Effectiveness of Respiratory Muscles Training by Voluntary Isocapnic Hyperpnea Versus Inspiratory Threshold Loading on Intercostales and Vastus Lateralis Muscles Deoxygenation Induced by Exercise in Physically Active Adults

SIMPLE SUMMARY: Respiratory muscle training (RMT) improves physical performance through increased efficiency of the muscles implicated in respiration, an aspect that preserves the blood flow, nutrients, and oxygen supply to the locomotor muscles. Whether this effect depends on the RMT method has yet to be discovered. The RMTs most employed in healthy subjects are based on inspiratory Threshold-load training (ITL), which trains mainly inspiratory muscles, and voluntary isocapnic hyperpnea (VIH), which trains both inspiratory and expiratory muscles. With the emergence of non-invasive technology that allows the assessment of the balance between the oxygen muscle supply and consumption, or muscle deoxygenation, as a reflection of changes in the blood flow at the microvascular tissue level target, it is possible to contrast the effect of ITL and VIH on the deoxygenation of locomotor muscles (m. vastus lateralis) and respiratory muscles (m. intercostales) simultaneously during exercise. This study showed that the deoxygenation of the intercostal muscles decreased after eight weeks of RMT independent of the type of training, with no effect on the vastus lateralis deoxygenation. ABSTRACT: Respiratory muscle training (RMT) improves physical performance, although it is still debated whether this effect depends on the type of training. The purpose of this study was to compare the effects of two different types of RMT, i.e., voluntary isocapnic hyperpnea (VIH) and inspiratory threshold loading (ITL), on the deoxygenation of intercostal (ΔSmO(2)-m. intercostales) and vastus lateralis (ΔSmO(2)-m. vastus lateralis) muscles during exercise. Twenty-four participants performed eight weeks of RMT by: (i) VIH (3 days·week(−1) for 12 min at 60% maximal voluntary ventilation) or (ii) ITL (5 sets·week(−1) of 30 breaths·minute(−1) at 60% maximal inspiratory pressure). Cardiopulmonary exercise testing (CPET) included ΔSmO(2) (the change from baseline to end of test) of intercostal and vastus lateralis muscles. After RMT, both groups showed decreased ΔSmO(2)-m. intercostales (VIH = 12.8 ± 14.6%, p = 0.04 (effect size, ES = 0.59), and ITL = 8.4 ± 9.8%, p = 0.04 (ES = 0.48)), without a coincident change of ∆SmO(2-)m. vastus lateralis. ITL training induced higher [Formula: see text] O(2-peak) absolute values than VIH (mean Δ post–pre, ITL = 229 ± 254 mL·min(−1) [95% CI 67–391] vs. VIH, 39 ± 153 mL·min(−1) [95% CI −58–136.0], p = 0.01). In conclusion, both RMT improved the balance between supply and oxygen consumption levels of m. intercostales during CPET, with ITL also inducing an increase of aerobic capacity.