Isolation of the lateral tricep head
I'm curious on some specific exercises to isolate the lateral head of the tricep muscle. That's the one most visible from a side perspective. I feel having this one(at least on my body) more developed than the other heads would be incredibly beneficial to shape both relaxed and especially flexed.
I understood while I was boxing, that this particular head is a bit more responsible when the arm is extending while the hand is rotating one way or another, fact or fiction?
Triceps Anatomy & Biomechanics
ANATOMY
The long head of the triceps originates in the glenoid cavity of the shoulder joint. The long head is on the inside of the arm, pressing up against the latissimus dorsi.
The lateral head is on the outside of the arm. It originates from the outer surface of the humerus near the shoulder.
Some people confusingly call the medial head the “inner head,” because it lies deeper than the other two heads. It's more accurate to call the long head the "inner head," because it's more prominent on the inner side of the arm. "Medial" is an anatomical word meaning: “related to, situated in, or extending to the middle.” This refers to the fact that the medial head is sandwiched between the lateral and long heads. The medial head is most prominent toward the elbow, but it actually originates toward the shoulder, on the rear of the arm, under the lateral and long heads.
All three heads of the triceps join to a single tendon that inserts into the elbow at the olecranon process of the ulna bone.
All three heads of the triceps cross the elbow joint, but only the long head crosses the shoulder joint. This is why overhead triceps extensions place more stretch on the long head, and are therefore more taxing on the long head. See below.
BIOMECHANICS
The impact of shoulder flexion/extension:
As a few other people have stated, triceps movements with the shoulders flexed (elbows above head) tend to target the long head. Triceps movements with the shoulders extended (elbows behind body) will in general target the lateral head. If you are finding this not to be the case with a certain exercise, it is likely that other factors are coming into play, such as those below.
Shoulder rotation:
In complex pressing movements like a benchpress, you typically emphasize the triceps by keeping the elbows close to the body. With the elbows in, this can be done either with an overhand grip (hands 6 inches to shoulder-width apart), or a wide underhand grip. In general, if you turn your elbows out, the emphasis will shift to your pecs. However, with pushups or dumbbell pressing movements at or below shoulder height, you can emphasize all three heads of the triceps by varying your shoulder rotation. For instance, if you do pushups with your elbows way out, i.e. with your fingers pointing toward each other, this will emphasize the medial head of the triceps by placing a little stretch on it. (Be careful as you experiment, as this movement can strain your shoulders.) Fingers forward is more neutral, and fingers pointed out will put more emphasis on the lateral head. As I think someone else mentioned, on triceps isolation exercises, you can also vary the width of your grip to emphasize either the long head or the lateral head.
Grip (supination/pronation):
Underhand (i.e. palms up, i.e. supinated) grips tend to emphasize the lateral head, while overhand (i.e. palms down, i.e. pronated) grips tend to emphasize the long head.
As I said in the anatomical section, all three heads of the triceps meet at a single tendon at the elbow. Therefore, while different movements are more taxing on different heads, experts disagree on how much of a difference it makes in the end. The size of your different triceps heads may have more to do with genetics than exercise form, but all you can do is experiment. I'm not a hardcore, experienced bodybuilder, so I haven't developed a strong opinion of my own yet. However, since some experts that I respect believe you can effectively target the different heads of the triceps, to me it makes sense to experiment and find out for myself.
If you want more information, I recommend checking out Eric Cressey's website, particularly his articles on "Bogus Biomechanics and Asinine Anatomy." He's an extremely educated guy, a high ranking powerlifter, and he coaches pro athletes. I also like the "perfect pushup technique" video on the website "bodyweightcoach." It demonstrates variations in shoulder rotation. Last but not least, Dr. Nick Evans has written a pretty respectable book on this subject, titled Bodybuilding Anatomy. It's available pretty much everywhere.