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Additionally, each athlete performed two submaximal 15-min outdoor runs at\u2009&gt;\u200970% of maximum heart rate, during which the smartwatch estimated V\u0307O\u2082\n                      <jats:sub>max<\/jats:sub>\n                      . Athletes were stratified into moderately trained (V\u0307O\u2082\n                      <jats:sub>max<\/jats:sub>\n                      \u2009\u2264\u200959.8 ml\u00b7min\u207b\n                      <jats:sup>1<\/jats:sup>\n                      \u00b7kg\u207b\n                      <jats:sup>1<\/jats:sup>\n                      ) and highly trained (V\u0307O\u2082\n                      <jats:sub>max<\/jats:sub>\n                      \u2009&gt;\u200959.8 ml\u00b7min\u207b\n                      <jats:sup>1<\/jats:sup>\n                      \u00b7kg\u207b\n                      <jats:sup>1<\/jats:sup>\n                      ) subgroups.\n                    <\/jats:p>\n                  <\/jats:sec>\n                  <jats:sec>\n                    <jats:title>Results<\/jats:title>\n                    <jats:p>\n                      Across all athletes, the smartwatch underestimated V\u0307O\u2082\n                      <jats:sub>max<\/jats:sub>\n                      [mean differences:\u2009\u2212\u20094.73 ml\u00b7min\u207b\n                      <jats:sup>1<\/jats:sup>\n                      \u00b7kg\u207b\n                      <jats:sup>1<\/jats:sup>\n                      (run 1), -4.05 ml\u00b7min\u207b\n                      <jats:sup>1<\/jats:sup>\n                      \u00b7kg\u207b\n                      <jats:sup>1<\/jats:sup>\n                      (run 2)]. Intraclass correlation coefficients (ICC) indicated moderate agreement between smartwatch and laboratory values (run 1: ICC\u2009=\u20090.71 [95% CI: 0.03\u20130.90]; run 2: ICC\u2009=\u20090.75 [95% CI: 0.17\u20130.91]), with mean absolute percentage errors (MAPE) of 7.9% and 7.2%. Subgroup analyses revealed better accuracy of smartwatch estimated V\u0307O\u2082\n                      <jats:sub>max<\/jats:sub>\n                      in moderately trained group (MAPE: 4.1\u20132.8%; ICC: 0.63\u20130.66 [95% CI: 0.09\u20130.87]), whereas in highly trained athletes, the smartwatch underestimated V\u0307O\u2082\n                      <jats:sub>max<\/jats:sub>\n                      by 6.3 ml\u00b7min\u207b\n                      <jats:sup>1<\/jats:sup>\n                      \u00b7kg\u207b\n                      <jats:sup>1<\/jats:sup>\n                      (MAPE: 10.4\u20139.4%; ICC: 0.34\u20130.41 [95% CI:\u2009\u2212\u20090.11\u20130.75]).\n                    <\/jats:p>\n                  <\/jats:sec>\n                  <jats:sec>\n                    <jats:title>Conclusion<\/jats:title>\n                    <jats:p>\n                      Smartwatch-derived V\u0307O\u2082\n                      <jats:sub>max<\/jats:sub>\n                      estimates are valid in moderately trained athletes but less valid in highly trained individuals. While smartwatches are useful for general monitoring, caution is warranted in their interpretation, particularly in highly trained individuals. Laboratory-based gas analysis remains the preferred method when precision is required.\n                    <\/jats:p>\n                  <\/jats:sec>","DOI":"10.1007\/s00421-025-05923-x","type":"journal-article","created":{"date-parts":[[2025,8,6]],"date-time":"2025-08-06T15:34:49Z","timestamp":1754494489000},"page":"591-603","update-policy":"https:\/\/doi.org\/10.1007\/springer_crossmark_policy","source":"Crossref","is-referenced-by-count":6,"title":["Validity of V\u0307O2max estimates from the forerunner 245 smartwatch in highly vs. moderately trained endurance athletes"],"prefix":"10.1007","volume":"126","author":[{"ORCID":"https:\/\/orcid.org\/0000-0003-0998-2277","authenticated-orcid":false,"given":"Florian A.","family":"Engel","sequence":"first","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Lukas","family":"Masur","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Billy","family":"Sperlich","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Peter","family":"D\u00fcking","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"297","published-online":{"date-parts":[[2025,8,6]]},"reference":[{"issue":"3","key":"5923_CR1","doi-asserted-by":"publisher","first-page":"67","DOI":"10.21315\/eimj2018.10.3.8","volume":"10","author":"WN Arifin","year":"2018","unstructured":"Arifin WN (2018) A web-based sample size calculator for reliability studies. 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