Have you ever heard of Brachialis or Brachioradialis? Yes? Very good! No? It's okay, just keep on reading and I will explain what a unicorn Brachialis is.
I also must say that this muscle is never neglected by the pros.
First of, let's look at the anatomy of these bad boys. Brachialis is located, simply speaking, between the outer biceps and triceps heads, and is in some cases popping out looking like a second biceps on an arm, while usually for most people biceps lies on top of Brachialis. Function of brachialis is to assist biceps in the pronated (palm faced down) position lifts. Also, if you are doing wide grip pull-ups - that motion will also be partially assisted by brachialis.
Well, Brachioradialis is connected to Brachialis and is basically the upper peak of the forearm.
Let's talk about Brachialis first.
Generally speaking, there are three major exercises that will target this beast.
Reverse grip curls
This is the major Brachialis exercise and can be done either with dumbbells or with an EZ curl grip bar/cable.
Dumbbells: grab a dumbbell palm facing down (pronated palm position). Lock yourself in a standing position elbow in front of you, relax your wrist and let it hang while holding the dumbbell, knuckles always facing directly forward. Now, without moving anything else, but your forearm - start bringing the dumbbell up until you make slightly over 90 degrees with your upper arm.
EZ curl bar: grab an EZ curl bar/handle depending if you are doing it with free weights or the cable pulley palm facing down (pronated palm position). Again, lock yourself in a standing position elbow in front of you, this time do not relax your wrist, and make it to be rigid with the forearm. Start bringing the bar up until you make slightly over 90 degrees with your upper arm.
These are hard, and frankly weird to perform, but might come in handy.
Technique: find a hanging bar, grip palms facing forward, grip at the width of about your palm width in between, elbows outwards until you are almost directly under the bar, now pull yourself up until your arms bend at about 100 degrees.
People usually like this exercise, because it lets their ego thrive, since you feel stronger in this exercise, mainly because you are just using more muscles than you think you are. This exercise works both Brachialis and Brachioradialis, although it will to some extent engage biceps as well.
Technique: stand up right, elbows in front of you, everything else relaxed. Grab dumbbells palm being in the neutral position (palms facing your body when at the body's sides). Start bringing your forearm up until it makes slightly greater than 90 degrees angle.
This is about it, for the Brachialis and Brachioradialis, but let me tell you a few words about the wrist curls
These might be useful to someone, although, I really don't see the need in these, since your forearms assist in most lifts and are responsible for the grip strength, so giving it extra of focused stress is most likely not needed. But anyways:
Technique: grab anything you want to do the wrist curls with, lock yourself and your arms in the position where nothing else moves, but your wrists. Start curling downwards for the inner part of forearms, and upwards for the upper part of the forearms.
Good luck adding extra size and peak to your arms.