The tricep’s primary job is to straighten the arm and it is the antagonist of the bicep (folds arm). A great way to improve muscularity and strength in your tricep (back of the upper arm) is tricep cable pull-downs.
How to do it:
Set yourself up 2-3 feet away from a cable system. Use the upper cable pulley and attach one of the various attachments (E-Z bar, Rope, straight bar, V handle, etc).
Keeping your elbows tight against the side of your body pull the attachment down until the arm is straight and next to the body. Squeeze and hold for 1-2 seconds and allow the attachment to rise again under control without moving the elbows.
Expert Advice- I like to use the rope attachment because it allows me to pull my hands apart at the bottom of the movement for the greatest range of motion. At the bottom I always hold and squeeze for 2 seconds to pump blood into the area.
Women- If you are looking to keep the under arm area tight be sure to train your triceps.
Men- If your goal is bigger arms be sure to train your triceps because the muscle group has more total mass in your arm than the bicep.