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Let’s compare Aroldis Chapman and Jared Weaver: Pelvic-Scapular-Separation-Style

Sean recently texted me an article talking about the concept of “Hip to Shoulder Separation” that has become popular in recent years and asked for my thoughts—there were plenty. It sent me down a pitching mechanics wormhole. Please enjoy.

First off, baseball players and especially coaches need to improve their anatomical vocabulary. We often talk about the hips when we are actually talking about the relationship between the pelvis and the spine rather than the femoral head and the acetabulum. When we say shoulder, are we talking about the gleno-humeral joint or the scapular-thoracic joint, or the whole shoulder complex? This lack of common and proper vocabulary adds unneeded confusion when trying to express our thoughts. I liken it to trying to write a book, but only having access to half of the alphabet. You may have a great story in your head, but if someone tried to read it they’d have no idea what you were trying to say.

The last anatomical critique I’ll make before jumping into an analysis of “Hip to Shoulder Separation” is the use of the word core and how most people think the core works. The core is more than just your abs. The core is a group of muscles around your spine, starting at the bottom with the pelvic floor and going all the way up to the deep neck flexors. It is comprised mainly of muscles that contain a high percentage of fibers that are built for stability and endurance. Your core is not going to be directly responsible for rapid motions. It is meant to be an anchor to give you proximal stability, which allows for you to have distal mobility. In other words, the more stable your core is, the greater your potential to get the legs and arms moving quicker and stronger and thus throwing and hitting harder.

Additionally, when you use the muscles meant for motion to stabilize, you put yourself at a greater risk of injury because these muscles will fatigue quicker and will no longer be able to provide the stability needed to complete whatever motion you are trying to accomplish. With the violence of the pitching motion, stability is of the utmost importance.

Now that I’m off my soapbox, let’s get into the nitty-gritty of what is going on when we create Pelvic-Scapular-Separation. One of the biggest proponents of this concept is Tom House, who is mentioned extensively in Matthew Schissel’s article at Inning’s Pitched (the one that set off a lot of feelings inside of me). I worked under a House-trained pitching coach between college and graduate school. His approach to pitching mechanics changed the way I look at baseball and greatly increased my desire to learn more about biomechanics. I’ve extrapolated a lot from his work and applied it to the swing.

“Hip-Shoulder-Separation” was one of the most important concepts. It makes sense on a fundamental level—by stretching the body you put it under tension like an elastic band, creating potential energy, which can be released as kinetic energy. Because energy cannot be created or destroyed, only transferred, by generating more potential energy you can release more kinetic energy and throw harder. I completely agree with this. As with much of my disagreement with House’s approach, the problem I have is with the rationale used to describe what is going on in this process and how we go about creating it.

As Schissel, puts it: “The hips open up and fire before the core engages, and then the shoulders and arm follows. So the hips need to be strong, mobile, and steady to keep everything else in line and on track.” It’s a great attempt at describing what is going on, but is severely misguided.

The hips (pelvis?) can’t efficiently open up and fire without the core being engaged already. By trying to get stability from the hip musculature, a pitcher is limiting their potential and setting themselves up for injury.  The stability we see comes from the core and allows the pelvis to open up.  In order for this to happen effectively, the core musculature must be at a proper length-tension relationship and alignment. In an efficient system, when a person changes their center of mass over their base of support, the core will kick in automatically. Below is a freeze frame of Aroldis Chapman at the very beginning of his leg kick (a change in base of support) before he throws the fastest pitch ever recorded. If anyone is ever going to have this proper relationship, it is this man, on this pitch.

Chapman1The quick and dirty way to analyze this is to look at the relationship between his xiphoid process (the little bony point at the bottom of the sternum) and the pubic symphysis (the point where the two pubic bones come together. These two points should be vertically on top of each other at the start of the motion towards the plate. The Reds’ jerseys make this very easy to do as there is a stripe going down the midline, which gives us an easy reference point to look at. This allows Chapman to make full use of his athleticism by activating a feed-forward mechanism that starts in the core and increases neuromuscular excitement throughout his system.

This alone won’t make you throw gas. Chapman also has the benefit of having ridiculous spinal and pelvic mobility. That is what actually enables the Pelvis-Scapular-Separation.


A lot of pitching coaches preach pinching the shoulder blades together to either increase stability or increase the potential energy that can be released when we protract the scapula from a starting position of retraction. Pinching the shoulder blades can do both of those things to an extent, but is not nearly the most efficient way to do so.

Humans are born with certain reflexes that help us survive, but as we develop they go away once we have control over our movement….unless we are put in situations with high levels of physiological stress (i.e. pitching). Chapman takes advantage of an infant reflex known as the asymmetrical tonic neck reflex that allows some of his motion to be processed in the spinal cord rather than having to travel all the way up to his brain. You can see that as Chapman turns his head towards the plate, his front elbow extends and his front scapula protracts while his back elbow flexes and his back scapula retracts. This allows him to fire those muscles quicker and increase his upper spinal mobility. Try rotating your spine with your shoulder blades in both positions and it should be pretty obvious which allows more motion to occur.


One last thing that Chapman does extremely well is that his initial movement is directly towards the plate and it is driven from his pelvis (a good indicator of efficient movement), which minimizes wasted energy. When combining this with a strong, engaged core and the ability to contract his “fast twitch” muscles freakishly quickly and forcefully, you get a fastball at 100+ mph.

Let’s use this framework to analyze a pitcher who doesn’t throw quite as hard, Jered Weaver.

Weaver has a good alignment of his xiphoid and pubic symphysis. However, his first movement is not forward, but rather it’s up. It also appears to be more hip driven than pelvis driven. This is difficult to see without a side-view. What is very evident though is that he has limited motion in his pelvis, especially when compared to Chapman. You can see this by comparing how the changes in heights of both pelvic bones in relation to each other.



We can tell that Weaver does not et nearly as much Pelvic-Scapular-Separation as Chapman. He attempts to get more by compensating through extension of his spine (my guess is his thoraco-lumbar junction, but because MLB rules require shirts, we can’t really tell). This is possible to see in the twisting that he does. While Chapman also has extension in his spine, it is in a better position to receive the compressive forces of gravity. If you were to draw a line straight down the spine into the ground, Chapman would be almost perfectly perpendicular, whereas Weaver is on a clear diagonal. This puts more weight into Weaver’s heel and causes him to “fall off” to the left during his delivery, as well as use muscles to do the job the spine is supposed to.

Lastly, we can see that Weaver does not take advantage of the ATNR the same way that Chapman does. Weaver starts with both elbows in extension and then moves both in flexion while retracting both scapula simultaneously, which limits his spinal rotation.

It should be pretty obvious why there is close to a 20 mph difference between these two pitchers—Chapman uses his body much more effectively and efficiently. Weaver used to be able to gas it up and his mechanics weren’t all that different. I have a feeling the drop in velocity later in his career is from using his body inefficiently, which caused impairments that have limited his mobility and in turn his velocity. He has remained effective because he is a great pitcher and his approach to batters has changed drastically. When it comes to getting batters out, there are many things that matter more than just velocity.

-Mike Plude

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