Brachialis: a flexor that lies in front of the lower part of the humerus whence it arises and is inserted into the ulna (https://www.merriam-webster.com/medical/brachialis).
Function: The brachialis muscle has a large cross sectional area, providing it with more strength than the biceps brachii and the coracobrachialis. In order to isolate the brachialis muscle the forearm needs to be in pronation, due to the biceps brachii's function as a supinator and flexor. By pronating the forearm the biceps is put into a mechanical disadvantage (https://www.physio-pedia.com/Brachialis#cite_note-:2-4).
Ok, now that we are all acquainted with this often-overlooked muscle, let’s discuss why it matters to bodybuilders. This little muscle located between the biceps and triceps actually pushes the two muscle groups further away from each other, leading to a wide looking arm. Bet you didn’t know that, huh?
I know what you’re thinking now…how do I train this magical muscle? Don’t worry, we’ve got the secret recipe!
Cross Body Hammer Curls (Windmill Curls)
Grab a dumbbell with a neutral grip (as if you were going to do a standard hammer curl). Instead of bringing the weight directly in front of you, curl the it across your body. Keep the dumbbell close to your body and end the rep at the top of your pec/shoulder. Focus on the squeeze at top of the rep and the stretch at the bottom.
Grab a dumbbell with a neutral grip, thumbs up. With your elbow tucked to your side, bring the weight straight up directly in front of you. Ensure that the elbow is the only thing moving. Squeeze at the top of the rep and stretch at the bottom.
Reverse-Grip Barbell Curl
Pick up a barbell with a reverse grip (palms facing down). Be sure to bring the thumbs over the top of the bar, not under it. With your elbows tucked, curl the weight directly in front of you. Squeeze both forearms as hard as possible at the top of each rep.
Well, there you have it…the secret that pros use to give the illusion of even bigger biceps and triceps than they actually have. You’re welcome.