Biomechanical Breakdown: Jack Weisenburger (Oakland Athletics)

Drafted in the 20th round of the 2019 MLB Amateur Draft by the Oakland A’s out of the University of Michigan, not a lot of people were talking about Jack Weisenburger. To tell you the truth I just recently learned about the guy too. But the buzz has started to pick up and I’ve done my fair share of research in the past few months to make up for a couple of years of neglecting the lower half of the A’s farm system and I come bearing gifts…in the form of biomechanical analysis. 

In a recent episode of the ‘Official Visit’ podcast, Weisenburger revealed that for the first two years of his three-year career, he was a two-way player at U of M. It was decided that the Michigan native would make the transition to being a PO for his Junior year after going to Driveline Baseball the summer before and topping out at 94 MPH – something he had never done before. He credits Sean Kenny (assistant coach) and Chris Fetter (pitching coach) at Michigan for introducing more routines, strength training, arm care, long toss, as well as what he worked on with Driveline, for his uptick in velocity. 

When he finally transitioned to being a PO, Weisenburger mostly performed set-up man duties out of the bullpen. He proved to be a solid young arm but lacked the ingredients to elevate himself to early draft hype. One of those ingredients being his lack of command and control around the strike-zone, posting 64 strikeouts and 48 walks in 63 IP as a Wolverine. After being selected by the A’s and pitching in the Fall Instructional League in Arizona – Weisenburger revealed that he “didn’t have the velocity [he] wanted”. So what’d he do? He booked a flight to Washington to work out with Driveline again in hopes of improving his velocity just like he did in college. The trip proved to be valuable, yielding yet another uptick in velocity to a consistent mid 90s fastball with plenty of horizontal movement that occasionally flirts with the upper 90s threshold. Additionally, the command issues that plagued Weisenburger’s college career have all but disappeared, at least in the first 12 appearances of his professional career – tossing 14.1 IP with 26 strikeouts to 3 walks and only 1 wild pitch at the time of writing this article. In a similar sample size during the Instructional League, he was similarly productive in terms of strikeouts but accumulated 7 more walks and 6 more wild pitches. 


Currently pitching for the Lansing Lugnuts in High-A, this is a small sample size to declare anything definitive about a player, however, Weisenburger has been extremely productive in that period and things are starting to look promising. If you want to know what the flashes of a future big-league closer look like, a glimmer is beginning to form beyond those overcast Lansing skies. 

So what’s the big deal?

The 23 year-old is mainly a two-pitch pitcher, featuring a mid 90s fastball, as stated earlier, and a swing-and-miss slider that is arguably his best pitch. I’d be remiss if I didn’t mention that he also has a changeup that he’ll sprinkle in, as far as I can tell this pitch is somewhat of a work in progress and will likely need to be further developed if he wants to advance to the highest levels within the A’s system. I’ll discuss his specific pitches in more detail later, but let’s take a look at the mechanics first. 

Despite Weisenburger being three inches taller, his mechanics resemble that of former A’s closer Liam Hendriks – heck I wouldn’t be too surprised if the reason the A’s rolled the dice on him in the 20th round was precisely because he possesses Hendriks-like tools that can hopefully be developed into something similar. Because of their similarities, I’ll be using Hendriks’ mechanics as an ancillary tool to understanding Weisenburger’s mechanics.

The similarities start with their leg lift, which simultaneously 1. transfers weight to the back leg while 2. beginning a forward drift motion. Drifting, simply put, is shifting the center of mass forward towards the plate before driving off the back leg. You’ll notice how both of these pitchers drift and drive – in that order. Although they both drive towards the plate with their back leg, it’s not a max effort move like Edwin Diaz or Max Scherzer who both actively engage their back leg to lunge towards the plate. For Hendriks and Weisenburger, their front foot is too essential to their mechanics to afford a waste of kinetic energy on that. Hendriks, in particular, relies on maintaining a clean, quiet, and upright posture up until front foot contact, where he then powerfully plants his front leg, using it as leverage to conduct the loop of potential energy generated from his weight transfer. Weisenburger shows elements of this as well. His posture is not as well maintained as Hendriks’, however, this isn’t a bad thing as long as he’s not rotating his trunk too early – in fact, it’s more of a feature than a bug. It seems obvious to acknowledge, but every pitcher has a different body and therefore requires different mechanics and this case is no exception. 

(ex: Daniel Winkler)

Moving to the upper half of the body, here is where more differences begin to appear. Hendriks does a fantastic job at delaying his arm until the last possible second (here’s a fantastic video that outlines how Jacob deGrom used this to increase his average velocity from mid-90s on average to the upper-90s where his average currently sits).

(Pre-Foot Contact)
(Start of Foot Contact – Heel)
(Foot Contact)

You’ll notice that during pre-foot contact, Hendriks’ arm is almost at a 270-degree angle and as his heel begins to touch the ground he’s still below the 180-degree parallel threshold. Exactly three frames after this point in time, Hendriks’ arm rotates 180-degrees upwards to a 90-degree angle. This is an incredible amount of movement in such a microscopic window of time and something that would be considered an advanced mechanical technique in pro ball. Some pitchers do this unconsciously, and some, like deGrom, need to actively integrate it into their motion over time. Weisenburger may adopt this mechanical aspect into his own game somewhere down the line, it’s worked wonders for Hendriks, deGrom, and many other high-velocity pitchers, not only to increase mechanical efficiency to limit the potential wear and tear on the arm itself, but also increasing arm speed, velocity, and deception. I’m not insinuating that Weisenburger’s mechanics are flawed or in desperate need of changing because they’re not – whatever he worked on at Driveline helped significantly from what he was doing prior. Unfortunately, I couldn’t find any great footage from college or anywhere before 2021 so we can’t exactly pin down what was tweaked.  

One could argue that this arm delaying is an inherently bad habit for a pitcher, from what I understand the stigma attached to it mainly stems from what’s referred to as the “Inverted W” where a pitcher will elevate his elbows above the shoulder which can cause wear and tear on the elbow and shoulder in particular. Hendriks doesn’t do this, his arm delay (for lack of a better term) is justified because his elbows are parallel to his shoulders up until his front foot strike. As long as the arm, specifically the elbow, is in the correct position, delaying the arm action isn’t harmful. The infamous Inverted W is nowhere to be seen with Weisenburger either, which is a good sign. 

The movement profile of Weisenburger’s breaking ball is once again reminiscent to that of Hendriks’, Jose Berrios, or even Sonny Gray, especially in terms of its efficacy when thrown hard back-foot to lefties.  The reason why I said it might be his best pitch was because of its putaway power, a High-A ball pitcher who primarily throws two pitches shouldn’t be too difficult to figure out – yet the fact that hitters continue to swing and miss is only a testament to its late-life and wicked horizontal movement. Although he can effectively throw his breaking ball to lefties, tape shows that righties see it more – particularly off the plate where it’s at peak swing-and-miss form.

As far as the fastball is concerned, Weisenburger seems comfortable and accurate enough to consistently hit the edges of the strike zone or wherever it’s called. The pitch has a great riding action and as mentioned before substantial arm-side movement. It’s not clear if an even greater uptick in velocity is in his future, it’s hard to look at 96 on the black and say that there’s anything left to be desired from a closer’s fastball. If I had to guess, I would probably bet that Don Schulze (Lansing Lugnuts pitching coach) is more concerned with maintaining control and command in the strike zone over pure velocity at this point. I should also add that just because I did quite a bit of comparing to Liam Hendriks, doesn’t mean that Jack Weisenburger is going to be Liam Hendriks. Seeing all the similar elements of Hendriks in Weisenburger gets me excited as an A’s fan but it’s not productive or realistic to use that in any kind of predictive way. They are both different pitchers who are at wildly different points in their respective careers and should be treated as such. Nonetheless, Jack Weisenburger’s journey through the minor leagues will be an interesting one to watch, and I wouldn’t be surprised if we see Weisenburger in an A’s uniform within the next year or two. 

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