There are lots of good Tricep exercises but here are some of the best ones for the Outer Triceps:
*Cable Pressdowns (with your palms facing down)
*Single Arm Behind Neck Dumbbell Tricep Extension
*Two Arm Behind Neck Dumbbell Tricep Extension
*Close Grip Bench Press
Lots of good options to choose from.
Coach John Kane CPT
The lateral head and the medial head of the triceps work in almost the exact same manner and extend the arm at the elbow. While changing the position of the upper arm or the forearm will result in the lateral head of the triceps being activated more or less in extension of the elbow, it is still simply extension of the elbow. And there is potential for elbow and shoulder injury if you are exceeding the limit of stability for these joints during weighted movement.
Isolated triceps extension added to the exercise program at the end of a resistance session can potentially enhance the development of this muscle. I would use equivalent biceps/flexor development that is reasonable to avoid strength imbalance between the two muscle groups.
And I would spend some time exploring the biomechanics of any exercise that I intended to have a client perform.
ACE recently did a study to determine which exercises activate the triceps muscles the most. Of the the 8 exercises studied, the top three are:
1. Triangle Push Up (Diamond/Close grip push up) (Activates 100% of long & short head)
2. Triceps Kickback
Take a look at the article: http://www.acefitness.org/certifiednewsarticle/1562/
You’ve got some pretty complete answers here, already. Keeping it short and simple, use your knowledge of biomechanics and have your client experiment with some of the same or familiar exercises done at different angles, perhaps with different grip/wrist positions. Keep the weight light to start off so that your client doesn’t strain too hard. Help your client to understand how body position throughout a range of motion can make all the difference.
The phrase “an inch makes a mile” has never been so true when it comes to form on any given exercise. By simply changing an angle on an exercise, you can increase/decrease load on specific muscles. Once you help your client to understand how to activate different muscles/muscle groups on command, it should be fairly simple to determine what will work best to get those target muscle groups working a little bit harder.
My advice is for you and your client to take some time and experiment with it. Make sure that whatever you and your client decide on is going to be safe and effective.