Pitching mechanics are a bit like long-snappers in football, in the sense that we hear about them only when something goes horribly wrong. Mechanics rarely enter the discussion until a pitcher gets hurt, but when an ace succumbs to injury, the village folk grab their torches and pitchforks to go on the hunt for blame.

Experience has taught me that there is rarely an isolated cause for a pitcher's injury, with confounding variables that include mechanics, conditioning, workloads, genetics, and plain old luck. The pitching delivery is a high-performance machine, with a multitude of moving parts that must work efficiently in concert for the system to perform at peak levels, and any weak link in the system can lead to a breakdown.

The crew at Baseball Prospectus has been at the forefront of injury research at the sabermetric level, with experts from Will Carroll to Corey Dawkins fighting the good fight to identify many of the risks associated with playing baseball. Among these discoveries is the concept of “cascade injuries,” a term coined by Carroll to describe the common scenario in which an athlete will alter their mechanics in order to compensate for an existing injury, touching off a ripple effect that leads to another injury elsewhere in the kinetic chain.

Carroll and Nate Silver pioneered an idea known as the “injury nexus,” which describes the phenomenon that pitchers under the age of 25 have a much greater risk of breaking down than their veteran counterparts. The heightened danger of the injury nexus could stem from the fact that these players are still growing at a time when their workloads are greatly increasing. Major-league teams have heeded the lessons of the injury nexus in recent years, monitoring the pitch counts and innings of pitchers until they reach physical baseball maturity.  

BP soldiers Keith Woolner and Rany Jazayerli came up with a technique to measure workloads, using the system of Pitcher Abuse Points (PAP) to quantify the labor imposed on arms throughout the league. The PAP system uses a baseline of 100 pitches and begins counting for each pitch that ventures past that boundary. The idea is that pitches are more taxing as a pitcher becomes fatigued, with the 100-pitch threshold serving as an approximation of when fatigue begins to set in. The system is an excellent proxy for relative workload, though it’s not perfect, as each individual player has a unique threshold for when fatigue truly sets in and that barrier is dynamic during the season. Randy Johnson may not have fatigued until he reached 120 pitches in his prime, while the exploits of Pedro Martinez past the 100-pitch mark are legendary.

The mechanical link to pitcher injuries is the main attraction, drawing attention away from most of the other risk factors when a great player goes under the knife. Sherlock Holmes will come out of the woodwork to point out the smoking gun behind the death of a pitcher, but consider me a conspiracy theorist who claims that the majority of pitcher injuries have multiple assailants. It may be more convenient to point fingers in a single direction, but a thorough investigation of an injury requires that we study all the evidence.

That said, there are a handful of mechanical patterns that have been identified as precursors to injury, including a pair of risk factors that we studied at the NPA. The first mechanical precursor that we found should be familiar to regular readers of Raising Aces, given the continual emphasis on postural instability. We discovered that pitchers with considerable spine tilt were more prone to arm injuries, and the research was taken a step further by our colleagues at ASMI, where Dr. Glenn Fleisig broke down both components of arm slot to determine the stress on the elbow. Fleisig found that peak elbow varus torque was impacted by both the angle of trunk tilt and the angle of shoulder abduction at release point, adding another injury precursor to the list and further damning the conventional coaching advice that says to “get on top of the ball.”

Another mechanical cue to injury risk is an elbow that drags behind the shoulder-line as the pitcher squares to the target, with the throwing arm laying back into maximum external rotation. A relatively new discovery, elbow-drag is difficult to see without hi-speed cameras, as the critical moment occurs in just a couple hundredths of a second. The elbow lag can result from improper timing, particularly from a hard-throwing pitcher who uses an excessive delay of trunk rotation to increase torque after foot strike. There’s a soft boundary between the velocity-related benefits of extra torque and creating so much delay that the arm fails to catch up to the rest of the body. The elbow lag often occurs in conjunction with a heavy scapular load, where a pitcher will effectively pinch the shoulders such that the elbows are positioned behind the shoulder-line when trunk rotation fires. The pictures below demonstrate the difference, with Cole Hamels (left) establishing a strong elbow position at maximum external rotation, but Chris Sale (right) demonstrating an elbow that drags behind the shoulder line.

Scapular loading is another technique that pitchers use to increase hip-shoulder separation and gain precious ticks on the radar gun, though it has also been identified as a risk factor for injury. The strategy is a sidekick to the modern-day legend of the “inverted W,” another potential precursor that has gained considerable traction among the all-consuming baseball masses. The inverted W describes a pitcher who raises his elbows above the shoulder line (for the biomechanics student, that's hyperabduction above the acromial line) as he hits foot strike. The I-W is thought to produce additional valgus stress on the elbow, particularly when trunk rotation triggers and the throwing arm transitions into maximum external rotation, putting the ulnar collateral ligament (UCL) at risk.

A scapular load is often paired with the inverted W, though pitchers such as Randy Johnson have displayed a massive scap-load while keeping the elbows below the shoulder-line. The jury is still out on the predictive value of the inverted W with respect to arm injuries, though the high-profile data points have been mounting with the likes of Adam Wainwright, Johan Santana, and Stephen Strasburg. The W might rear its ugly head at foot strike, but the risk occurs when the pitcher initiates the rotational elements of the delivery, and the physical raising of the elbows above the shoulder line is less harmful than the elbow-position at the start of trunk rotation. Not to keep picking on Chris Sale, but the young southpaw has a pronounced inverted W in his delivery (right), offering a stark contrast to the abduction angles of Greg Maddux (left) in the pictures below. 

The greatest risks occur during the arm acceleration phase of the delivery, beginning with trunk rotation through max external rotation and into peak velocity at release point, where joint integrity is tested at maximum intensity. The hardest throwers are at the greatest risk of injury, by virtue of the additional kinetic energy as well as the heavier workloads bestowed upon aces. Increasing the kinetic energy in the system is going to put more stress on the joints, and the key to healthy performance is to maximize mechanical efficiency and structural stability. The injury precursors that have been identified are merely indicators of risk, far from perfectly reliable, and they are vulnerable to the influence of surrounding chain-links, a point which is driven home by the frequent appearance of pitchers who display a scapular load that precipitates an inverted W, leading to elbow-drag and making it nearly impossible to isolate a singular cause of injury.


The Prior Strikes Back
Not to sound overly dramatic, but the injury history of Mark Prior is this generation's most frustrating example of conventional wisdom gone awry.

After being selected second overall in the 2001 draft, the right-handed phenom needed just nine starts in the minors before he was mowing down hitters in the Show. Prior dominated immediately, but a career that started on the fast track to the Hall of Fame was derailed by injuries before he could reach the next station. Prior's mechanics were labeled as flawless by anyone with a pair of eyes and a slant toward player evaluation, with a delivery that encapsulated the visual representation of scout-speak favorites such as “smooth” and “effortless.”  The disappointment that resulted from Prior's continued trips to the trainer's table created a cloud of judgment that hovered above the masses who had assumed that perfect pitching mechanics were an impenetrable shield to injury, and the public backlash was fueled by angst that was directed toward his supposedly impervious delivery.

Mark Prior had a slight inverted-W paired with a strong scapular load, and hindsight evaluations have honed in on these elements in a rush to blame Prior's mechanics for his injuries. Such theories have persisted despite the presence of several other well-known risk factors, with Prior toting red flags related to age, workload, and the cascade effect of injuries. In addition, any insistence on blaming his delivery completely ignores the fact that Prior's major injuries did not occur while throwing a pitch.

In July of 2003, Prior collided with Marcus Giles while running the bases, suffering a strain in his throwing shoulder that would keep him out of his first All-Star game. The Cub right-hander would miss only a few starts before returning to the mound in August, and he dominated over the season's final weeks. The Cubs were undeterred by any risk of cascade injury and rode their 22-year-old ace hard down the stretch, with Prior posting pitch counts of 131, 129, 109, 124, 131, and 133 over six September starts. Prior would toss another 133 pitches in the 2003 NLDS against the Braves and would exceed 115 pitches in each of his NLCS starts versus the Marlins.

All told, Mark Prior threw 235 innings in '03, violating Tom Verducci's “Year-After Effect” while sitting right in the thick of the injury nexus, putting in his most strenuous work at the end of his longest season as a pro after coming back from an arm injury during the high-pressure environment of a pennant run. Prior compiled the third-highest PAP total in baseball in '03, despite the missed starts and before counting the 43,000 PAP that he accrued in his playoff appearances.

Under such circumstances, it is a miracle that Prior pitched at all in 2004. The injuries started early, with a busted Achilles' tendon in spring training, an ailment that sidelined him until June. Prior was limited to just 21 starts and a disappointing stat line, though that can be understood if there were mitigating factors occurring inside his arm.

The wunderkind came back with a vengeance in 2005, cruising through the first several weeks until disaster struck again in late May, when a 100-mph line drive off the bat of Brad Hawpe struck Prior in the elbow, resulting in a compression fracture that put the right-hander back on the shelf. Ever resilient, the tenacious Cub came back once again in rapid fashion, taking just a one-month hiatus before making his return to Wrigley. Prior would finish the '05 season with the third-highest PAP in the league for the second time in three seasons, again having pitched a half-dozen fewer games than the other leaders, all while he was still climbing out of the injury nexus at the age of 24. 

A strained shoulder would crop up the following spring and delay his arrival until June, with continued shoulder woes ending his 2006 season after just nine forgettable mid-summer starts. The right shoulder has never been the same, and multiple surgeries have uncovered existing damage to Prior's prized pitching arm. A 2007 exploratory surgery by Dr. James Andrews revealed structural damage to the shoulder that may have existed for years, while a 2008 procedure addressed a torn anterior capsule in the shoulder, which is one of the toughest injuries to identify via MRI. Johan Santana is one of the first examples of a pitcher to successfully come back from the injury, and the diagnostic obstacles indicate that other pitchers may have unknowingly dealt with capsule tears that went undetected.

It has been nearly six years since Prior's last major-league pitch, and though we are left to guess at the root cause of his fragility, the sheer multitude of risk factors is nothing short of overwhelming. It leaves one wondering how Prior's pitching mechanics could possibly overshadow the usual suspects that are sitting right in front of us. Prior fell victim to virtually every known precursor, from extreme workloads during the injury nexus to traumatic collisions and injury cascades, yet the consensus view of his career is one of mechanical failure.

Many people are ignoring the facts in favor of selective reasoning to blame mechanics alone for Prior’s health struggles, despite these myriad risk factors that splatter the landscape of his injury-laced career. His mechanics GPA was off the chart, with elite marks in every category across the board: ideal balance and posture, excellent momentum, plus-plus torque, and amazing repetition of timing. It is certainly possible that mechanics contributed to Prior's injury risk. However, it is also a tribute to his mechanical efficiency that he was able to perform so well under such harsh conditions. The collective understanding of pitching has devolved thanks to the popular misconception that Mark Prior had bad mechanics.


“You have to be open-minded. Closed minds don't make progress.”—Nolan Ryan

I am an admitted skeptic of conventional wisdom, particularly those theories that have been widely accepted in the face of glaring evidence to the contrary. The issue takes on additional heat when discussing injuries, especially in a competitive environment where athletes constantly challenge their own physical boundaries in order to improve performance. A heavy scapular load might be harder on the body, but the potential reward will drive pitchers to put themselves at risk in order to gain a few ticks on the fastball. My unconventional advice for these pitchers is to buy a surfboard and hit the waves, as paddling is an ideal exercise for building the back-side shoulder muscles that are critical to scapular load. As we used to say at the NPA in San Diego, “we never had a bad-armed surfer.”

Pitching mechanics support function as well as safety, but the lines blur at the extremes of player performance. A pitcher might have exceptional mechanical efficiency that produces 100-mph fastballs on a consistent beeline to the target. However, the player will only last as long as his body can handle the kinetic toll.

Thank you for reading

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I thought the Verducci effect had been pretty comprehensively debunked - not to say that there isn't an issue with overuse of young pitchers, especially if the workload is increased too quickly - but the specifics of Verducci don't appear to stand up to much scrutiny.
The specifics of the Verducci effect are debatable, particularly the specificity of the 30-inning jump. That said, I do think a 68-inning jump at the age of 22 is noteworthy, especially when sandwiched around an arm injury.
Great stuff Doug. I enjoyed all of the links as well.
The discussions of pitching mechanics and the corresponding comparisons never seem to fascinate me. I actually watch games differently now. At live games (MLB, college and high school games) I will occassionally record pitchers from different angles to compare at a later date (for my own curiosity).
Great call on the different angles. I will walk all over the place when working with a pitcher in the bullpen in order to get a good look at each piece of the mechanics equation, and I will take strategic seats when evaluating a game performance.

The behind-the-plate POV is best for evaluating left-right balance into foot strike and for posture at release point, as well as hip-rotation and its contribution to torque. It is also the best angle for evaluating velocity, movement, and pitch trajectory from the hitter's POV.

The side view (behind 1B or 3B) is ideal for momentum, stride, release distance, front-back balance, and shoulder load. It is probably the best POV for assessing timing and repetition, as well.

An angle closer to 45-degrees (dugout POV) will give the most complete view of torque and the relative contributions of hips vs. shoulders. The ideal angle to measure torque is actually a bird's-eye view from above, but we will not be able to see those until they install NFL-style cameras that fly around the stadium.

The typical game feed is far from ideal for evaluating pitching mechanics, with shots coming from beyond the outfield wall at an off-centered angle. We can see a little bit of everything from that angle, but we can't get a great look at anything. Unfortunately, it is pretty much all we have to work with when it comes to what is available at our fingertips, which helps to explain why I tend to lean on pictures over GIF's in my articles unless I am trying to display a timing element or the results of a pitch.
"A relatively new discovery, elbow-drag is difficult to see without hi-speed cameras, as the critical moment occurs in just a couple hundredths of a second."

My hunch from studying pitching mechanics a bit in college, is that almost everything needs to be analyzed with a high speed camera because of that small time when things peak and scouting mechanics is going to be largely ineffective.

Great articles!
Your hunch is dead on - the higher frame-rate of video you have, the better you will be able to capture what is really happening in the delivery. Of course not everyone has access to hi-speed cameras, let alone 10-camera setups for 3-D analysis like we had at the NPA, and there is still much to be gleaned from regular 32-fps video.

The effectiveness of scouting mechanics depends largely on the eyes + mind of the one doing the scouting, though certain elements of mechanics are nearly impossible to evaluate when using just our eyeballs given the time-frame limitations that you mentioned - this is especially true for aspects such as peak torque or maximum external rotation.
Could you clarify what NPA is? Maybe it could get a glossary definition. Same goes for ASMI, which is easier to google. NPA comes up with a choice for National Parking Association, National Powerball Auctions, and Natural Products Association. Apologies in advance (AIA) if it is in the article and I missed it. I hope it stands for Notable Pitcher Abuse.
It's Nation Pitching Association.
Given that shoulder injuries are generally tougher to recover from than elbow injuries, are there any risk factors that you discuss in this article that are more likely to cause injury to the shoulder, or do the shoulder and elbow share the risk somewhat equally?
There is certainly crossover in the injury influence, due to the mechanism of the kinetic chain. Often times a pitcher is putting additional stress on both elbow and shoulder, but the structure that is structurally weakest is the one that will break down first.

The risk is not necessarily equal, and certain precursors do foretell more specific damage. Elbow-drag is an example that is pretty much elbow-specific, but the scapular load that often precipitates elbow-drag can also be a risk factor for shoulders (just ask Jake Peavy).

I think it is widely held that pitching has vastly improved over the past 10-20 years. Specifically when it comes to the amount of pitchers with elite velocity and stuff. How much of this can be attributed pitchers focusing more on conditioning and muscle strength?

However, does the extra strength/conditioning/flexibility lend itself to pitcher putting more stress on their arms because of the additional torque they are able to create due to this advanced fitness? Or does the added conditioning outweigh the injury risk caused by the additional torque?

I'd imagine this is different for every pitcher, but I'd like to hear your thoughts on it.

Basically, yes on all points.

The conditioning factor is somewhat of a double-edged sword - conditioning will better prepare the body to withstand the rigors of pitching, while at the same time enabling the body to perform at increasingly high levels kinetic energy. Conditioning essentially limits the underlying structural risk while increasing the peak-intensity risk.

I think the most dangerous aspect is the lack of balance in most pitchers' conditioning regimen, such that they might be conditioning certain muscles and joints in preference over others. A pitcher might train his body to throw harder through specific exercises, but a lack of muscular balance can create relative weak links in the kinetic chain. Think of a car, where you might tune an engine to generate higher RPM's, but the machine will fall apart if the belts are too weak to support the increased workload.

As you say, it differs for every pitcher, but the global trends are fascinating in their own rite. Great question.
Also, I have developed a morbid curiosity to see Chris Sale spontaneously combust on the mound like he was a drummer from Spinal Tap.
Truly fantasy article. The last sentence sums up everything perfectly.
"Under such circumstances, it is a miracle that Prior pitched at all in 2004."

Um what? If PAP was so perfectly predictive of next season injury, don't you think we would have seen a massive drop in pitcher injuries over the past 10 years?

Also, the "injury nexus" is, in my opinion, very likely to be a selection effect -- not an age-related effect. That is, certain physiologies can withstand chronic abuse better than others; naturally the differentiation won't show itself until sufficient abuse has been accumulated -- typically the pitcher's early 20s. The only ones who make in past that age intact are those with unusually resilient ligaments/muscles, etc. I think this drives most, if not all of the observed effect.
Please pardon my hyperbole in that line, and understand that I was referring to more than just PAP.

"All told, Mark Prior threw 235 innings in '03, violating Tom Verducci's “Year-After Effect” while sitting right in the thick of the injury nexus, putting in his most strenuous work at the end of his longest season as a pro after coming back from an arm injury during the high-pressure environment of a pennant run."

That is a lethal combination of factors, particularly when you consider what he did post-injury (i.e. risk of cascade). He was venturing into uncharted territory with innings and enduring his heaviest per-game workloads of the season, even before considering the age-related factors. Pitchers also have a tendency to kick things into high gear in high-pressure situations, and a 2nd-year player trying to carry his team into the playoffs would certainly qualify as high-pressure, and that's before you consider that this was a Cubs team that hadn't won the Series in 100 years.

You are certainly entitled to your opinion on the injury nexus, but I disagree with the absolute nature of your conclusion. There might be some selection bias, as you mentioned, and individual physiology certainly plays a role, but age is also a factor. Stats like PAP look at the collective population for individual trends, but of course they break down on an individual basis (hence Pedro v. Randy), and players develop physically at different rates. The injury stats do highlight general risk factors, and accumulating significant workloads during stages of physical development is one of those risk factors.

I am also curious as to what you consider to be "sufficient abuse," as I have seen plenty of amateur players that were completely abused long before reaching their 20's. Saying that "the only ones who make it past that age intact are those with unusually resilient ligaments/muscles" essentially assumes that all pitchers are enduring the same workloads, which is far from reality (especially at the high school and college levels). It also ignores the conditioning factor, which is an underlying part of the injury risk for any individual pitcher, with most amateur players having no idea how to properly train their bodies to throw a baseball.

If age is not a factor, then should we just tell 14-year olds to toss 110 pitches per game, with the hope that they have resilient enough ligaments/muscles to withstand such abuse?

"If age is not a factor, then should we just tell 14-year olds to toss 110 pitches per game, with the hope that they have resilient enough ligaments/muscles to withstand such abuse? "

This is completely misreading my comment. My comment says nothing about changing practices. It merely makes the observation that you have two possible explanatory variables, and your article is assuming that just one (age) is sufficient. Pitchers may be getting hurt in clusters around a certain age because:

1) There is something unique to that age that drives injury.
2) The majority of pitchers of that age have just recently received sufficient lifetime workload to cause body parts to break down for the first time.

My point is that there is nothing terribly special about the age; it's the cumulative arm use that matters.

Once a player has shown that he can survive long-term use without injury (by whatever exact measure used and by whatever age this may occur), it's reasonable to guess that he has technique or physiology that is better than average.

The only way to show that age alone is a significant factor would be to measure injury rates by age after controlling for lifetime workloads. I actually think that would be an interesting study that could yield useful fundings.
A little off topic, but you have got me slow-motioning a lot of pitchers on tv. Curious as to Morrow's grade on posture. I remember you also mentioning Weaver as a poor posture pitcher. What is his grade also?