Brachialis
Concept
A deep muscle in the upper arm that, when developed, pushes the bicep outwards, making the entire arm appear larger.
Mentioned in 2 videos
Videos Mentioning Brachialis

Turkesterone, Carb Overfeeding, and Commonly Neglected Muscles (Ep 92)
Stronger By Science
A deep muscle in the upper arm that, when developed, pushes the bicep outwards, making the entire arm appear larger.

Elbow: discussion + exam with orthopedic surgeon, Alton Barron, M.D.
Peter Attia MD
A muscle in the upper arm that lies underneath the biceps, primarily responsible for elbow flexion and contributing to basic curling strength.