Background <p>Extension-type low back pain (LBP) is common among baseball players, but immediate exercise responses may differ among athletes. Evidence on exercise-specific acute responses in adolescent athletes remains limited. This retrospective observational study examined immediate changes in trunk extension angle and pain following a five-exercise program and explored response patterns.</p> Methods <p>Annual medical checkup data from 6,105 male high school baseball players were retrospectively analyzed. Athletes reporting pain-related limitations (score 3 or 4) on a four-point LBP screening item (from no pain to inability to participate) underwent a two-step clinical examination; 54 athletes clinically diagnosed with extension-type LBP were included. The program included Sit-up Keep, Relax the Rectus Abdominis, Abdominal Draw-in Maneuver, Seated Trunk Rotation, and Hip Flexion Exercise. Trunk extension angle and relative pain intensity were assessed before the program and after each exercise. Relative pain intensity was assessed on an 11-point relative scale from 0 to 10, with 0 indicating no pain and 10 representing pre-program pain. Exploratory factor analysis and Ward’s hierarchical cluster analysis used exercise-specific trunk extension changes.</p> Results <p>Mean trunk extension angle increased from 60.7 ± 13.9° to 78.4 ± 16.0° after the exercise sequence. Mean relative pain intensity decreased from 10.0 to 2.4 ± 2.8. Exercise-specific analyses showed that the significant immediate responses differed by outcome, with Sit-up Keep showing the largest mean incremental change in trunk extension angle. Exploratory cluster analysis identified three response patterns: an Affected Area Response Pattern, a Non-Affected Area Response Pattern, and a Transversus Abdominis Response Pattern. Because the Transversus Abdominis Response Pattern included only four participants, this pattern should be interpreted cautiously.</p> Conclusions <p>A five-exercise program was associated with immediate within-session improvements in trunk extension angle and relative pain intensity. The exploratory response patterns suggest heterogeneity in acute functional responses and may support individualized assessment. Given the retrospective design, no control group, fixed exercise order, and unvalidated relative pain scale, the findings should not be interpreted as treatment efficacy. </p> Trial registration <p>Not applicable. This study was a retrospective analysis of data collected during annual medical checkups and was not registered as a clinical trial.</p>