Objectives: Throwing-related elbow injuries, particularly ulnar collateral ligament (UCL) tears, are highly prevalent among baseball pitchers and remain a major cause of time lost from sport. Pitch counts are the most widely used workload-limiting tool, yet they do not directly reflect physiologic fatigue or the function of dynamic stabilizers of the medial elbow. The flexor digitorum superficialis (FDS) is critically involved in finger pinch motion and has been shown in biomechanical and cadaveric studies to provide dynamic valgus stability by reducing medial elbow stress and protecting the UCL. Clinical observations further suggest that compromised FDS function may predispose pitchers to medial elbow injury. However, it remains unknown whether finger pinch strength changes during a single pitching bout, how these changes differ by age, and which physical or pitching-related factors are associated with pinch strength change. Clarifying these relationships may provide novel, objective markers of fatigue to complement pitch-count rules and contribute to UCL injury prevention.The Objectives of this study are to prospectively compare age-dependent changes in pulp and tip pinch strength during repetitive pitching among elementary, high school, and collegiate pitchers, and to explore which physical and pitching-related factors are associated with pinch strength change. Methods: A prospective laboratory study was conducted from 2021 to 2024 at Teikyo University, Tokyo, Japan. Fifteen elementary (mean age 12), fifteen high school (17), and ten collegiate pitchers (19) were enrolled. Elementary pitchers threw 70 fastballs, whereas older cohorts threw 100. Pulp and tip pinch strength (kgf) were measured at baseline and every 10 pitches using a pinch sensor (Sakai Medical Co., Tokyo, Japan). Muscle stiffness of the forearm and upper arm (mm/10), fatigue visual analog scale (VAS), and Borg scale were also assessed. Ball velocity and spin rate were measured for every pitch using the Rapsodo Pitching 2.0 system (Rapsodo Inc., St. Louis, MO, USA), and 10-pitch mean values were used for analysis.Percent changes from baseline were calculated, and group × pitch count interactions were analyzed using mixed-effects models. Exploratory correlation analyses were performed to examine associations between pinch strength change and baseline physical as well as pitching-related factors. Results: Pinch strength showed no significant change through 70 pitches in elementary pitchers. In contrast, high school pitchers demonstrated declines beginning at around 60 pitches, with more pronounced reductions beyond 80 (pulp −16%, tip −14% at 100). Collegiate pitchers exhibited the earliest and steepest declines, evident by 10-20 pitches (pulp −22%, tip −24% at 100). Group × pitch count interaction confirmed significant age-related differences (p < 0.01).Exploratory analyses demonstrated that ball velocity and spin rate were significantly correlated with pinch strength decline, whereas muscle stiffness (forearm and upper arm), fatigue VAS, and Borg scale were not. Specifically, higher ball velocity was associated with greater decline (pulp r = −0.61, p < 0.001; tip r = −0.49, p = 0.001), and higher spin rate was also associated with greater decline (pulp r = −0.64, p < 0.001; tip r = −0.50, p = 0.001). These findings suggest that pitchers capable of throwing faster pitches with greater spin are more likely to experience substantial pinch strength decline during a single outing. Conclusions: Pinch strength change during pitching is strongly age-dependent. Collegiate pitchers show early and continuous loss, high school pitchers demonstrate declines after 60-80 pitches, and elementary pitchers remain stable. Exploratory analyses identified ball velocity and spin rate, but not muscle stiffness, fatigue, or perceived exertion, as factors associated with pinch strength decline. Monitoring pinch strength-functionally linked to FDS activity-together with velocity and spin data provides a physiology-based adjunct to conventional pitch counts, enabling individualized fatigue assessment and offering novel strategies for UCL injury prevention in youth pitchers.
