triceps brachii is made up of 3 heads, lateral, medial, and long head. lateral and medial just extend the elbow while the long head also adducts the arm(brings the arm closer to your torso) and extends the shoulder(brings the arm back). kickback is a great exercise to target all these parts.
although you need to keep your upper arm(from shoulder to elbow) steady in kickback and not extend it back, still you are using the long head to hold that position(isometric training).
make sure you avoid going extra heavy on this exercise. control the weight and movement and also try to squeeze your triceps to the fullest so there is no bend in the elbow, this way you target the medial part the best. also do not bring your hands more than perpendicular to the ground since anything past that is not going to affect the triceps.