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Factors that increase a pitcher’s injury risk: A focus on Sixto Sánchez

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Concerns about Sánchez’s velocity, overuse, and mechanics may be overblown

@sixto03_sanchez/Instagram

With the trade of J.T. Realmuto to the Philadelphia Phillies, the Miami Marlins received a player worthy of fans’ attention: Sixto Sánchez. Heading into the 2019 season, MLB.com’s MLB Pipeline ranks Sánchez as the 27th-best prospect in the minors, while Baseball Prospectus ranks him 23rd. He is widely regarded as the finest asset in the Marlins farm system.

Here at Fish Stripes, we already detailed why Sánchez is an exciting young prospect—his combination of velocity and pinpoint accuracy are special.

Yummy. However, concerns have been raised about his durability. In 2018, Sánchez only had eight appearances before being shut down in early June with right elbow inflammation.

For all his potential, is Sánchez at risk for an injury-plagued career? And for that matter, can we even identify pitchers at high risk for injury? Do we know ANYTHING?

The short answer? Determining pitcher injury risk is an inexact science. Thankfully, a growing body of research has begun to shed light on the root cause of shoulder and elbow injuries.

Below, I am going to take you all on a lovely journey through three potential aspects that could predispose Sánchez to injury. I’m going to be busting myths, raising some concerns, and at the end you will either love me or love me. You read that right. Let’s begin.

Pitching Velocity

Sánchez throws hard, like triple digits hard. Is that a concern?

Everyone who has been around baseball has heard something like this before: “X/Y/Z pitcher throws pretty hard. I’m worried about his elbow. Might as well call the doc now for Tommy John surgery, it is only a matter of time.”

Velocity is a hot-button issue in the pitcher injury world. In one FanGraphs post, Baseball Prospectus Milwaukee’s Julien Assouline used data from Baseball Prospectus and FanGraphs amassing a data set of close to 1,600 pitchers to look at how velocity relates to pitcher injury. In the article, Assouline groups pitchers into “velocity buckets,” determining the proportion of pitchers who received TJ surgery in each “bucket.” He concluded that a higher proportion of players who pitched faster received surgery.

Well, I have an objection to this type of analysis: it does not have an appropriate design to pinpoint velocity as the primary reason these pitchers needed Tommy John surgery. In scientific research, it is important to have a study population—in this case, “Pitchers who underwent Tommy John surgery”—matched to an appropriate comparison group (“Pitchers who did not undergo Tommy John surgery”). Pitchers in each group should be as physically similar as possible, except for the fact that one group received Tommy John surgery and the other did not. Groups should be similar in ways you might not even think about, including age, weight, mechanics, etc.

Brent Suter post-surgery
Photo by Stacy Revere/Getty Images

So, why do we try and match characteristics of study groups and comparison groups? In this analysis, failing to do so makes it more difficult to understand what is increasing injury risk. For example, rookie right-hander Michael Kopech and journeyman southpaw Brent Suter each required Tommy John late last season. Yes, they are on opposite ends of the fastball velo spectrum, but we cannot overlook the countless differences in their styles and pedigree.

Luckily for us, a different study published in 2016 in the Journal of Shoulder and Elbow Surgery actually asked the very question we are pondering: is velocity associated with risk of Tommy John injury? They used a Tommy John surgery group and a comparison group matched for pitcher weight, height, age, handedness, pitcher role (starter/reliever), MLB experience, and number of innings pitched. Our savior is here!!!

What did they find? MLB pitchers requiring Tommy John surgery did not pitch faster than healthy pitchers not requiring the surgery. However, they did find that pitchers requiring surgery pitched a higher percentage of fastballs overall (46.7% vs. 39.4%).

I tend to gravitate toward this study due to its effective design. It tells us that velocity alone is unlikely to be a major factor in predisposing a pitcher to injury. Still, the association with high fastball usage and risk for injury does raise some eyebrows. We can only speculate why this is the case. Could it be mechanics related, or does throwing hard at a certain frequency risk injury?

So, for everyone worried about Sánchez hitting triple digits, fear not. Let the kid throw!

Overuse at a young age

This section is going to be quick, but I do think it is worth mentioning that overuse in youth can increase a pitcher’s risk for injury. One 2011 study published in the American Journal of Sports Medicine found that of 481 children aged 9-14, those who threw greater than 100 pitches per year were 3.5 times more likely to be injured within 10 years.

Overuse increases the wear and tear on the elbow/shoulder. At a younger age, our bodies are more malleable, meaning that certain forces such as the repeated stress of throwing can even change a child’s anatomy.

Another research study found that 94% of youth pitchers they analyzed had radiographic evidence (such as X-ray, etc.) of anatomic changes to their elbow. It is possible that these types of anatomic changes could increase these pitchers’ risk for injury later in life.

Was Sánchez overused in his youth? That is a question we can’t answer. It may have contributed to his right elbow injury in June 2018. Definitely something to monitor, but for now I wouldn’t be too worried. Or...maybe do be worried? I can’t control you.

Pitching Mechanics

Now moving on to what I find to be the most interesting part of this discussion.

Back in 2016, I wrote two articles for Fish Stripes that remain relevant, one covering Wei-Yin Chen’s mechanics, and the other analyzing the mechanics of the late great José Fernández. I am going to steal from my Chen article because I think it has an accurate description of how pitching mechanics can predispose a player to injury:

Ideal pitching mechanics effectively transfer energy from the legs, to the torso, to the arm. Timing each movement perfectly is critical to minimize stress on the throwing shoulder/elbow. Research has shown that it is especially important to synchronize a pitcher’s trunk/torso rotation with the throwing arm. A 2015 study in the Orthopaedic Journal of Sports Medicine at Medstar Washington Hospital Center found that early trunk rotation, defined as “ trunk rotation before the stride foot hit the ground and occurrence of non-vertical arm position at initiation of trunk rotation,” increases risk of shoulder and/or elbow injuries in MLB pitchers.

Here is a sketch of early trunk rotation from the 2015 paper:

If the trunk begins to rotate toward home plate before the arm is vertical, there is an improper energy transfer into the throw. This can cause a pitcher to overthrow in order maintain velocity, which can be dangerous. It also causes rotational strain on the elbow and shoulder due to an excessive whipping motion of the non-vertical throwing arm, as it is forced to quickly move into a vertical position and explode through a throw.

Below is a five-panel snapshot of Sánchez’s mechanics. Let’s take a look to see if we think he has early trunk rotation:

We see some encouraging signs here, and I’ve labeled them in the third panel above. Sánchez’s arm position is vertical as his stride foot touches down, his shoulder is still raised, and perhaps most importantly, his trunk has not or only just begun to rotate towards home plate. These mechanics appear to effectively transfer energy from his legs to his torso to his arm.

Sánchez is not overly reliant on his arm to produce fastball velocity; he has an effectively coordinated sequence of movements that include his entire body. This should decrease the stress on his throwing shoulder and elbow. I like what I’m seeing!


Now, as I mentioned earlier, determining pitcher injury risk is an inexact science. It is difficult to predict whether Sánchez will suffer a major injury in the future, as many factors—some beyond the pitcher’s control, others still undiscovered—contribute to pitcher injuries.

However, in Sánchez’s case, I do think that the “velocity concern” is overblown. More importantly, it does appear that Sánchez’s delivery effectively minimizes stress on his throwing shoulder/arm. This indicates he may have long-term durability.

I get why many of us feel uncomfortable about Sánchez’s 2018 injury, and the fact that he only pitched eight games last year. I am bothered by that, too. What was the issue? Was it overuse, or is there something mechanical we are not seeing?

Simply put, these guys throw baseballs for a living. It’s an unnatural motion for the human body, and repeatedly doing it will always lead to wear and tear. Name me a pitcher who hasn’t suffered from elbow soreness in his career...I’m waiting.