What 86,000 Days on the Injured List Tell Us About Arm Angles (Slot)

Baseball has spent a decade trying to solve the arm. Pitch counts, innings limits, biomechanics labs, wearable sleeves that measure elbow torque in real time. And yet shoulder and elbow injuries have kept climbing. A study published in the Orthopaedic Journal of Sports Medicine on August 26th takes a different angle on the problem literally.

Researchers pulled Hawk-Eye tracking data on every MLB pitcher who threw at least 1,000 pitches between 2020 and 2025, 927 arms in total and measured each one's average arm angle at ball release. Then they matched those angles against injured list time for shoulder and elbow problems over the same six seasons.

The cohort accumulated 86,412 combined IL days: 25,710 from shoulders, 60,702 from elbows. And the pattern in that pile of missed time was consistent. For every one standard deviation a pitcher's arm slot moved toward overhand about 10.9 degrees injury burden rose. Combined shoulder-and-elbow burden went up 13%, shoulder burden 14%, elbow burden 12%. Those figures held after the researchers adjusted for total pitches thrown, share of games started, fastball velocity, and pitch quality. In other words, it wasn't simply that overhand pitchers throw harder or throw more.

The part that makes this hard to dismiss

Injury findings like this usually come with an implied trade-off: sure, it's safer, but you'll give up stuff. That's not what the data showed. Per standard deviation toward overhand, Stuff+ decreased by about 1.09 points, expected FIP rose 0.05, and hard-hit rate rose half a percentage point. The lower slots weren't just holding up better — they were, on average, grading out slightly better as pitchers.

One nuance worth holding onto: shoulder burden rose in a straight line with a more overhand slot, but elbow burden didn't. It was non-linear, peaking within the overhand group at roughly 45 to 57 degrees. Shoulder and elbow are not responding to arm angle the same way, and lumping them into "arm injuries" hides that.

How we use this at Rival

The first thing I'd say is what this study does not license: taking a healthy pitcher with a clean delivery and dropping his slot ten degrees. Mechanical overhauls carry their own injury risk during the transition, and a cohort study doesn't support that intervention.

What it does support is how we already think about throwers when they come through our doors:

• Arm slot is a screening variable, not a stylistic preference. When we build a risk profile for a throwing athlete, release angle sits alongside workload, velocity, and range of motion. It costs nothing to measure and it's now backed by the largest dataset we have.

• We don't treat a naturally low slot as a flaw to coach out. A lot of young pitchers get pushed "over the top" on the assumption that it's cleaner. We've never worked that way, and this is strong evidence that the assumption deserves scrutiny.

• We separate shoulder and elbow in monitoring. That non-linear elbow pattern means a pitcher sitting in the 45–57° band warrants different attention than one at 60°. Treating "arm health" as one number hides the thing you need to see.

• For a pitcher already exploring a lower slot for stuff, for platoon reasons, or coming back from injury this is evidence the change isn't a durability compromise. That's a conversation we have often, and now we have better data behind it.

• We screen more variables, such as internal vs. external rotation strength and range of motion. This will give us a better all around view of the athlete to know if they are REALLY prone to injury.

The honest limitations

This is a Level 3 cohort study, and association is not causation. The injury data came from public IL reports, which means diagnoses are broad categories rather than specific pathology, and some injuries were likely misclassified or missed entirely. Arm slot is a composite measure — it doesn't tell you which joint positions produced it. Submarine pitchers were excluded for small sample size. And there's a selection effect the study can't rule out: pitchers who reach and stay in MLB throwing sidearm may differ in ways that have nothing to do with the angle itself.

The authors say plainly that this warrants prospective study. That's the right call. But 927 pitchers and six seasons is a serious signal, and it points the opposite direction from how a lot of throwing has been coached.

This is the kind of thing we read every week, because the athletes we work with deserve a plan built on current evidence rather than on what everybody's always said about the overhand delivery.

If you're a throwing athlete in Northern New Jersey (or a parent of one) and you want an arm assessment that looks at mechanics, workload, capacity, strength and range of motion rather than just counting pitches, that's what we do at Rival Physical Therapy. We handle both sides of it: rehab when something's already wrong, and performance work to keep it from going wrong in the first place.

We've had athletes drive in from as far as Mount Olive (1hour away) to get it right. Reach out and we'll take a look.