Inspiratory muscle training and cardiac autonomic function in healthy adults: a systematic review of acute and chronic responses.

BACKGROUND: Reduced heart rate variability (HRV) is an early marker of cardiac autonomic decline and predicts arrhythmia, sudden cardiac death, and heart failure. Inspiratory muscle training (IMT) is a low-cost, self-administered intervention that may modulate autonomic function, but prior reviews have focused on clinical populations or older adults. No synthesis has examined HRV responses to IMT in healthy adults or distinguished acute from chronic effects. This review evaluated the effects of acute and chronic IMT on HRV in healthy adults and explored dose-response relationships. METHODS: Six databases (PubMed, Scopus, Web of Science, MEDLINE, EMBASE, Dimensions) were searched from inception to April 2026. Controlled trials of IMT in healthy adults (≥18 years) reporting autonomic outcomes were included. Reporting followed PRISMA 2020; risk of bias was assessed with RoB 2 and ROBINS-I. Given substantial methodological heterogeneity, evidence was synthesized narratively (PROSPERO CRD420261352310). RESULTS: Thirteen trials (2013-2024; 10 independent cohorts; 214 participants, aged 18-80 years) were included, comprising six chronic (4-11 weeks) and seven acute reports. IMT modulated HRV in an intensity-, duration-, and population-dependent manner. Low-to-moderate loading (30%-60% MIP) was associated with increases in vagal-related HRV indices; however, these changes may partly reflect alterations in breathing pattern and respiratory mechanics. Changes were detectable from week 2 in chronic studies and were reversed following detraining. Loading above ∼60% MIP activated the inspiratory muscle metaboreflex and was associated with a relative shift toward higher normalized LF and lower HF power. The proximate mechanism appeared to be remodeled breathing pattern and enhanced cardiorespiratory coupling rather than altered baroreflex sensitivity. Certainty of evidence was very low across outcomes, reflecting small samples and a narrow demographic base. CONCLUSIONS: In healthy adults, IMT at 30%-60% MIP was associated with favorable changes in vagal-related HRV indices, although whether these reflect true vagal adaptation or altered respiratory mechanics remains unresolved. A protocol of 50% MIP, 30 breaths, five sessions weekly for ≥4 weeks, with standardized paced breathing during HRV assessment to minimize respiratory confounding, is proposed as a reference condition for future IMT-HRV trials. SYSTEMATIC REVIEW REGISTRATION: https://www.crd.york.ac.uk/PROSPERO/view/CRD420261352310, PROSPERO (CRD420261352310).