There is a comforting lie that lives inside every velocity reading. The lie goes like this: if the number on the gun is holding, the shoulder underneath it must be fine. We have built entire development models on that assumption. Throw hard, keep throwing hard, and as long as the velo does not drop, nothing is wrong. But what if the most important thing happening in a throwing shoulder is the thing that never shows up on the gun?

A 2019 study published in Medicine & Science in Sports & Exercise set out to answer a version of that question, and while it was done in tennis players rather than pitchers, the mechanics are close enough that every coach working with throwers should pay attention. Gillet and colleagues took 15 competitive male tennis players and used 25 minutes of electrical muscle stimulation to selectively fatigue one muscle, the lower trapezius. Then they had the athletes serve before and after, capturing humeral and scapular kinematics with motion capture and recording the activity of 13 shoulder muscles using a mix of surface and indwelling EMG. They were not interested in what happens when an athlete is globally exhausted. They wanted to know what happens when one specific link in the scapular control chain gets weak while everything else stays intact. That is a much more honest model of what we actually see in our athletes, because weakness is rarely whole-body. It is almost always local, specific, and quietly hidden.

What the Study Found

What happened next is the part of this study that should make every throwing coach a little uncomfortable, because the deficit the researchers created was both real and almost entirely invisible. The stimulation worked exactly as intended, dropping lower trapezius strength by 22.5%, which is a meaningful deficit, the kind of number that should set off alarm bells. And yet, when the players went back out to serve, their maximum racket speed barely moved (84.6 km/h before, 82.7 km/h after, with a p value of 0.19 that tells us the difference was statistical noise). The performance metric, the thing everyone watches, said nothing was wrong. If you had been standing there with a radar gun and a clipboard, you would have written down "no change" and sent the athlete back to work.

The motion capture told the same reassuring story at first glance. Humerothoracic joint kinematics, meaning the gross motion of the arm relative to the trunk, did not change. The arm still got where it needed to go. But the scapula, the foundation that arm is moving on, told a completely different story. Scapular upward rotation increased significantly during the acceleration phase (p=0.02) and again during early follow-through (p=0.01). The shoulder blade was compensating, tilting and rotating differently to keep the arm path looking normal. To be honest, this reminds me of watching a building settle on a weak foundation. From the street the walls still look straight, but somewhere underneath, the load is being redistributed in ways that nobody can see until something cracks.

The EMG data is where the real warning lives. As the lower trapezius fatigued, its activation dropped during early cocking, acceleration, and early follow-through, which you would expect. But it dragged friends down with it. During the acceleration phase, the anterior deltoid, pectoralis major, and subscapularis all reduced their activation too. These are not minor players. The subscapularis in particular is a primary stabilizer of the humeral head, and the anterior muscles are part of how the shoulder stays centered under enormous load. And here is the part that genuinely surprised me. The upper trapezius, the muscle we would most expect to barge in and take over, did not increase its activity at all. There was no rescue. The system did not recruit a backup. It simply absorbed the deficit and changed the way the scapula moved. The authors note that this combination, more scapular upward rotation paired with less anterior muscle support, could reduce the subacromial space and compromise humeral head stability. In plain language, the roof of the shoulder may be getting a little closer to the structures underneath it, right at the moment of maximum stress.

Why This Information Is Important

The reason this matters so much for pitchers is that we have anchored our entire monitoring system to the wrong signals. We watch velocity. We count pitches. We assume that if those numbers look fine, the athlete is fine. This study is a clean demonstration that an athlete can lose a fifth of the strength in a key stabilizer and keep producing the same output, with the cost paid quietly in altered mechanics rather than loudly in performance.

And this is not an isolated finding. To be honest, this reminds me of the work by Laughlin and colleagues in the American Journal of Sports Medicine, who studied pitchers with a history of SLAP repair. Those athletes had roughly 10 degrees less external rotation and about half the horizontal abduction of healthy controls, real and significant losses in their arm path, yet their ball velocity did not suffer at all. Velocity held while the underlying mechanics had clearly changed. Same lesson, different population. The radar gun is a wonderful tool for telling you how hard someone threw and a terrible tool for telling you how they did it or what it cost them.

Even fatigue itself shows up earlier and more subtly than our thresholds suggest. Johnson and colleagues, also in 2025, found that adolescent pitchers showed measurable losses in hip and torso mechanics, reduced torso rotation, smaller hip-to-shoulder separation, and altered sequencing, after roughly 35 to 40 pitches. That is well before any traditional pitch limit would flag a problem. Put these studies next to each other and a pattern emerges. The body is very, very good at hiding its deficits behind a normal-looking output, right up until it cannot.

How Can This Information Be Applied

So what do we actually do with this? The first and most obvious move is to start screening for lower trapezius strength rather than assuming it is fine because the velo is fine. If a 22.5% drop in this one muscle can reshape scapular mechanics without touching performance, then knowing where an athlete's lower trap strength sits is far more useful than any number on the gun for understanding their risk. This is the diagnostic mindset over the pass-fail mindset. The point is not to assign a grade. The point is to understand why the scapula moves the way it does for that specific athlete.

The second move is to train the whole upward rotation synergy, not just the lower trap in isolation. The study showed us that the system did not compensate cleanly when one link failed, which tells us the links matter individually. I do wish the researchers had measured serratus anterior activity, because I suspect the serratus may have been quietly picking up some of the slack to maintain upward rotation, and that synergy is exactly what we want to build deliberately rather than leave to chance.

The third move connects to how we manage the deceleration phase, and here the work of Ishigaki and colleagues is instructive. They found that after 100 full-effort pitches, internal rotation losses persisted for at least 24 hours, and that pitchers with higher arm speed suffered greater deficits, likely from the eccentric stress placed on the external rotators during deceleration. The follow-through is not a passive coast to the finish. It is a violent braking event, and the scapular stabilizers are part of the brake. If the lower trap is the muscle eccentrically controlling the scapula through that phase, and it is weak, the cost compounds throw after throw, season after season.

None of this means we abandon velocity or stop counting pitches. All training, and all monitoring, is stress management. The question is never whether a tool is good or bad, it is whether you understand what the tool is actually telling you and, just as importantly, what it is not. Velocity tells you about output. It says almost nothing about the integrity of the system producing that output.

Conclusion

The athlete in front of you can lose real strength in a key stabilizer, keep throwing the same speed, and still be walking a road toward a rotator cuff problem that no radar gun will ever warn you about. That is the uncomfortable takeaway here, and it is one I think we need to sit with. We have spent years optimizing the metrics we can see and ignoring the deficits we cannot. This study, done in tennis players but speaking directly to throwers, is a reminder that a normal-looking output is not the same as a healthy system. The compensation is the warning sign, and the compensation is invisible unless you go looking for it. So I will leave you with the same question the data leaves me with. You know how hard your athletes throw. Do you know how strong their lower traps are?

References

Gillet B, Rogowski I, Monga-Dubreuil E, Begon M. Lower Trapezius Weakness and Shoulder Complex Biomechanics during the Tennis Serve. Medicine & Science in Sports & Exercise. 2019;51(12):2531-2539.

Laughlin WA, Fleisig GS, Scillia AJ, Aune KT, Cain EL Jr, Dugas JR. Deficiencies in pitching biomechanics in baseball players with a history of superior labrum anterior-posterior repair. The American Journal of Sports Medicine. 2014.

Johnson AL, Kokott W, Dziuk C, Cross JA. Assessment of Muscular Fatigue on Hip and Torso Biomechanics in Adolescent Baseball Pitchers. 2025.

Ishigaki T, Kurisuga Y, Sato R, et al. Changes in glenohumeral range of motion by repetitive pitching and their relationship with arm speed during pitching. Sports Biomechanics. 2025.