Stephen Strasburg at Ball Release

Stephen Strasburg at Ball Release

Before we get into the mechanical analysis and discussion on what causes UCL rupture, I want to visit a quote by the Nationals’ General Manager, Mike Rizzo:

Was Strasburg injured on one pitch or was this from wear and tear? Rizzo called the earlier MRIs on Strasburg “pristine,” then said:

“I listen to the opinion of the doctors and they feel it was probably a one-pitch event. The MRI we took [Thursday] was dramatically changed from the MRI we took when we drafted him.”

Yes, it’s true that the changeup that he threw was the pitch that caused the rupture in his ulnar collateral ligament, but it was not the only thing that caused the issue. Single pitches do not rupture ligaments and require Tommy John surgery – the UCL is torn microscopically over time in a flawed pitching delivery. Let’s please dispel this notion that a single pitch thrown incorrectly caused the injury.

What Causes Elbow Injuries?

To discuss Stephen Strasburg’s UCL rupture injury (and the injuries of every pitcher before him with a similar injury), we must understand what causes UCL rupture. Here’s how Medscape describes it:

The acceleration phase of the overhead throw causes the greatest amount of valgus stress to the elbow. Extension occurs at a rate of up to 2500º per second and continues to 20º of flexion. During this phase, the forearm lags behind the upper arm and generates valgus stress while the elbow is primarily dependent on the anterior band of the UCL for stability. During the acceleration phase, valgus stress can exceed 60 Newton meters (Nm), which is significantly higher than the measured strength of the UCL in cadavers. The valgus force can, therefore, overcome the tensile strength of the UCL and cause either chronic microscopic tears or acute rupture

In other words, as the forearm lays back in external rotation and reaches Maximum External Rotation (MER), valgus stress is pulling the bones in the elbow apart while the UCL stabilizes it. Over time, this can cause the UCL to tear and eventually rupture.

Another interesting phrase in that description is:

During the acceleration phase, valgus stress can exceed 60 Newton meters (Nm), which is significantly higher than the measured strength of the UCL in cadavers.

That’s right – the valgus stress that pitchers experience in their elbows often exceeds the measured strength of the UCL in cadavers! So, how is it possible to pitch without suffering traumatic injury every single time out there? In Biomechanics of the elbow during baseball pitching by Werner et al., they conclude that:

Triceps, wrist flexorpronator, and anconeus activity during peak valgus stress suggests that these muscles may act as dynamic stabilizers to assist the ulnar collateral ligament in preventing valgus extension overload.

The idea that the surrounding muscles can support the UCL and prevent injury is nothing new, but one that is often ignored by pitching coaches around the world. Strength training has a positive benefit on injury prevention as well as performance!

For more information on the ulnar collateral ligament, read Trip Somers’ post about it on his blog,

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Pitching Mechanics and Elbow Injuries

If elbow valgus stress is responsible for UCL damage, then it makes sense that we should seek to limit it when possible, right? Well, not necessarily. As with most complex systems, the neuromuscular system is not a single-input single-output machine. Reducing one variable that correlates with injury could theoretically reduce fastball velocity, and this wouldn’t necessarily be a good thing. For example: We know that torque around the shoulder increases the chance of injury but also increases fastball velocity, and vice versa.

A groundbreaking paper was published in 2002 titled Relationship between throwing mechanics and elbow valgus in professional baseball pitchers (Werner et al.) and it sought to correlate variables with elbow valgus. Here is the abstract of the paper (emphasis mine):

Valgus elbow stress leads to medial tension and lateral compression injuries in baseball pitchers of all ages. This study was undertaken to investigate the relationship between elbow stress in professional baseball pitchers and the kinematic parameters of pitching mechanics. This was done in an attempt to understand valgus extension overload better and in an effort to improve preventive and rehabilitative protocols. High-speed video data were collected on 40 professional pitchers in game situations during the 1998 and 1999 Cactus League season in Arizona, as part of Major League Baseball Spring Training. A multiple linear regression analysis was used to relate elbow valgus to kinematic parameters of pitching mechanics. The resulting analysis produced an adjusted multiple R2 value of 0.974, indicating that nearly 100% of the variance in valgus stress on the elbow was explained by the parameters in the regression equation. This ability to explain over 97% of the variance in valgus stress is significant. The parameters of pitching mechanics related to elbow valgus may be assessed and optimized, if necessary, in order to decrease the magnitude of elbow stress in pitching.

The variables that correlated with elbow valgus were:

  • Shoulder abduction angle at instant of stride foot contact (positive correlation)
  • Peak shoulder horizontal adduction angular velocity (positive correlation)
  • Elbow angle at instant of peak valgus torque (negative correlation)
  • Maximum shoulder external rotation torque (negative correlation)

Werner et al. measured thirty-seven (37) total variables, including ball velocity, max hip angular velocity, trunk tilt angle, and stride length. The four variables above were responsible for 97% of the variance in elbow valgus stress. Though cause and effect relationships can’t be made based on regression analyses, this is an excellent starting point when discussing how to reduce elbow valgus stress in pitchers. Note that ball velocity was NOT a significant variable!

Combining this study with my favorite research paper on baseball throwing – Humeral Torque in Professional Baseball Pitchers (Sabick et al.) – gives you a baseline of what specific mechanics can lead to high rates of elbow valgus stress and high rates of humeral loading. Let’s talk about one of those variables above, what it means, and how we can possibly alter someone’s mechanics to reduce elbow valgus stress.

Peak Shoulder Horizontal Adduction Angular Velocity

Shoulder horizontal adduction is best described as the action you perform when you are doing dumbbell flys:

DB Fly

The phrase “peak shoulder horizontal adduction angular velocity is correlated with elbow valgus stress” is a fancy way of saying that the faster the arm moves in that pattern, the more elbow valgus stress you get. So, what mechanical cues can increase this value? Well, “scapular loading” is one of them.

Mark Prior - Scapular Loading

Mark Prior – Scapular Loading

When the arm is taken behind the body in this position, it must return to the neutral/anterior portion of the trunk to deliver the ball to the plate. As a result, this will cause peak shoulder horizontal adduction angular velocity to increase, which causes elbow valgus to increase.

“Scapular loading” is a popular term first posited as a mechanical cue that would help improve fastball velocity by Paul Nyman of SETPRO. While he has since taken down his website, you can find this quote on it using The Wayback Machine (all typos his):

Fact: SETPRO was the first to sunderstand and show the difference between HORIZONTAL ADDUCTION or what SETPRO call “SCAPULA LOADING” and HYPEFLEXING. Almost all power pitchers SCAPULA LOAD. SCAPULA LOADING is vital to developing stretch reflex, storage of elastic energy and increasing the Range Of Motion (ROM) of the delivery.

Paul Nyman himself says that scapular loading is horizontal adduction (technically it’s horizontal abduction followed by fast horizontal adduction during the acceleration phase) and that he teaches it to all of his clients as it is vital to “developing stretch reflex.” That’s a story for another blog post, but the end result is absolutely clear: Scapular loading causes higher rates of peak shoulder horizontal adduction angular velocity and therefore causes greater elbow valgus, which we know is positively correlated with UCL rupture.

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Bringing it Back to Stephen Strasburg

As we described above, high rates of peak shoulder horizontal adduction angular velocity are correlated with elbow valgus and injury. One way to get these high rates is to intentionally force the elbows behind the body in a position of “scapular loading” so the arm must return from behind the body to the front of it to deliver the ball. Or, in a picture format…

Strasburg's Scapular Loading

Strasburg’s Scapular Loading

Yes, Strasburg has a lot of “scapular loading” going on in his delivery. Let’s turn to the animated GIF to see it in action:

Strasburg vs. Phillies

Strasburg vs. Phillies

I’m not saying that the large amount of “scapular loading” is solely responsible for Stephen Strasburg’s UCL rupture requiring Tommy John surgery, but it’s definitely one of the variables that contributed to high elbow valgus stress in his motion.

At Driveline Baseball, we use high-speed video to measure these kinematic variables to build our pitchers a better, more efficient, and safer delivery. We compiled over 6 years of research into our elite training guide, Hacking The Kinetic Chain.