Hammer Bicep Curl - Phase 2 - Day 4 - Strength. The hammer curl works the outer head of the biceps & the brachialis. This exercise helps you achieve full development of the upper arm, and it also works the forearms, however to a lesser extent. Ensure that you pull your navel in to the spine to activate the core throughout the entire exercise, and keep the weight controlled throughout so that you not only optimise the result, but minimise the risk of injury. Nick Jones - World Champion Bodybuilder & Brad Ryan - Bodybuilder.