IntroductionInspiratory muscle training (IMT) protocols are typically performed using pressure threshold loading with inspirations initiated from residual volume (RV). We aimed to compare effects of three different IMT protocols on maximal inspiratory pressures (PImax) and maximal inspiratory flow (V̇Imax) at three different lung volumes. We hypothesized that threshold loading performed from functional residual capacity (FRC) or tapered flow resistive loading (initiated from RV) would improve inspiratory muscle function over a larger range of lung volumes in comparison with the standard protocol.Methods48 healthy volunteers (42% male, age: 48 ± 9 years, PImax: 110 ± 28%pred, [mean ± SD]) were randomly assigned to perform three daily IMT sessions of pressure threshold loading (either initiated from RV or from FRC) or tapered flow resistive loading (initiated from RV) for 4 weeks. Sessions consisted of 30 breaths against the highest tolerable load. Before and after the training period, PImax was measured at RV, FRC, and midway between FRC and total lung capacity (1/2 IC). V̇Imax was measured at the same lung volumes against a range of external threshold loads.ResultsWhile PImax increased significantly at RV and at FRC in the group performing the standard training protocol (pressure threshold loading from RV), it increased significantly at all lung volumes in the two other training groups (all p < 0.05). No significant changes in V̇Imax were observed in the group performing the standard protocol. Increases of V̇Imax were significantly larger at all lung volumes after tapered flow resistive loading, and at higher lung volumes (i.e., FRC and 1/2 IC) after pressure threshold loading from FRC in comparison with the standard protocol (all p < 0.05).ConclusionOnly training with tapered flow resistive loading and pressure threshold loading from functional residual capacity resulted in consistent improvements in respiratory muscle function at higher lung volumes, whereas improvements after the standard protocol (pressure threshold loading from residual volume) were restricted to gains in PImax at lower lung volumes. Further research is warranted to investigate whether these results can be confirmed in larger samples of both healthy subjects and patients.