What 100 Pitches Actually Do to a High School Pitcher's Elbow

For anyone involved in youth baseball, the health of a pitcher's elbow is a constant concern. We've all heard the stories about players, especially youth players whose careers have been derailed in some way or ended due to a UCL injury. We know pitching is stressful, but a recent study in the International Journal of Sports Physical Therapy gives us an unprecedented look at the immediate, measurable problem this stress creates inside a high school pitcher's arm.

Using MSK ultrasound, researchers looked at the elbows of young athletes right after they threw 100 pitches, aiming documenting the toll of a single session. This article distills the three most impactful findings from this research. First, we will establish the clear and immediate findings: the physical changes that happen inside the elbow. Then, we will uncover the two most surprising revelations from the study, the deeper causes found in the shoulder and spine that explain how an athlete’s body fails, or succeeds, in managing that stress.

The Immediate Aftermath: What 100 Pitches Actually Does to an Elbow

In the study, 26 high school baseball players threw 100 pitches in a simulated session. Researchers used MSK ultrasound to take precise measurements of their throwing elbow right before and immediately after. The results established a clear problem:

• The Medial Joint Gap Widened: The space on the inner side of the elbow joint grew from an average of 3.93 mm to 4.77 mm.

• The Main Ligament Lost Stiffness: The ulnar collateral ligament (UCL), the crucial stabilizer replaced during Tommy John surgery, became significantly less stiff.

• The Supporting Muscles Also Weakened: The forearm flexor pronator muscles (FPMs), which act as dynamic stabilizers for the elbow, also showed a significant decrease in stiffness.

These findings quantify the acute stress of pitching. They are the measurable signs of an elbow pushed to its short-term limit. But the most important question the researchers asked was not what changed, but why.

It’s Not Your Elbow, It’s Your Shoulder?

Interestingly, when the researchers analyzed what predicted these changes, the answers began to point away from the elbow itself. While the loss of UCL stiffness was linked to the rotational force on the elbow (valgus torque), it was also independently predicted by a pitcher’s shoulder strength.

The study’s authors explain the mechanics behind this: when shoulder abductor strength is reduced, shoulder stability can’t be maintained during repetitive pitching. This increases the arm’s susceptibility to fatigue, leading to subtle but damaging changes in arm position that transfer even more load directly onto the UCL. Crucially, the study found that the elbow torque was a predictor only for the changes in the UCL, not for the other signs of fatigue like joint-gap widening or forearm muscle strain.

The study suggests that if shoulder stability isn't maintained, fatigue-related changes in a pitcher's mechanics may increase the load on the elbow joint, directly affecting the Ulnar Collateral Ligament.

Look to the Spine: Why Your Upper Back Health is Crucial for Your Forearm

The most profound discovery came when researchers investigated the cause of forearm muscle fatigue. The change in the forearm muscles was not linked to the torque on the elbow. Instead, it was significantly predicted by the pitcher’s thoracic spine.

The researchers found two red flags: 1) a pitcher’s rounded posture at the point of maximum arm layback, and 2) how much their spine had to move to get there from a resting stance.

This suggests that both poor static posture and a lack of dynamic mobility can overload the forearm. This discovery is even more significant when you recall that elbow torque wasn't a predictor of forearm muscle fatigue…. but spinal posture was.

When thoracic mobility is limited, the kinetic chain breaks down, and the demand shifts downstream to the smaller forearm muscles, which become overworked trying to stabilize the elbow. Think of the kinetic chain like a whip. To get a loud 'crack' at the tip (the hand), the movement must start with a powerful, fluid motion at the handle (the hips and spine). If the handle's movement is stiff or jerky, the whip itself will absorb damaging force. The same is true for a pitcher.

A Broader View of Arm Care

Here are a coupe of take home points and things to consider. Protecting a pitcher’s elbow requires comprehensive full body programming. While 100 pitches create immediate, measurable stress on the elbow, the body’s ability to withstand that stress is built in the trunk and shoulders. It’s no longer enough to focus on the elbow and shoulder in isolation. The evidence demonstrates that a pitcher's durability is built from the ground up, with spinal mobility and core stability serving as the true foundation for a healthy arm.

The question is not just if young athletes are strengthening their shoulders and backs, but why these programs are still seen as secondary to simply throwing more.

References:

1  Okamura S, Hattori H, Iida K, Sakaguchi K, Akasaka K. Changes in the Medial Elbow Joint During Repetitive Pitching and Related Factors in High School Baseball Players: An Analysis of Physical Characteristics Including Maximum Elbow Valgus Torque. Int J Sports Phys Ther. 2025;20(11):1574-1581. Published 2025 Nov 2. doi:10.26603/001c.146053