Introduction: Placing a load on the chest during physical activity reduces efficiency and increases the work of the respiratory muscles. A change in the mechanics of respiration leads to accelerated fatigue of the respiratory muscles. The purpose of the research work was to evaluate the effects of inspiratory muscle strength training on physical performance while carrying loaded backpac in members of the Special Operations Unit of the Slovenian Army. Methods: Volunteers were divided into two groups by lot. The experimental group practiced continuously for six weeks with the POWERbreathe® device against incremental inspiratory resistance. The comparison group performed equal training protocol with sham inspiratory resistance. The initial and final evaluation of subjects comprised aerobic capacity assessment, respiratory volumes measurement, general fitness test, 60-minute walk test with a backpack, maximal respiratory muscle pressure tests before and after the walking test, and total and respiratory perceived effort assessment before and after the walking test,. All tests and measurements were repeated at after the training intervention. Results: After the intervention, the strength of inspiratory muscles statistically significantly increased in the experimental group both before (47 ± 13%) and after (58 ± 20%) the walking test, while strength of expiratory muscles increased only after the walking test (19 ± 18%). The fatigue of inspiratory muscles significantly decreased by 67% and of the expiratory muscles by 47%. No statistically significant changes in cardiovascular responses and perceived effort were observed in the experimental group. In the comparison group, no statistically significant changes were detected in any of the measured parameters. Discussion and conclusion: Exercise of the inspiratory respiratory muscles with the incremental inspiratory resistance improves strength of inspiratory muscles and reduces their fatigue, while it also has positive effect on capacity of expiratory muscles. We assume that potential changes in cardiovascular responses and percived effort during the walking test were not detected due to insuficient test intensity. Based on the results, we conclude that inspiratory muscles training is effective for improving respiratory muscle function in people with physically demanding occupations or those who perform intense recreational physical activity.
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