Acute effects of different inspiratory muscle training intensities on intercostal muscle oxygenation in healthy adults: A randomized crossover study

Background: Increased inspiratory loading increases oxygen consumption and metabolic demand in the respiratory muscles, affecting muscle oxygenation. However, evidence regarding different inspiratory muscle training (IMT) intensities on intercostal muscle oxygenation remains limited. This study compared acute effects of different IMT intensities on intercostal muscle oxygenation and cardiorespiratory responses. Methods: This randomized, single-blind, crossover study included 30 healthy individuals. Participants performed IMT at 0% (sham), 30% maximal inspiratory pressure (MIP) (endurance-based), and 70% MIP (strength-based) in randomized order. Intercostal muscle oxygen saturation (SmO) and total hemoglobin (THb) were assessed using near-infrared spectroscopy. Respiratory muscle strength (mouth pressure device), endurance (incremental threshold loading test), heart rate, peripheral oxygen saturation [pulse oximeter (SpO)] and dyspnea and fatigue (Modified Borg Scale) were evaluated. Results: No significant difference was observed in resting SmO among the training intensities (p=0.337). Minimum SmO, maximum SmO, and ΔSmO differed significantly according to training intensity (p<0.001). The 70% MIP protocol resulted in lower minimum and maximum SmO values and a greater ΔSmO than the sham and 30% MIP protocols. ΔTHb was also significantly higher during the 70% MIP protocol. ΔHR, perceived dyspnea, and fatigue were significantly greater during the 70% MIP protocol than during the sham and 30% MIP protocols (p<0.05). Conclusions: High-intensity IMT (70% MIP) induced greater intercostal muscle deoxygenation and increase in total hemoglobin than sham and endurance IMT. High-intensity loading also induced greater increases in heart rate, perceived dyspnea, and fatigue. These findings suggest that physiological responses to different IMT intensities should be considered when designing IMT programs.