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At Baseball Rebellion, we often teach and write about the importance of the lower half of the body because kinetic energy works from the ground and proceeds through the body, eventually reaching the bat. While this is still the priority, it is also important understand how to fire the upper body explosively when the time is right. The major movement we use to do this is adduction of the back arm. Adduction is simply the motion of bringing your arm closer to the midline of your body. You can remember this because adduction is ADDing your arm to your body. Below is a diagram showing adduction as well as a video where I take a look at Aaron Judge adducting his arm in a swing.
Now that we know that adduction is important in the baseball swing, lets discuss the major muscles that act to adduct the arm. In my my opinion, there are three muscles on our back that we we want to use maximally (Latissimus Dorsi, Teres Major, and Subscapularis) and one that we want to use less (Pec Major).
The Latissimus Dorsi is a big, strong muscle on the back that plays many roles including including adduction of the back arm. As mentioned in the video of Aaron Judge above, it is important to set up the use of our back muscles by retracting our scapula so that the adduction of the arm happens over top of these muscles instead of in line or in front of our rib cage. This can be seen in this picture:
The Teres Major Muscle originates on the bottom of the scapula and inserts onto the front of the humerus (Upper arm). It has many of the same actions as the Latissimus Dorsi above. As you can see in the diagram above, the Teres Major muscle wraps around the arm. Because of this, the back can act to adduct the arm forcefully. As with the the Lat, it is important to set up the use of this muscle by reacting the arm/scapula prior to initiating the upper body in the swing.
The Subscapularis muscle is the most anterior muscle that we want to use maximally when adducting the back arm in the baseball swing. It is located on the front side of the scapula and attaches to the front of the upper arm. As you can see from the diagrams of all three muscles mentioned those far, they show how the big strong back muscles can act on the arm to adduct it quickly and powerfully. This can be a very advantageous move for hitters.
While the Pec Major muscle isn’t necessarily bad to use in addition to the three major adductors mentioned above, we don’t want to use it by itself to adduct the arm for a couple of reasons. The first is that you simply will not generate the same amount of force when compared to adduction using the Lat, Teres Major, and Subscapularis muscles. Using the Pectorals Major muscle as the primary adductor often leads to bat drag because there is nothing to stop the back elbow from sliding forward and across the stomach. We typically see bat drag in younger kids for precisely this reason. They often do not have big back musculature yet and have little coordination and control of their scapula. Allowing young hitters to understand where their muscles are and how to use them can be highly effective in creating better movement patterns. Without the important setup/retraction move mentioned earlier, the Pectoralis Major muscle will most likely take over as the primary adductor and lead to inefficient upper body mechanics.
I hope this has helped you understand your body and your swing.
Thank you for reading.
Gabe Dimock – Baseball Rebellion Hitting Instructor