The effect of inspiratory muscle training on the inspiratory muscle metaboreflex: A systematic review

Background: Increases in respiratory work and, consequently, the development of respiratory muscle fatigue can result in activation of the inspiratory muscle metaboreflex (IMM). This central nervous system response includes redirection of blood flow from skeletal muscles to respiratory muscles, which can limit exercise capacity. Therefore, developing approaches that may minimize the need for IMM is important. Recent studies have suggested that inspiratory muscle training (IMT) may reduce the IMM response. This review aims to synthesize studies that examine the efficacy of IMT in reducing IMM. Methods: Databases were systematically searched for randomized controlled clinical trials (RCTs) and non-randomized controlled trials evaluating the effect of IMT on the IMM. Searches were performed in MEDLINE/PubMed, SCOPUS, Web of Science and LILACS from inception to December 2023. Assessment of the methodological quality of the included studies was guided by the PEDro scale. Results: Four studies met the inclusion criteria, two RCTs and two non-randomized controlled trials, collectively including 76 subjects. Three studies included healthy subjects, and one included patients with heart failure. In all studies, IMT demonstrated a significant attenuating effect on the IMM, increasing inspiratory muscle strength and time of exercise tolerance (Tlim). Conclusion: Current evidence suggests that IMT may provide an effective approach for attenuating the IMM and increasing inspiratory muscle strength and Tlim. However, the small collective sample size and heterogeneity across studies limit current recommendations.