Scouting Grades: Power

June 22, 2020 • Scouting

It's been four months since I published the first entry in the Scouting Grades series, and it's fair to say that motivation to continue blogging about baseball has been limited in that time. With the 2020 draft in the rearview, I found myself thinking about scouting grades again.

If you haven't already, it's probably a good time to review the post on Speed. Now, let's talk about POWER.

Power wins games. Power puts butts in seats. No other tool has a showcase -- the home run derby -- that exists specifically to elevate and celebrate a specific tool. It should come as no surprise that power also draws big signing bonuses and buys prospects more time to develop.

In this context, development most often means translating raw power into game power, the actual production of runs. That development generally takes the form of increased plate discipline which is not limited to swinging at fewer balls but also includes learning and taking advantage of pitch locations that really allow the hitter to get to that power.

Raw Power vs Game Power

Before player- and ball-tracking technology, raw power grades were determined by watching a hitter unload on baseballs -- usually during batting practice, a scout's best opportunity to see a lot of swings in a short amount of time. Raw power was essentially how hard a player could hit the ball, frequently evidenced by distance.

[Note: batting practice is not the only input, obviously. Game swings matter more and commonly result in harder contact than batting practice swings. However, scouts will almost always see fewer game swings than BP swings, and plenty of hitters will show you what they've got during BP.]

The Raw Power Scouting Scale (roughly):

  • 20 – Even with a favorable gale force wind, the ball probably isn't going over the fence.
  • 30 – A stiff breeze should do it.
  • 40 – Doesn't need help, but you don't know it's over the fence until it is.
  • 50 – Hey, there's some pop!
  • 60 – Stadium vendors and people on the concourse should pay attention.
  • 70 – The ball hits weird parts of the stadium, and occasionally leaves to check out other places.
  • 80 – The ball has a greater than zero chance of escaping Earth's atmosphere.

Game power is an assessment of how often a player is going to tap into that raw power against MLB pitching. There's no escaping the subjectivity required for a game power grade since it relies on, among other things, the player's future hit tool and future raw power which are both projected by the scout.

This produces an unbelievably wide spectrum of projections and results. There are guys you've never even heard of with top shelf, 80-grade raw power that projected to hit approximately 0.000 at the highest level. Then you've got freaks like Frank Thomas and Miguel Cabrera that are just as likely to win a big league batting title as they are to win a homerun crown.

The Game Power Scouting Scale (roughly, thrown into disarray by the new baseball; not funny):

  • 20 – 5 homeruns in a single season would be incredible.
  • 30 – ~5 homeruns per season.
  • 40 – ~11 homeruns per season.
  • 50 – ~18 homeruns per season.
  • 60 – ~26 homeruns per season.
  • 70 – ~35 homeruns per season.
  • 80 – Homerun crown contender.

Of the obvious variables that limit raw power in games -- pitch recognition, plate discipline, swing length, and others which will be discussed in the Hitting post -- there is one in particular that can limit a hitter with plus hitting and plus raw power to mediocre or even below average production: launch angle.

Exit Velocity & Launch Angle

Three names immediately jump to mind when I think about how a low launch angle stifles power: Nick Markakis, Eric Hosmer, and Yandy Diaz. That's three big dudes with big raw power, decent batting averages, and just not very many homeruns. Depending on your age, you may recall that, as prospects, Markakis and Hosmer were fairly widely believed to be potential perennial 40-homerun hitters. Diaz, who arrived just in time for launch angle to become a hot button issue, has been frequently noted as a guy that hits the ball as hard as anyone else in the game.

Last season, all three were in the Top 50 in Hard Hit % (95+ MPH) -- Hosmer #32, Diaz #43, Markakis #48 -- just ahead of Mike Trout (#49) and Fernando Tatis, Jr (#50).

Last Season, all three were in the Top 60 in Average Exit Velocity -- Diaz #20, Markakis #30, Hosmer #57 -- ahead of Anthony Rendon (#64). Trout checked in at #51. (Stats according to StatCast via Baseball Savant.)

In 469 plate appearances, Markakis hit 9 homeruns. In 667 plate appearances, Hosmer hit 22 homeruns. In 347 plate appearances, Diaz hit 14 homeruns. That's 45 homeruns in just under 1,500 plate appearances.

In 600 plate appearances, Trout hit... 45 homeruns.

Given 2.5x the number of plate appearances, three hitters that are ostensibly better at hitting the ball hard than Mike Trout combined to produce the exact same number of homeruns as Mike Trout alone.

The key difference (and admittedly not the only difference) is launch angle.

Eric Homser had a 2.1° average launch angle. Yandy Diaz had a 5.7° average launch angle. Nick Markakis had a 7.3° average launch angle.

The average launch angle for all MLB batted balls was 11.2°.

Mike Trout had a 22.2° average launch angle.

The point of all of this is to illustrate that translating raw power into game power requires hitting the ball in the air. A player can hit 100 MPH ground balls all day, but it won't result in power production.

Context and Projection

As scouting evolves thanks to the ever expanding use of technology, the scout's role will increasingly be to provide context for objective data rather than authoring almost entirely subjective reports. With the proliferation of Trackman, HitTrax, and similar systems, for example, it's a matter of course to have objective data for a player's exit velocity and launch angle. A scout provides little of use by assigning a 20-80 grade for present raw power.

A player's future raw power, on the other hand, is a contextual grade usually backed by an assessment of physical projection. Physical projection offers insight into a player's potential through continued growth and added speed/strength. An 18-year-old that could still grow a couple of inches and has never lifted a day in his life could get a 2-grade bump, while a 23-year-old with a mature build generally gets none.

When it comes to game power, the hit tool is the big separator. It is easily the most complex tool grade and is frequently broken into several sub-grades, and the realization of hitting potential often relies on additional external factors that are not easy for a scout to assess. That really is a topic for a future post, but a few quick examples will illustrate the type of context scouts should be chasing.

The path of the barrel, including its overall length and depth, plays a key role for both power and hitting. While a long, flat path might lead to a lot of overall contact, it isn't likely to lead to much hard contact. A shorter path on plane with the pitch has a better chance of producing hard contact, and being on plane with the pitch has the added benefit of an increased launch angle.

The length of the swing ties into timing which has downstream effects on pitch recognition and plate discipline. All other things equal, a hitter with a longer path to contact has to start to swing earlier, giving the hitter less time to see the ball before launching a swing. Less time to see the ball means worse recognition which will lead to bad swings and bad takes.

When timing is more easily disrupted, the contact point "moves" to different parts of the swing. Generally speaking, if the swing is late, the ball is more likely to be popped up because the point of contact has "moved" deeper where the barrel hasn't come back up to the anticipated point of contact yet. If the swing is early -- particularly on slower pitches that drop more -- the ball is more likely to be hit on the ground because the point of contact has "moved" out front where the barrel has already risen past the anticipated point of contact.

These issues are amplified more in swings that are further from the plane of the pitch.

Projecting a future game power grade can seem a lot like witchcraft, but it boils down to just a couple of questions:

  • Will the player's raw power increase or decrease?
  • How will the player hit against top-level pitching?
  • Will the player hit the ball in the air?

At this point, projecting a player falls into the gap between scouting and player development. Amateur scouts can get to know a guy well enough to have a good idea of the player's development potential ahead of the draft, but the same can't be always be said for pro scouts who are watching another ball club's players. This development gap is a complex topic and will hopefully be the focus of a future post.


Scouting Grades: Speed

February 20, 2020 • Scouting

Some grades are straight-forward, objective evaluations, but others are quite subjective and open to varying degrees of bias. The Scouting Grades series aims to discuss different tools and how they are evaluated.

Speed is one of the more objective evaluations, but it isn't as straight-forward as one might think. Let's start with a fairly common chart that some of you have probably seen before. This chart contains the generally accepted guidelines for objectively converting a batter's Time to 1B into a scouting grade for speed.

Time to 1B Grades (seconds)
LHB Grade RHB
3.9- 80 4.0-
4.0 70 4.1
4.1 60 4.2
4.2 50 4.3
4.3 40 4.4
4.4 30 4.5
4.5+ 20 4.6+

Grades are typically meant to represent a normal distribution centered around 50 as MLB average (or 5 on a 2-8 scale) where 40 is 1 standard deviation below average and 60 is 1 standard deviation above average. A quick look at the StatCast baserunning sprint speed numbers for 2019 blows that idea out of the water.

128 batters averaged 4.55+ seconds on competitive runs to first base. That's almost exactly 25% of the 510-player sample objectively sitting at or well below the bottom of the scale. The median time of 4.38 seconds is roughly a 35 according to this scale. For comparison, normally distributed speed grades would put 68.2% of players between 40 and 60, with an additional 15.9% above 60, leaving 15.9% below 40, and only 2.1% below 30!

I took a quick crack at dividing the Savant list into separate tabs for left, right, and switch hitters and posted it in a Google Sheets document - 2019 MLB Baserunning Speed. I may have missed a leftie or two (#manualData) as right-handed (the default starting point), but a few lefties sneaking into the righties data won't ruin the samples. Here's what the chart would look like based on the split data with 333 right-handed batters and 132 left-handed batters. (44 switch hitters were left out since their times were presumably mixed.)

2019 MLB - Time to 1B Grades (seconds)
LHB Grade RHB
3.794- 80 3.794-
3.985 70 4.009
4.176 60 4.224
4.367 50 4.439
4.558 40 4.654
4.749 30 4.869
4.940+ 20 5.084+

This chart is a more accurate representation of the actual Time to 1B for MLB players with the one exception being that no one would qualify as an 80 runner.

MLB clubs have undoubtedly been aware of this disconnect since stopwatch times were first compiled on a spreadsheet. An educated assumption here would be that clubs are more or less ignoring current speed grades in favor of objective measures from the player tracking technology deployed across ballparks all over the baseball world. Yet the guidelines remain in effect in most scouting contexts.

Where the scouting element may actually come into play is in projecting future speed. More on that later.

The Before Time / The Long, Long Ago

Before player tracking systems were everywhere, measuring every runner on every play, the only way to get objective speed measurements came from scouts' stopwatches. Time to 1B is a standard because it is a fixed-distance sprint that is run by every batter.

To properly capture a Time to 1B, a scout anticipates contact, attemping to start the stopwatch at the exact moment that bat hits ball, and then reads the batter's steps, attempting to stop the stopwatch at the exact moment the batter's foot touches the base.

As error prone as this might sound, you may be surprised to learn that scouts actually get pretty good at this with practice. Is it as good as a player tracking system? Obviously not. Does it get the job done anyway? Somewhat surprisingly, yes.

Speed elements and application, the downside of relying on Time to 1B

What does Time to 1B really tell you about a player's speed? It doesn't really tell you his peak speed, and it doesn't really tell you how quickly he accelerates either. What it does tell you is a decent approximation of the two.

Take another look at the spreadsheet I prepared. You can get a decent idea of who accelerates well by looking for players that are faster to 1B than their sprint speed peers. For instance, it's pretty interesting to see Jeff McNeil average a 4.10 with average sprint speed.

For Time to 1B, peak speed would need to be significantly faster to make up for below average acceleration, given the relatively short sprint. Peak speed really comes into play over longer sprints: gap fly balls, doubles and other two-base sprints, and especially triples. In other words, Player A might be slower than Player B to 1B but faster than Player B to 2B on a double. Does Time to 1B actually tell you which player is faster?

Speed impacts defense almost purely as it relates to range. Outfield defense typically falls into the same acceleration-and-peak mix as baserunning, but for most infield defense, acceleration is far more important, with peak speed generally only coming into play on pop-ups in No Man's Land.

But there's something else about Time to 1B that you may have not considered yet. Different batters have different swings and require different adjustments to transition from swinging to sprinting. Balance, momentum, stance, and overall effort each affect that batter's ability to recover from the swing and get moving toward 1B, and every batter's Time to 1B has this swing effect rolled into it. 2 players could have the exact same acceleration and peak speed but different Times to 1B because of different swings!

There is no swing effect on defense, and that may be the only thing that really prevents Time to 1B from acting as a near-perfect proxy for an outfielder's defensive range. For infield range, Time to 1B would seem to have little or no correlation. (NOTE: I'd enjoy looking at a study that digs into this idea, and I'll cover range more completely in the Defense entry in this series.)

Projecting Future Speed - A Case Study

This could probably be an entire series of articles by itself, so we're going to floor it for a bit, then slam on the brakes, get out of the car, and do a walk-around.

The one thing to keep in mind is that, absent an alternative guideline, a speed projection should target the player's speed at physical maturity, not the end of the player's career. Physical projection plays an immensely important role in projecting speed.

Nomar Mazara made his Double-A debut with Frisco late in the 2014 season. He was 19 years old and had a wire frame on which you could hang a lot of mass. He showed coordination but lacked any sense of athletic explosiveness. He routinely ran 4.70+ to 1B.

He was reportedly 6' 5" when he signed (source) as a 16-year-old and was now listed at 6' 4", so a scout could fairly assume that Mazara had been that tall for at least 3 years, all while working with professional strength coaches and trainers. By all accounts, he would get stronger and heavier as he got older.

Mazara was a 20 runner with a profile that screamed for a negative speed projection, so of course he returned to Double-A Frisco in 2015 running sub-4.40.

There's a brightside, though. Having jumped two grades in one off-season, there was now room for negative projection! That may sound like a joke, but sticking with the negative projection on the 2015 report is the right call. The better Time to 1B obviously indicates more explosiveness, but everything else in the projection is still true.

If you thought he was going to be a large, lumbering fellow at maturity in your original projection, the only difference in the new projection would be some lighter lumbering.

What else could this two-grade jump indicate? It takes a lot of work to jump a grade in anything, and Mazara jumped two grades in a single off-season! A scout would be crazy to positively project Mazara again, but the jump alone is arguably enough of an indicator that the negative projection should be smaller than originally projected.

Positive speed projections are extremely rare outside of young, undeveloped athletes. In Mazara's case, it seems reasonable to conclude retrospectively that Mazara still fell into that categry, but between 2014 being his third full year as a professional and the rarity of positive projections even within that category, a positive projection would have been met with skepticism.

TL;DR

  • Scouting for speed is probably going the way of the dodo -- if it hasn't already -- thanks to tracking technology that puts a scout's stopwatch to shame, but physical projection is still the scout's domain.
  • A competitive Time to 1B -- measured from bat-on-ball to foot-on-bag -- represents a good-enough estimation of most practical applications of speed in baseball.
  • Projecting speed is the art of projecting physical maturity against present ability.

Another post about Brandon McCarthy

February 25, 2010 • Scouting

If you're a betting man, you should know that the odds are good that this won't be my last article featuring the mechanics and health of the Texas Rangers starting pitcher Brandon McCarthy.

As my favorite subject, his mechanics have spent a lot of time on my computer monitor playing forward and backward, in slow motion, and in still shots. As a result, I have a small tendency to see a little bit of McCarthy in just about every pitcher. Every once in a while I run into a pitcher whose mechanics have a lot in common with him.

Meet University of Texas at Dallas junior Marvin Prestridge.

In light of recent mechanical changes, Prestridge doesn't look much like McCarthy does these days [Edit: this may not actually be true since I haven't seen high-speed video of McCarthy's new mechanics], but when I pulled up the video I shot of McCarthy last spring, the similarities were striking. The angles aren't quite the same, so you may have to use a little imagination in places.

McCarthy (left) and Prestridge (right) at the top of their leg kicks.

They don't look too similar at the top of their leg kicks, but they appear to have a similar degree of reverse rotation (turning their backs to the plate). McCarthy is more compact, and Prestridge lifts his knee much higher.

McCarthy and Prestridge at hand-break.

At hand-break, their mechanics are starting to run together. McCarthy sits a little lower on his back leg. Prestridge breaks his hands much closer to his body.

McCarthy and Prestridge right before their forearms start to turn over.

Before foot plant, this is the frame where their elbows stop moving upward and backward (toward 1B), and their arms begin external rotation. You can clearly see McCarthy's inverted W and that Prestridge's arm is below shoulder level with an extended elbow. Both pitchers have their arms well behind their shoulders.

I much prefer Prestridge's method of picking up the baseball to McCarthy's method from last spring. As a part of the changes he has made to his mechanics over the past 9 months or so, McCarthy's current pick-up features a full arm swing that positions his pitching arm much like Prestridge's arm.

McCarthy and Prestridge at foot plant.

By the time they hit foot plant, there's only one evident difference between the two: Prestridge is pulling his glove arm back toward second base. McCarthy's glove arm is essentially dead weight, while Prestridge's arm helps create additional rotational force through his shoulders.

McCarthy and Prestridge at peak elbow height just before elbow extension.

Again, the only difference is the glove arm action and position, though it appears that Prestridge has a greater degree of trunk tilt toward 1B.

McCarthy and Prestridge at full arm extension just prior to release.

At this point, the pitchers are literally inches away from letting go of the baseball. Prestridge is able to reach a little more toward vertical, thanks to his 1B-side trunk tilt.

McCarthy and Prestridge after primary arm deceleration.

After release, the pitching arm continues internal rotation while the body tries to keep the arm from flying out of socket. This frame attempts to capture the moment where internal rotation stops.

What's clear in this frame is that McCarthy's arm continued to fly forward, winding up closer to his head than to his chest. Prestridge's arm, on the other hand, is still essentially at shoulder level. This is the most significant difference between the two deliveries.

With McCarthy's arm positioned like this, the head of his humerus is placed in an anatomically questionable position while his rotator cuff applies extreme compressive force at the glenohumeral joint, driving the humerus awkwardly into the scapula.

Prestridge's arm is in a more natural position at this point, and as a result, I do not view his mechanics as risky despite their on-the-surface similarity to McCarthy's old, problematic mechanics.

McCarthy and Prestridge after complete deceleration of the arm.
McCarthy and Prestridge during the recovery stage after their follow-throughs.

[Edit: For reference, here's a link to the video I shot of McCarthy at spring training in 2009.]


Draft Prospect: Tyler Matzek, LHP, Capistrano Valley HS

June 5, 2009 • Scouting

Unlike Matthew Purke, Tyler Matzek is routinely regarded as a pitcher with an easy, repeatable delivery. Matzek is able to throw 90-94 mph with ease and has recently been as high as 98 mph according to several reports. He throws a curveball that has plus potential, but scouts from the Major League Scouting Bureau have a lesser opinion of its current quality than Baseball America does.

Dr. Mike Marshall weighed in on Matzek in his Questions/Answers 2009 file. If you are at all familiar with Dr. Marshall's view of the 'traditional' pitching motion, you'll recognize that his comments border on praise. Dr. Marshall had the following to say:

Mr. Matzek's version of the 'traditional' baseball pitching motion is not as injurious as many I have seen... I doubt that he will suffer a serious pitching injury. However, he will never be as good as he should have been.

As with Purke, I was intrigued by the generic mechanical comments I'd read in scouting reports. This time, though, the comments were about an easy, repeatable delivery. The comments from Dr. Marshall further piqued my interest. I dug around and found a video similar to the one I used for my Purke article.

Here's the video I looked at, courtesy again of Baseball Factory:

Even though it's a warm-up pitch, I chose to use the first pitch in the video as my example. The stills in the two images below were taken from this same pitch.

Important stages of Matzek's arm action.
Important stages of Matzek's arm action. Click to view larger image.

In the first frame, you can clearly see that Matzek takes the ball primarily toward second base during his pick-up. If you looked at my review of Purke's mechanics, you'll remember that he took the ball primarily toward third base. Skipping ahead to the fourth frame (approximately the release point), you can see that Matzek has much less side-to-side movement than Purke. His driveline to the plate is very direct compared to most 'traditional' pitchers, extremely efficient.

The second frame shows Matzek's body position at foot-plant, in the 'traditional' cocked position, where he seems to have a nearly straight-forward stride. At 90° of flexion, the ulnar collateral ligament is at its most vulnerable. At this stage Matzek's arm is fairly straight, so the late forearm turnover and reverse forearm bounce that follow are less of a problem. This is part of why Dr. Marshall views Matzek's arm action as less injurious.

As Matzek increases his elbow flexion, he also tilts his shoulders to the glove side and raises his elbow. This action helps limit his forearm flyout by effectively straightening his elbow's path. At his approximate release point, you can see that his forearm is nearly vertical.

In the video Dr. Marshall reviewed (of a side session), he mentioned that he was concerned about Matzek's forearm flyout. The video that I reviewed seems to show that Matzek's forearm flyout is minimal.

Matzek's follow-through from release to finish.
Matzek's follow-through from release to finish. Click to view larger image.

The first frame in this image is from shortly after release. Matzek's elbow is flexed and his wrist is pronated to the point where his palm is nearly facing up. This indicates two things: (1) Matzek pronates his release very powerfully, and (2) Matzek may be using his latissimus dorsi to internally rotate his arm instead of using his pectoralis major to horizontally flex his arm.

At this normal frame rate, it is practically impossible to tell for certain whether or not Matzek pronates into release, but he sure appears to be doing so.

Dr. Marshall says that because Matzek's stride is "too closed," he must be using his pectoralis major to horizontally flex his pitching arm. If he were actually horizontally flexing his arm to throw the baseball, I don't believe that Matzek could achieve the arm position shown in the first frame of this image.

By the second frame, his primary arm deceleration phase is done. The continuation of his body action in the next two frames causes his arm to wrap across his body. I believe the appearance of recoil is an illusion created when Matzek stands up to field his position.

So... what are you saying?

Matzek throws some high-quality pitches from a relatively safe, easy, and repeatable delivery. From an objective perspective, there are fewer risk factors than most 'traditional' pitchers. For a high school pitcher, he's a lot more polished than I would have expected.

I have reason to believe that he throws three different fastballs and a pronated curveball. After doing my research, I like him more than I did, and I'm kind of upset that there's no chance he'll be around when the Texas Rangers pick at #14. I guess I can hope, though.

You might also want to check out dirtberry's YouTube channel for more on Tyler Matzek.


Draft Prospect: Matthew Purke, LHP, Klein HS

June 2, 2009 • Scouting

Spring, TX - a suburb of Houston - has produced quite a number of early-round draft picks over the past several years. It looks like Klein's Matthew Purke will join a list that includes former Klein players Josh Barfield, Chris George, and David Murphy, as well as Josh Beckett, Sam Demel, and Daryl Jones from Spring High School.

Purke stands at 6-foot-3 and weighs 180 pounds. Scouts like his projectability and believe he could add velocity as he gets stronger. He already sits at 92-94 MPH, occasionally throwing a tick or two harder. His primary off-speed pitch is a slurve-type offering called a slider by Baseball America and a curve by the Major League Scouting Bureau. Whatever you call it, it's one of the best breaking balls in this year's high school draft class.

Conflicting reports from the same two sources have his change up somewhere between "unknown" and "good."

Purke is believed to have a strong commitment to Texas Christian University in Fort Worth, so signability has become a concern for some clubs. Rumors of his signing bonus demands have ranged from $2 million to $7 million, but the most recent rumors have him in the neighborhood of $3 million.

Almost every report on Purke has mentioned something about questionable mechanics. Typically, high school deliveries are full of flaws and quirks, but rarely do these scouting reports go out of their way to mention them.

Baseball America mentions Purke's slinging action, saying that it is neither violent nor smooth. I guess that would be... Average? Typical? Expected? Outside of this specific mentioning of a flaw, I could only find generic references to his mechanics. I wanted to have a look for myself.

I located a solid video on YouTube, looked at it, and made some still photos. Here's the video, courtesy of Baseball Factory: [video has been made private and can no longer be embedded]

There are two big things that jump out at me, but first, I want to say that outside of his arm action, there really isn't much to complain about. He steps nearly straight forward, landing only a few inches closed.

Purke stays closed very well and gets great hip rotation and shoulder rotation. He even stands relatively tall through his release. His core does its job very well.

If I had to pick one part of his body action to complain about, it would have to be his somewhat stiff front leg. That might be the source of the reported inability to repeat his delivery consistently.

The still shots below were taken from the same pitch, the first one in the video.

Matthew Purke's arm action at four key points in his delivery.
Matthew Purke's arm action at four key points in his delivery. Click to enlarge.

Now, here are the flaws in his arm action as I see them. In the first frame, you can see that Purke's entire pitching arm has been moved about as far toward third base as possible. Skipping ahead to the final frame, you get an idea of how much horizontal acceleration takes place. When his arm finally starts moving forward, the centripetal force from the curved path results in forearm flyout. The Baseball America "slinging" comment is dead-on.

Pronated releases can help protect against the negative effects of forearm flyout, but there aren't enough frames available to be able to determine if, or to what extent, Purke pronates into his release.

The second frame shows a massive scapular load at foot plant. This puts extra stress on the anterior capsule of the shoulder, and the extra movement of the head of the humerus places the glenoid labrum at risk.

Still in the second frame, his forearm is past horizontal, avoiding an inverted arm position, but his forearm is almost 180° of external rotation from the throwing position shown in frame three. This causes a late forearm turnover.

Because his elbow is flexed to near 90° during his late forearm turnover, he experiences a pretty intense reverse forearm bounce which puts his ulnar collateral ligament at risk.

Purke's follow-through.
Purke's follow-through. Click to enlarge.

Here's an overly simplistic look at his follow-through. The first frame shows where his arm winds up after primary deceleration. His arm continues in the curved path and winds up finishing hard toward third base. There is no recoil evident at this frame rate, and his arm winds up tucked in softly at his waist.

There could be some extra stuff going on in his shoulder, but the standard 30 frames per second video does not reveal it.

So... what are you saying?

Overall, Purke's mechanics could be a lot worse. That said, I'm not a fan of his arm action at all. The slinging action reported by Baseball America is clearly present, and he puts a lot of torque on his elbow. Long term, he has almost no chance to stay healthy with these mechanics.

Any team that drafts Purke will have to ask themselves which risk they want to take: leave his mechanics alone and risk his arm falling apart - or - change his mechanics and risk his stuff dropping off.