The Injury Epidemic

More Arms Are Breaking Than Ever

In 2019, 32 MLB pitchers underwent UCL reconstruction (Tommy John surgery). In 2024, that number hit 40. Through the projection window for 2025, the count is tracking toward 44. The trend line has no bend in it.

Minor league numbers are worse. UCL surgeries across affiliated baseball topped 120 in 2024, up from 88 in 2019. The arms breaking are younger, throwing harder, and further from the big leagues when the ligament gives out.

Velocity and Ligament Tears Move Together

Average four-seam velocity has risen every year since 2008. So has the UCL surgery rate. The correlation is 0.78. That number alone should end the conversation about whether throwing harder causes more injuries. It does.

The biomechanics are straightforward. The UCL stabilizes the elbow during the arm acceleration phase of throwing. Higher velocity means more torque on the elbow. More torque means more stress on the ligament. At some point the tissue fails. The variation is in timing, not in whether it happens.

Sliders Are the Highest-Risk Pitch

Sliders generate more elbow valgus stress than any other pitch type. A 2023 study from the American Sports Medicine Institute measured the torque load by pitch and found sliders at 64 N·m, compared to 58 N·m for four-seam fastballs and 52 N·m for changeups. The difference is the combination of velocity and wrist pronation angle at release.

Slider usage league-wide went from 15% in 2015 to 25% in 2025. Sweeper-style sliders, which use even more horizontal wrist torque, added another 7% on top. The pitch that puts the most stress on elbows is also the pitch that keeps gaining usage share.

The incentive structure makes this predictable. Sliders produce a 34% whiff rate versus 23% for fastballs. They allow fewer hits, generate more strikeouts, and improve a pitcher's surface-level stats. Organizations reward the outcome. The elbow absorbs the cost.

Pitchers Throw Fewer Pitches but Break Down More

Average pitch count per start dropped from 97 in 2015 to 86 in 2025. Average innings per start fell from 6.0 to 5.3. By every traditional workload metric, pitchers are being managed more carefully than at any point in history. And they're getting hurt more often.

Total appearances are also up. Relievers throw fewer pitches per outing but appear more frequently. A reliever who throws 18 pitches three times a week at max effort accumulates more spike-stress events than a starter who throws 100 pitches once every five days. The recovery window shrank faster than the workload.

The Market Pays for the Behavior That Breaks Arms

Free agent pitchers who averaged 95+ mph received contracts worth an average of $18.2 million annually in the 2024-25 offseason. Those below 95 mph averaged $8.7 million. The velocity premium is a 2.1x multiplier. The market signal is unambiguous: throw harder, get paid more.

The 95+ mph starter earns twice as much and has a 28% UCL surgery rate. The below-92 starter earns half as much with a 16% surgery rate. From the pitcher's perspective, the math still favors throwing hard. The career earnings difference over 6 years dwarfs the cost of surgery and rehab. The incentive to push velocity remains rational at the individual level even when the collective outcome is an epidemic.

The Solutions Nobody Wants to Implement

Three interventions could reduce UCL injury rates. None are popular. First: limit slider usage to 20% of total pitches. This directly reduces the highest-stress pitch. Second: enforce minimum rest between relief appearances. Current rules allow pitchers to appear on consecutive days indefinitely. Third: cap developmental velocity targets. Organizations push prospects to add velocity because it predicts success. A ceiling on training intensity would slow the arms race.

The problem with all three is that they limit competitive advantage. No team will voluntarily reduce slider usage while opponents use theirs freely. No front office will rest its best reliever while the other team deploys theirs nightly. And no development program will cap velocity when the draft rewards it.

The Arms Race Has a Human Cost

Baseball optimized for strikeouts, velocity, and breaking-ball dominance. It got all three. It also got a 25% increase in torn ligaments, younger pitchers breaking down before they reach the majors, and a rehabilitation industry that has become as central to the sport as scouting.

The data connects the dots cleanly. Higher velocity correlates with more injuries (r = 0.78). Slider usage correlates with UCL stress. The market rewards the behaviors that produce the injuries. And the workload reductions that were supposed to protect pitchers didn't account for the intensity increases that accompanied them.

Jesse Walker

Jesse Walker writes about baseball through data. He played outfield in high school, found his real position behind a spreadsheet, and hasn't stopped building models since.