Modeling Tendon Adaptation Under High-Intensity, Short-Duration Loads in Baseball Pitching

Baseball pitching produces brief impulses of extremely high mechanical stress on the ulnar collateral ligament (UCL) and the surrounding flexor–pronator tendon complex. To quantify how these repeated loads influence tissue integrity, a high-level systems‑biology qualitative ODE model was developed that links mechanical stress, cumulative fatigue, viscoelastic relaxation, fibroblast activity, IL‑1β–mediated inflammation, and macrophage polarization to extracellular‑matrix (ECM) turnover. Using parameters derived from the literature and a representative workload of 100 pitches thrown over 2 hours, the model predicts (i) rapid ECM synthesis outpaces degradation in the early hours after a single outing, (ii) a transient peak in IL‑1β concentration (~2 × 10^-3 a.u.) after a moderate amount of time that coincides with a pro‑to‑anti‑inflammatory macrophage shift, and (iii) a saturation of cumulative fatigue after the first pitching session. These results highlight that recovery windows may be a dominant lever for maintaining tendon and ligament integrity, and they establish a qualitative foundation for real‑time workload management in pitchers.