This study investigated whether the addition of eight weeks of inspiratory muscle training (IMT) to a regular preseason soccer training program, including incremental endurance training (IET), would change pulmonary function, lung ventilation, and aerobic performance in young soccer players. Sixteen club-level competitive junior soccer players (mean age 17.63 ± 0.48 years, height 182 ± 0.05 cm, body mass 68.88 ± 4.48 kg) participated in the study. Participants were randomly assigned into two groups: experimental (n = 8) and control (n = 8). Both groups performed regular preseason soccer training, including endurance workouts as IET. In addition to this training, the experimental group performed additional IMT for eigght weeks with a commercially available respiratory muscle trainer (Threshold IMT), with a total of 80 inhalations (twice per day, five days per week). Preand post-intervention tests of pulmonary function, maximal inspiratory pressure, and the Cooper test were implemented. Eight weeks of IMT had a positive impact on expiratory muscle strength (p = 0.001); however, there was no significant effect on respiratory function parameters. The results also indicate increased efficiency of the inspiratory muscles, contributing to an improvement in aerobic endurance, measured by VO 2 max estimated from running distance in the cardiorespiratory Cooper test (p < 0.005). Author Contributions: Conceptualisation, K.M., K.R.-P., M.C.; methodology, K.M., K.R.-P., M.K.; software, M.K., P.O.; validation, K.R.-P., M.K., J.S.; formal analysis, K.M., K.R.-P, M.C.; investigation, M.C., K.M., J.S.; resources, J.S.; data curation, K.R.-P., P.O., K.M.; writing-original draft preparation, K.R.-P., K.M., M.C.; writing-review and editing, K.R.-P., K.M., P.O.; visualisation, M.K., M.C., P.O.; supervision, KM., K.R.-P. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding.
Conflicts of Interest:The authors declare no conflict of interest. The results do not constitute endorsement of any product or device. The authors would like to thank the soccer players who participated in this study.