A chin up is a variant of a pull up where your forearms are supinated, allowing you to grab the bar with an undergrip. The main muscle working here is still the latissimus dorsi, and it's still a compound exercise which uses a lot of helper muscles, but there are two key differences. Firstly, in a chin up, your biceps and brachialis are perfectly aligned with the line of grip tension, which allows them to engage really hard and help the movement. Secondly, because the elbows are in front of your

Veiny Forearms? Fix Small Forearms with This Muscle