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HRV was assessed from standardized supine morning recordings summarized as 7-day means at Baseline, Post-intervention, and Follow-up. Primary outcomes were lnRMSSD and lnHF, analyzed using linear mixed-effects models with autoregressive residual structure, condition-specific heterogeneous variances, and Holm-adjusted contrasts.<\/jats:p>\n                  <\/jats:sec>\n                  <jats:sec>\n                    <jats:title>Results<\/jats:title>\n                    <jats:p>\n                      Significant Condition \u00d7 Time interactions were observed for lnRMSSD (\u03c7\u00b2(2)\u2009=\u200916.27,\n                      <jats:italic>p<\/jats:italic>\n                      \u2009&lt;\u20090.001) and lnHF (\u03c7\u00b2(2)\u2009=\u200918.18,\n                      <jats:italic>p<\/jats:italic>\n                      \u2009&lt;\u20090.001), reflecting divergent autonomic trajectories across conditions. Within IHNT, lnRMSSD increased by +\u200913.6% (\n                      <jats:italic>p<\/jats:italic>\n                      \u2009&lt;\u20090.001) and lnHF by +\u200917.9% (\n                      <jats:italic>p<\/jats:italic>\n                      \u2009&lt;\u20090.001) Post-intervention, with partial retention at Follow-up (+\u20094.5% and +\u20097.0%, respectively). Sham showed significant lnHF decreases at Post (\u2212\u200920.5%) and Follow-up (\u2212\u200921.3%; both\n                      <jats:italic>p<\/jats:italic>\n                      \u2009=\u20090.030), while lnRMSSD remained stable. Pairwise between-condition contrasts did not reach significance after Holm\u2013Bonferroni adjustment. A baseline-adjusted sensitivity analysis confirmed the lnHF interaction (\u03c7\u00b2(1)\u2009=\u200912.04,\n                      <jats:italic>p<\/jats:italic>\n                      \u2009&lt;\u20090.001). Blinding was maintained (52.4% accuracy,\n                      <jats:italic>p<\/jats:italic>\n                      \u2009=\u20090.287).\n                    <\/jats:p>\n                  <\/jats:sec>\n                  <jats:sec>\n                    <jats:title>Conclusion<\/jats:title>\n                    <jats:p>IHNT at rest was associated with divergent cardiac autonomic trajectories versus sham in master cyclists, with consistent within-IHNT increases in parasympathetic indices and concurrent vagal withdrawal under sham. These proof-of-concept findings support a physiological signal warranting confirmation in adequately powered trials.<\/jats:p>\n                  <\/jats:sec>","DOI":"10.1007\/s00421-026-06280-z","type":"journal-article","created":{"date-parts":[[2026,5,29]],"date-time":"2026-05-29T12:20:55Z","timestamp":1780057255000},"page":"4615-4627","update-policy":"https:\/\/doi.org\/10.1007\/springer_crossmark_policy","source":"Crossref","is-referenced-by-count":0,"title":["Intermittent normobaric hypoxia at rest is associated with divergent cardiac autonomic trajectories in master cyclists"],"prefix":"10.1007","volume":"126","author":[{"ORCID":"https:\/\/orcid.org\/0000-0002-4805-7108","authenticated-orcid":false,"given":"Pedro","family":"Navarro-G\u00f3mez","sequence":"first","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"297","published-online":{"date-parts":[[2026,5,29]]},"reference":[{"key":"6280_CR1","doi-asserted-by":"publisher","DOI":"10.3389\/fphys.2020.01072","volume":"11","author":"MR Aebi","year":"2020","unstructured":"Aebi MR, Bourdillon N, Bron D, Millet GP (2020) Minimal influence of hypobaria on heart rate variability in hypoxia and normoxia. 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JAMA 333:71\u201374. https:\/\/doi.org\/10.1001\/jama.2024.21972","journal-title":"JAMA"}],"container-title":["European Journal of Applied Physiology"],"original-title":[],"language":"en","link":[{"URL":"https:\/\/link.springer.com\/content\/pdf\/10.1007\/s00421-026-06280-z.pdf","content-type":"application\/pdf","content-version":"vor","intended-application":"text-mining"},{"URL":"https:\/\/link.springer.com\/article\/10.1007\/s00421-026-06280-z","content-type":"text\/html","content-version":"vor","intended-application":"text-mining"},{"URL":"https:\/\/link.springer.com\/content\/pdf\/10.1007\/s00421-026-06280-z.pdf","content-type":"application\/pdf","content-version":"vor","intended-application":"similarity-checking"}],"deposited":{"date-parts":[[2026,8,7]],"date-time":"2026-08-07T12:21:45Z","timestamp":1786105305000},"score":1,"resource":{"primary":{"URL":"https:\/\/link.springer.com\/10.1007\/s00421-026-06280-z"}},"subtitle":[],"short-title":[],"issued":{"date-parts":[[2026,5,29]]},"references-count":48,"journal-issue":{"issue":"8","published-print":{"date-parts":[[2026,8]]}},"alternative-id":["6280"],"URL":"https:\/\/doi.org\/10.1007\/s00421-026-06280-z","relation":{},"ISSN":["1439-6319","1439-6327"],"issn-type":[{"value":"1439-6319","type":"print"},{"value":"1439-6327","type":"electronic"}],"subject":[],"published":{"date-parts":[[2026,5,29]]},"assertion":[{"value":"14 March 2026","order":1,"name":"received","label":"Received","group":{"name":"ArticleHistory","label":"Article History"}},{"value":"13 May 2026","order":2,"name":"accepted","label":"Accepted","group":{"name":"ArticleHistory","label":"Article History"}},{"value":"29 May 2026","order":3,"name":"first_online","label":"First Online","group":{"name":"ArticleHistory","label":"Article History"}},{"value":"The author declares no financial or non-financial conflicts of interest. The Longfian JAY-10H hypoxic generator and BCOxygen Oxysleep pulse oximeter were purchased by the institution at market price. The manufacturers had no involvement in study design, data collection, analysis, interpretation, or manuscript preparation.","order":1,"name":"Ethics","label":"Conflict of interest","group":{"name":"EthicsHeading","label":"Declarations"}},{"value":"During the preparation of this manuscript, the author used generative AI tools to assist with English language editing, writing support, and manuscript revision. The author reviewed and edited all content and takes full responsibility for the accuracy, integrity, and originality of the work.","order":2,"name":"Ethics","label":"Declaration of generative AI and AI-assisted technologies in the writing process","group":{"name":"EthicsHeading","label":"Declarations"}}]}}