Bart Van Trigt

In baseball pitching, high performance is closely related to injuries. The baseball pitch is a rapid, full-body throwing motion that culminates in a ballistic motion of the throwing arm, creating high ball velocity but exposing the elbow to significant loads. As a result, injuries to the medial side of the elbow involving the Ulnar Collateral Ligament (UCL) are currently a major concern in baseball pitchers at all levels of play. UCL injuries are recently prevalent among youth pitchers and injury rates have gradually increased over the years. It is important to prevent injuries in (youth) pitchers, not only to attain healthy pitching performance but also to avoid injuries at older ages. The general aim underlying the present dissertation is to establish biomechanical injury mechanisms related to the Ulnar Collateral Ligament in baseball pitchers. Knowledge of these mechanisms can eventually be used to develop an ‘early warning system’ to safeguard baseball pitchers from UCL injuries. This dissertation is divided into three parts. The single pitch In the first part, we aim to describe the relationship between the UCL properties and elbow stabilizers with the UCL loading during pitching. The elbow load during pitching is approximately 5Nm to 85Nm higher than the ultimate in-vitro (cadaver) UCL torque. This mismatch raises the question of why not every UCL is torn during a single pitch and thus why ‘only’ 16% of the pitchers sustain a UCL injury (Chapter 2). Underestimation of the effect of other structures in in-vivo studies is the most likely explanation for this mismatch because the calculated in-vivo external valgus torque also includes possible contributions of functional and structural stabilizers. The elbow muscles (functional stabilizers) show activity at the critical instant of elbow loading during pitching (Chapter 3). We conclude that the elbow muscles are important in shielding the UCL (structural stabilizer) during pitching and should be included when quantifying the UCL load. Repetitive pitching In the second part, we investigated the effect of repetitive pitching in relation to elbow load. The central question was: Why does one pitcher sustain a UCL injury, and another does not? A simple explanatory injury model was developed to illustrate the relationship between within-individual load variability and injury risk, illustrating how pitchers with a higher load variability are more likely to sustain an injury compared to pitchers with less load variability (Chapter 4). In addition, this model shows that all pitchers are at risk of sustaining an injury; higher load variability, higher magnitude, and longer exposure all increase this risk. Furthermore, the model comprises the (theoretical) effect of fatigue on load variability and injury threshold