dissertation).