The triceps muscle group is composed of three separate muscles (hence the prefix tri-), being the medial, lateral, and long heads. The long head is biarticulate, meaning it crosses two joints. Working at the shoulder and elbow joints, it helps to extend the elbow and also adduct the shoulder (arms moved down and towards the body). Due to this arrangement it cannot be active at both joint at the same time. So, for presses and dips, the long head is for the most part just hanging out. You must go overhead to stimulate the long head, and thus the entire triceps, so make sure to throw in at least one overhead movement into your triceps/arm routine.
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