{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,4,30]],"date-time":"2026-04-30T01:08:00Z","timestamp":1777511280249,"version":"3.51.4"},"reference-count":46,"publisher":"MDPI AG","issue":"4","license":[{"start":{"date-parts":[[2023,2,10]],"date-time":"2023-02-10T00:00:00Z","timestamp":1675987200000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Sensors"],"abstract":"<jats:p>Identifying exercise intensity boundaries has been shown to be important during endurance training for performance enhancement and rehabilitation. Unfortunately, even though surrogate markers show promise when assessed on a group level, substantial deviation from gold standards can be present in each individual. The aim of this study was to evaluate whether combining two surrogate intensity markers improved this agreement. Electrocardiogram (ECG) and gas exchange data were obtained from 21 participants who performed an incremental cycling ramp to exhaustion and evaluated for first (VT1) and second (VT2) ventilatory thresholds, heart rate (HR) variability (HRV), and ECG derived respiratory frequency (EDR). HRV thresholds (HRVT) were based on the non-linear index a1 of a Detrended Fluctuation Analysis (DFA a1) and EDR thresholds (EDRT) upon the second derivative of the sixth-order polynomial of EDR over time. The average of HRVT and EDRT HR was set as the combined threshold (Combo). Mean VT1 was reached at a HR of 141 \u00b1 15, HRVT1 at 152 \u00b1 14 (p &lt; 0.001), EDRT1 at 133 \u00b1 12 (p &lt; 0.001), and Combo1 at 140 \u00b1 13 (p = 0.36) bpm with Pearson\u2019s r of 0.83, 0.78, and 0.84, respectively, for comparisons to VT1. A Bland\u2013Altman analysis showed mean biases of 8.3 \u00b1 7.9, \u22128.3 \u00b1 9.5, and \u22121.7 \u00b1 8.3 bpm, respectively. A mean VT2 was reached at a HR of 165 \u00b1 13, HRVT2 at 167 \u00b1 10 (p = 0.89), EDRT2 at 164 \u00b1 14 (p = 0.36), and Combo2 at 164 \u00b1 13 (p = 0.59) bpm with Pearson\u2019s r of 0.58, 0.95, and 0.94, respectively, for comparisons to VT2. A Bland\u2013Altman analysis showed mean biases of \u22120.3 \u00b1 8.9, \u22121.0 \u00b1 4.6, and \u22120.6 \u00b1 4.6 bpm, respectively. Both the DFA a1 and EDR intensity thresholds based on HR taken individually had moderate agreement to targets derived through gas exchange measurements. By combining both non-invasive approaches, there was improved correlation, reduced bias, and limits of agreement to the respective corresponding HRs at VT1 and VT2.<\/jats:p>","DOI":"10.3390\/s23041973","type":"journal-article","created":{"date-parts":[[2023,2,10]],"date-time":"2023-02-10T02:09:59Z","timestamp":1675994999000},"page":"1973","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":12,"title":["Improved Estimation of Exercise Intensity Thresholds by Combining Dual Non-Invasive Biomarker Concepts: Correlation Properties of Heart Rate Variability and Respiratory Frequency"],"prefix":"10.3390","volume":"23","author":[{"ORCID":"https:\/\/orcid.org\/0000-0001-8458-4709","authenticated-orcid":false,"given":"Bruce","family":"Rogers","sequence":"first","affiliation":[{"name":"College of Medicine, University of Central Florida, 6850 Lake Nona Boulevard, Orlando, FL 32827-7408, USA"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-8330-8817","authenticated-orcid":false,"given":"Marcelle","family":"Schaffarczyk","sequence":"additional","affiliation":[{"name":"Interdisciplinary Institute of Exercise Science and Sports Medicine, MSH Medical School Hamburg, University of Applied Sciences and Medical University, Am Kaiserkai 1, 20457 Hamburg, Germany"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-5610-6013","authenticated-orcid":false,"given":"Thomas","family":"Gronwald","sequence":"additional","affiliation":[{"name":"Interdisciplinary Institute of Exercise Science and Sports Medicine, MSH Medical School Hamburg, University of Applied Sciences and Medical University, Am Kaiserkai 1, 20457 Hamburg, Germany"}]}],"member":"1968","published-online":{"date-parts":[[2023,2,10]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"1729","DOI":"10.1007\/s40279-020-01322-8","article-title":"An Examination and Critique of Current Methods to Determine Exercise Intensity","volume":"50","author":"Jamnick","year":"2020","journal-title":"Sports Med."},{"key":"ref_2","doi-asserted-by":"crossref","first-page":"2411","DOI":"10.1007\/s40279-021-01476-z","article-title":"Evaluating the Accuracy of Using Fixed Ranges of METs to Categorize Exertional Intensity in a Heterogeneous Group of Healthy Individuals: Implications for Cardiorespiratory Fitness and Health Outcomes","volume":"51","author":"Iannetta","year":"2021","journal-title":"Sports Med."},{"key":"ref_3","doi-asserted-by":"crossref","first-page":"466","DOI":"10.1249\/MSS.0000000000002147","article-title":"A Critical Evaluation of Current Methods for Exercise Prescription in Women and Men","volume":"52","author":"Iannetta","year":"2020","journal-title":"Med. Sci. Sports Exerc."},{"key":"ref_4","doi-asserted-by":"crossref","first-page":"323","DOI":"10.1152\/japplphysiol.00764.2021","article-title":"Neuromuscular and perceptual mechanisms of fatigue accompanying task failure in response to moderate-, heavy-, severe-, and extreme-intensity cycling","volume":"133","author":"Iannetta","year":"2022","journal-title":"J. Appl. Physiol."},{"key":"ref_5","doi-asserted-by":"crossref","first-page":"2481","DOI":"10.1111\/sms.13280","article-title":"Metabolic and performance-related consequences of exercising at and slightly above MLSS","volume":"28","author":"Iannetta","year":"2018","journal-title":"Scand. J. Med. Sci. Sports"},{"key":"ref_6","doi-asserted-by":"crossref","first-page":"1932","DOI":"10.1249\/MSS.0000000000000613","article-title":"Exercise Intensity Thresholds: Identifying the Boundaries of Sustainable Performance","volume":"47","author":"Keir","year":"2015","journal-title":"Med. Sci. Sports Exerc."},{"key":"ref_7","doi-asserted-by":"crossref","first-page":"592","DOI":"10.3389\/fphys.2019.00592","article-title":"Editorial: Training Intensity, Volume and Recovery Distribution Among Elite and Recreational Endurance Athletes","volume":"10","author":"Sperlich","year":"2019","journal-title":"Front. Physiol."},{"key":"ref_8","doi-asserted-by":"crossref","first-page":"337","DOI":"10.3389\/fphys.2017.00337","article-title":"Intensity- and Duration-Based Options to Regulate Endurance Training","volume":"8","author":"Hofmann","year":"2017","journal-title":"Front. Physiol."},{"key":"ref_9","doi-asserted-by":"crossref","first-page":"1208","DOI":"10.1249\/MSS.0b013e3182828ab2","article-title":"Determination of Maximal Lactate Steady State in Healthy Adults: Can NIRS help?","volume":"45","author":"Bellotti","year":"2013","journal-title":"Med. Sci. Sports Exerc."},{"key":"ref_10","doi-asserted-by":"crossref","first-page":"596567","DOI":"10.3389\/fphys.2020.596567","article-title":"A New Detection Method Defining the Aerobic Threshold for Endurance Exercise and Training Prescription Based on Fractal Correlation Properties of Heart Rate Variability","volume":"11","author":"Rogers","year":"2021","journal-title":"Front. Physiol."},{"key":"ref_11","doi-asserted-by":"crossref","unstructured":"Rogers, B., Giles, D., Draper, N., Mourot, L., and Gronwald, T. (2021). Detection of the Anaerobic Threshold in Endurance Sports: Validation of a New Method Using Correlation Properties of Heart Rate Variability. J. Funct. Morphol. Kinesiol., 6.","DOI":"10.3390\/jfmk6020038"},{"key":"ref_12","doi-asserted-by":"crossref","first-page":"2085","DOI":"10.1007\/s40279-022-01690-3","article-title":"An Examination and Critique of Subjective Methods to Determine Exercise Intensity: The Talk Test, Feeling Scale, and Rating of Perceived Exertion","volume":"52","author":"Bok","year":"2022","journal-title":"Sports Med."},{"key":"ref_13","doi-asserted-by":"crossref","first-page":"613","DOI":"10.1007\/s40279-013-0045-x","article-title":"Methods of prescribing relative exercise intensity: Physiological and practical considerations","volume":"43","author":"Mann","year":"2013","journal-title":"Sports Med."},{"key":"ref_14","doi-asserted-by":"crossref","first-page":"742","DOI":"10.1016\/j.jsams.2017.11.015","article-title":"Critical power: How different protocols and models affect its determination","volume":"21","author":"Maturana","year":"2018","journal-title":"J. Sci. Med. Sport"},{"key":"ref_15","doi-asserted-by":"crossref","first-page":"1029","DOI":"10.1123\/ijspp.2018-0572","article-title":"Is the Functional Threshold Power Interchangeable with the Maximal Lactate Steady State in Trained Cyclists?","volume":"14","year":"2019","journal-title":"Int. J. Sports Physiol. Perform."},{"key":"ref_16","doi-asserted-by":"crossref","unstructured":"Rogers, B., Giles, D., Draper, N., Mourot, L., and Gronwald, T. (2021). Influence of Artefact Correction and Recording Device Type on the Practical Application of a Non-Linear Heart Rate Variability Biomarker for Aerobic Threshold Determination. Sensors, 21.","DOI":"10.3390\/s21030821"},{"key":"ref_17","doi-asserted-by":"crossref","first-page":"463","DOI":"10.1139\/h04-031","article-title":"Principles, Techniques, and Limitations of Near Infrared Spectroscopy","volume":"29","author":"Ferrari","year":"2004","journal-title":"Can. J. Appl. Physiol."},{"key":"ref_18","doi-asserted-by":"crossref","first-page":"875","DOI":"10.1055\/a-1828-8499","article-title":"Validity and Reliability of Two Near-infrared Spectroscopy Devices to Measure Resting Hemoglobin in Elite Cyclists","volume":"43","author":"Javaloyes","year":"2022","journal-title":"Int. J. Sports Med."},{"key":"ref_19","doi-asserted-by":"crossref","first-page":"1841","DOI":"10.1097\/00005768-200111000-00007","article-title":"Validity and reliability of combining three methods to determine ventilatory threshold","volume":"33","author":"Gaskill","year":"2001","journal-title":"Med. Sci. Sports Exerc."},{"key":"ref_20","doi-asserted-by":"crossref","first-page":"879071","DOI":"10.3389\/fphys.2022.879071","article-title":"Fractal Correlation Properties of Heart Rate Variability as a Biomarker for Intensity Distribution and Training Prescription in Endurance Exercise: An Update","volume":"13","author":"Rogers","year":"2022","journal-title":"Front. Physiol."},{"key":"ref_21","doi-asserted-by":"crossref","first-page":"299","DOI":"10.1007\/s00421-022-05050-x","article-title":"Validation of a non-linear index of heart rate variability to determine aerobic and anaerobic thresholds during incremental cycling exercise in women","volume":"123","author":"Schaffarczyk","year":"2022","journal-title":"Eur. J. Appl. Physiol."},{"key":"ref_22","doi-asserted-by":"crossref","first-page":"301","DOI":"10.3389\/fphys.2017.00301","article-title":"Cardiac Autonomic Responses during Exercise and Post-exercise Recovery Using Heart Rate Variability and Systolic Time Intervals\u2014A Review","volume":"8","author":"Michael","year":"2017","journal-title":"Front. Physiol."},{"key":"ref_23","doi-asserted-by":"crossref","unstructured":"Rogers, B., Schaffarczyk, M., and Gronwald, T. (2022). Estimation of Respiratory Frequency in Women and Men by Kubios HRV Software Using the Polar H10 or Movesense Medical ECG Sensor during an Exercise Ramp. Sensors, 22.","DOI":"10.3390\/s22197156"},{"key":"ref_24","doi-asserted-by":"crossref","first-page":"1067","DOI":"10.1007\/s00421-011-2055-4","article-title":"Evidence of break-points in breathing pattern at the gas-exchange thresholds during incremental cycling in young, healthy subjects","volume":"112","author":"Cross","year":"2012","journal-title":"Eur. J. Appl. Physiol."},{"key":"ref_25","doi-asserted-by":"crossref","unstructured":"Garc\u00eda, D., Kontaxis, S., Hern\u00e1ndez-Vicente, A., Hernando, D., Milagro, J., Pueyo, E., Garatachea, N., Bailon, R., and L\u00e1zaro, J. (2021, January 13\u201315). Ventilatory Thresholds Estimation Based on ECG-derived Respiratory Rate. Proceedings of the 2021 Computing in Cardiology (CinC), Brno, Czech Republic.","DOI":"10.23919\/CinC53138.2021.9662701"},{"key":"ref_26","doi-asserted-by":"crossref","first-page":"5729","DOI":"10.1113\/JP274596","article-title":"The human ventilatory response to stress: Rate or depth?","volume":"595","author":"Tipton","year":"2017","journal-title":"J. Physiol."},{"key":"ref_27","first-page":"215","article-title":"Differential control of respiratory frequency and tidal volume during exercise","volume":"123","author":"Sacchetti","year":"2022","journal-title":"Eur. J. Appl. Physiol."},{"key":"ref_28","first-page":"164","article-title":"Higher Precision of Heart Rate Compared with VO2 to Predict Exercise Intensity in Endurance-Trained Runners","volume":"10","author":"Reis","year":"2011","journal-title":"J. Sports Sci. Med."},{"key":"ref_29","doi-asserted-by":"crossref","first-page":"726","DOI":"10.1097\/HJR.0b013e328304fed4","article-title":"Methodological approach to the first and second lactate threshold in incremental cardiopulmonary exercise testing","volume":"15","author":"Binder","year":"2008","journal-title":"Eur. J. Cardiovasc. Prev. Rehabil."},{"key":"ref_30","first-page":"614","article-title":"Biostatistics 104: Correlational analysis","volume":"44","author":"Chan","year":"2003","journal-title":"Singapore Med. J."},{"key":"ref_31","doi-asserted-by":"crossref","first-page":"284","DOI":"10.1037\/1040-3590.6.4.284","article-title":"Guidelines, criteria, and rules of thumb for evaluating normed and standardized assessment instruments in psychology","volume":"6","author":"Cicchetti","year":"1994","journal-title":"Psychol. Assess."},{"key":"ref_32","doi-asserted-by":"crossref","first-page":"135","DOI":"10.1177\/096228029900800204","article-title":"Measuring agreement in method comparison studies","volume":"8","author":"Bland","year":"1999","journal-title":"Stat. Methods Med. Res."},{"key":"ref_33","doi-asserted-by":"crossref","first-page":"143","DOI":"10.1111\/j.1440-1681.2009.05288.x","article-title":"Confidence in Altman-Bland plots: A critical review of the method of differences","volume":"37","author":"Ludbrook","year":"2010","journal-title":"Clin. Exp. Pharmacol. Physiol."},{"key":"ref_34","doi-asserted-by":"crossref","unstructured":"Mateo-March, M., Moya-Ram\u00f3n, M., Javaloyes, A., S\u00e1nchez-Mu\u00f1oz, C., and Clemente-Su\u00e1rez, V.J. (2022). Validity of Detrended Fluctuation Analysis of Heart Rate Variability to Determine Intensity Thresholds in Elite Cyclists. Eur. J. Sport Sci., 1\u20138.","DOI":"10.1080\/17461391.2022.2047228"},{"key":"ref_35","doi-asserted-by":"crossref","unstructured":"Rogers, B., Berk, S., and Gronwald, T. (2022). An Index of Non-Linear HRV as a Proxy of the Aerobic Threshold Based on Blood Lactate Concentration in Elite Triathletes. Sports, 10.","DOI":"10.51224\/SRXIV.124"},{"key":"ref_36","doi-asserted-by":"crossref","first-page":"550572","DOI":"10.3389\/fphys.2020.550572","article-title":"Fractal Correlation Properties of Heart Rate Variability: A New Biomarker for Intensity Distribution in Endurance Exercise and Training Prescription?","volume":"11","author":"Gronwald","year":"2020","journal-title":"Front. Physiol."},{"key":"ref_37","doi-asserted-by":"crossref","first-page":"611550","DOI":"10.3389\/fphys.2020.611550","article-title":"Network Physiology of Exercise: Vision and Perspectives","volume":"11","author":"Hristovski","year":"2020","journal-title":"Front. Physiol."},{"key":"ref_38","doi-asserted-by":"crossref","first-page":"24","DOI":"10.1016\/j.autneu.2016.06.003","article-title":"Cardiac vagal preganglionic neurones: An update","volume":"199","author":"Gourine","year":"2016","journal-title":"Auton. Neurosci."},{"key":"ref_39","doi-asserted-by":"crossref","first-page":"240","DOI":"10.3389\/fpubh.2017.00240","article-title":"Heart-Rate Variability\u2014More than Heart Beats?","volume":"5","author":"Ernst","year":"2017","journal-title":"Front. Public Health"},{"key":"ref_40","doi-asserted-by":"crossref","first-page":"959815","DOI":"10.3389\/fcvm.2022.959815","article-title":"Convergent cardiorespiratory neurons represent a significant portion of cardiac and respiratory neurons in the vagal ganglia","volume":"9","author":"Devarajan","year":"2022","journal-title":"Front. Cardiovasc. Med."},{"key":"ref_41","doi-asserted-by":"crossref","unstructured":"Rogers, B., Schaffarczyk, M., Clau\u00df, M., Mourot, L., and Gronwald, T. (2022). The Movesense Medical Sensor Chest Belt Device as Single Channel ECG for RR Interval Detection and HRV Analysis during Resting State and Incremental Exercise: A Cross-Sectional Validation Study. Sensors, 22.","DOI":"10.3390\/s22052032"},{"key":"ref_42","doi-asserted-by":"crossref","first-page":"1178","DOI":"10.1152\/jappl.1983.55.4.1178","article-title":"\u201cAnaerobic threshold\u201d: Problems of determination and validation","volume":"55","author":"Yeh","year":"1983","journal-title":"J. Appl. Physiol."},{"key":"ref_43","doi-asserted-by":"crossref","first-page":"237","DOI":"10.1007\/s40279-021-01581-z","article-title":"Identification of Non-Invasive Exercise Thresholds: Methods, Strategies, and an Online App","volume":"52","author":"Keir","year":"2022","journal-title":"Sports Med."},{"key":"ref_44","doi-asserted-by":"crossref","unstructured":"Pallar\u00e9s, J.G., Mor\u00e1n-Navarro, R., Ortega, J.F., Fern\u00e1ndez-El\u00edas, V.E., and Mora-Rodriguez, R. (2016). Validity and Reliability of Ventilatory and Blood Lactate Thresholds in Well-Trained Cyclists. PLoS ONE, 11.","DOI":"10.1371\/journal.pone.0163389"},{"key":"ref_45","doi-asserted-by":"crossref","first-page":"967","DOI":"10.1080\/02640410801910269","article-title":"Do regression-based computer algorithms for determining the ventilatory threshold agree?","volume":"26","author":"Ekkekakis","year":"2008","journal-title":"J. Sports Sci."},{"key":"ref_46","doi-asserted-by":"crossref","first-page":"469","DOI":"10.2165\/00007256-200939060-00003","article-title":"Lactate Threshold Concepts","volume":"39","author":"Faude","year":"2009","journal-title":"Sports Med."}],"container-title":["Sensors"],"original-title":[],"language":"en","link":[{"URL":"https:\/\/www.mdpi.com\/1424-8220\/23\/4\/1973\/pdf","content-type":"unspecified","content-version":"vor","intended-application":"similarity-checking"}],"deposited":{"date-parts":[[2025,10,10]],"date-time":"2025-10-10T18:29:36Z","timestamp":1760120976000},"score":1,"resource":{"primary":{"URL":"https:\/\/www.mdpi.com\/1424-8220\/23\/4\/1973"}},"subtitle":[],"short-title":[],"issued":{"date-parts":[[2023,2,10]]},"references-count":46,"journal-issue":{"issue":"4","published-online":{"date-parts":[[2023,2]]}},"alternative-id":["s23041973"],"URL":"https:\/\/doi.org\/10.3390\/s23041973","relation":{},"ISSN":["1424-8220"],"issn-type":[{"value":"1424-8220","type":"electronic"}],"subject":[],"published":{"date-parts":[[2023,2,10]]}}}