Inspiratory muscle training in the healthy adult: The relationship between load, perception, and oxygen consumption

Abstract

Background: Inspiratory muscle training (IMT) is used in a broad range of populations to improve the strength and endurance of the respiratory muscles, to improve both athletic performance and clinical outcomes. However, the optimal approach to IMT remains uncertain, and IMT is frequently declined in the clinical setting. This study aimed to measure oxygen consumption (VO2) and perceived difficulty and unpleasantness during commonly cited IMT loads.

Methods: Thirty participants performed IMT at 4cmH2O and 30%, 50% and 80% of their maximal inspiratory strength (PImax). VO2 was measured using indirect calorimetry. After each load, a visual analogue scale was used to rate breathing difficulty (VAS-D) and unpleasantness (VAS-U)

Results: Median (IQR) VO2 was 4.42 (3.36–4.82) mL/min/kg at baseline, increasing to 4.90 (4.11–5.03) mL/min/kg, 4.38 (3.69–5.23) mL/min/kg, 4.64 (4.09–5.28) mL/min/kg and (4.82–6.51) mL/min/kg after IMT at 4cmH2O and 30, 50 and 80% PImax respectively (Friedman's ANOVA p < 0.001). VO2 increased by 0.013 mL/kg/min for every 1% of PImax increase in IMT load. Perceived difficulty and unpleasantness increased with IMT load. PImax significantly influenced the load-perception relationship: slope (95% CI) of load versus VAS-D in the combined model 0.37 (0.09–0.65)mm/%PImax, p = 0.01), additional influence of baseline PImax 0.003 (0.001–0.005) mm/%PImax/cmH2O, p = 0.009.

Conclusions: IMT causes a load-dependent increase in VO2, with marked increases in breathing difficulty and unpleasantness at higher loads. The additional impact of the absolute magnitude of load provides insight into the perception of respiratory effort. These data help understand the factors that influence IMT prescription, in terms of exercise response and acceptability.