What is a moment arm and how will it help me?
Let’s talk about using internal moment arms to inform exercise selection.
What is an internal moment arm? It’s the distance between a muscle acting on a joint and the joint itself. A large moment arm means that the muscle can create more force through better leverage on the joint.
The goal is to find those exercises that create large internal moment arms for the deltoids to be challenged.
The anterior and middle heads of the deltoid have both shoulder flexion moment arms. These moment arms are the longest when the arms are raised in front of the body (like in a front raise) or when the arms are overhead (like in an overhead press).
A dumbbell overhead press with the elbows to the side will target both the anterior and middle heads of the deltoid roughly equally.
The posterior head has a shoulder extension moment arm which is greatest when the arms are at your side like, for example, in a horizontal row. Unlike the lats that also extend the shoulders, the delts maintain a meaningful extension moment arm even at very high degrees of shoulder flexion. Exercises like pullovers can also train the posterior heads of the deltoid.
The middle deltoid has a significant shoulder abduction moment arm. The anterior delt kicks in to assist with shoulder adduction as the arm moves further away from the body. To focus on the middle head of the delt, it’s a better idea to do lateral raises only to about shoulder height. That is the position that it’s moment arm is the longest as well as the external moment arm of the weight on the hand.
The anterior delt works in horizontal adduction (sometimes called horizontal flexion) along with the pec major so exercises like dumbbell flys will preferentially target the anterior deltoid at the bottom of the movement.
The posterior delt has the opposite moment arm so exercises like reverse flys will preferentially target the posterior delt at the top.