Effects of twelve-week inspiratory muscle vs. plyometric training on pulmonary function and pupillary light reflex in youth middle-distance runners—a three-arm randomized controlled trial

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Background Inspiratory muscle training (IMT) and plyometric exercise are established conditioning modalities that elicit distinct physiological adaptations, yet their comparative effects on respiratory function and autonomic regulation in youth athletes remain unclear. The pupillary light reflex (PLR) provides a non-invasive index of autonomic activity and may offer new insight into training-induced adaptations. This study examined the effects of a 12-week IMT versus plyometric training program on pulmonary function and PLR dynamics in youth middle-distance runners. Methods Ninety youth middle-distance runners (aged 14–18 years) were randomly assigned to an IMT group (n = 30), a plyometric training group (n = 30), or a control group (n = 30). The prespecified primary outcome was forced expiratory volume in 1 second (FEV 1 ); secondary outcomes included other spirometry parameters (forced expiratory volume [FEV 1 ], forced vital capacity [FVC], forced expiratory volume/forced vital capacity [FEV 1 /FVC] ratio, peak expiratory flow [PEF], and maximal inspiratory pressure [MIP]) and PLR metrics (initial diameter, constriction velocity, maximum constriction, re-dilation velocity) served as secondary/exploratory outcomes. Assessments were conducted at baseline and 12 weeks (no interim timepoint was assessed). Of 90 randomized participants, 86 (95.6%) completed the study and were included in the complete-case primary analysis. Results IMT led to marked gains in FEV 1 (4.12 ± 0.23 to 4.45 ± 0.25 L, p < 0.001), FVC (4.89 ± 0.31 to 5.28 ± 0.33 L, p < 0.001), and PEF (628 ± 45 to 672 ± 48 L/min, p < 0.001). PLR constriction velocity also improved significantly (5.2 ± 0.8 to 6.1 ± 0.9 mm/s, p < 0.01). In comparison, plyometric training yielded modest but significant improvements in FEV 1 (4.08 ± 0.26 to 4.25 ± 0.28 L, p < 0.05) and increased PLR re-dilation velocity (3.1 ± 0.6 to 3.8 ± 0.7 mm/s, p < 0.05). No significant changes were observed in the control group. Conclusion In this complete-case analysis, IMT produced greater adjusted improvements in pulmonary function than plyometric training in youth middle-distance runners, while both modalities were associated with distinct changes in pupillary light reflex indices (constriction velocity with IMT; re-dilation velocity with plyometric training) that may reflect differential autonomic adaptation. As running performance was not assessed, these findings should be interpreted as physiological rather than performance evidence, and the potential value of combining both modalities warrants prospective testing.