Exercises that bring the arms forward in front of the body, such as the lying extension, emphasize the long head of the triceps by stretching it.
To fully stretch the triceps long head, it is critical to bring the arms overhead, as in overhead extensions.
Exercises that bring the arms up overhead stretch the long head of the triceps, because it is the only one that is not attached to the humerus (upper arm) bone; it attaches to the scapula.
When you stretch a muscle, it can contract with more force. Therefore, when you stretch the long head during overhead extensions, it becomes the stronger muscle head and takes on the majority of the load.
Although lying extensions do stretch the long head to some degree, research shows that they better emphasize the lateral head.
VERDICT: OVERHEAD EXTENSIONS
To bring up the long head of the triceps and add more mass to your arms, be sure to perform overhead extensions.
Use a variety of overhead extensions in your triceps workouts. Consider starting your workouts with dumbbell or barbell overhead extensions, while you’re strongest, to place the most emphasis on the long head. Then, after doing exercises that hit the lateral and medial heads, consider adding another exercise for the long head, such as one-arm dumbbell overhead extensions or cable overhead extensions. – FLEX