“Crossing the Acromial Line” is a term that I have heard quite a few pitching coaches mention, yet the term was first used by Dr. Mike Marshall many years ago. However, I believe Ron Wolforth is the coach to make it a more popular term that’s become a staple in many pitching coaches vocabularies.
What is the Acromial Line?
The acromion is one of 2 bony processes located on the scapula, along with the coracoid process. It connects to the clavicle, by a ligament, to form the acromioclavicular joint, otherwise known as the “AC joint”. When this joint is sprained it’s referred to as a shoulder separation (not to be confused with shoulder dislocation).
So now that you know that much, the acromial line is simply an imaginary line that crosses each acromion. Below is an example of what that would look like, from a superior (top) view.
Why is crossing the Acromial Line bad?
Dr. Marshall believes that by crossing this line with our hand(s), during the early cocking phase, prior to “forearm turnover”, adds stress on the shoulder, including stretching of the glenohumeral ligament and damage to the labrum. I’ve also read that he believes it can lead to forearm flyout, which would make sense. Even for those that don’t, it could cause early fatigue to muscles that protect the UCL, including the bicep and flexor-pronator mass.
While I don’t always agree with what Dr. Marshall believes, I tend to agree with him on this topic. There’s really no reason why a pitcher would need a big swinging arm action during the early cocking phase. It’s wise to clean up these types of arm action(s) to so that you can help your pitcher(s) have a longer career.
When you cross the acromial line during this long arm action you put a lot of stress on the GH ligament and added stress on the labrum, something that could create serious long-term damage. Although the weight of a baseball is ~5 ounces, the force behind a fastball adds many pounds, depending on the speed of the arm, of course. I have read that the force behind a 90 MPH is somewhere along the lines of 22 pounds, although I’ve also read less than that and much higher than that. I’m too lazy to even attempt to figure that one out, so I’ll let you decide on whether or not that matters.
What to do if you cross the acromial line?
Clean up your arm action or the arm action of the pitcher you’re working with. There’s a lot of theory behind this, but you could use a tool such as the Core Velocity Belt, or a connection ball if you feel the need to.
I think there are definitely use case scenarios for the connection ball, although I don’t use it much, if at all these days. For some of you, it could be very useful here, especially those who have more severe cases.