Background: The second heart rate (HR) turn point has been extensively studied, however there are few studies determining the first HR turn point. Also, the use of mathematical and statistical models for determining changes in dynamic characteristics of physiological variables during an incremental cardiopulmonary test has been suggested. Objectives: To determine the first turn point by analysis of HR, surface electromyography (sEMG), and carbon dioxide output (VCO2) using two mathematical models and to compare the results to those of the visual method. Method: Ten sedentary middle-aged men (53.9±3.2 years old) were submitted to cardiopulmonary exercise testing on an electromagnetic cycle ergometer until exhaustion. Ventilatory variables, HR, and sEMG of the vastus lateralis were obtained in real time. Three methods were used to determine the first turn point: 1) visual analysis based on loss of parallelism between VCO2 and oxygen uptake (VO2); 2) the linear-linear model, based on fitting the curves to the set of VCO2 data (Lin-LinVCO2); 3) a bi-segmental linear regression of Hinkley's algorithm applied to HR (HMM-HR), VCO2 (HMM-VCO2), and sEMG data (HMM-RMS). Results: There were no differences between workload, HR, and ventilatory variable values at the first ventilatory turn point as determined by the five studied parameters (p>0.05). The Bland-Altman plot showed an even distribution of the visual analysis method with Lin-LinVCO2, HMM-HR, HMM-VCO2, and HMM-RMS. Conclusion: The proposed mathematical models were effective in determining the first turn point since they detected the linear pattern change and the deflection point of VCO2, HR responses, and sEMG.176614622Wasserman, K., Hansen, J.E., Sue, D., Whipp, B.J., Casaburi, R., (1999) Principles of exercise testing and interpretation, , 3rd ed. Philadelphia: Williams & WilkinsSkinner, J.S., McLellan, T.H., The transition from aerobic to anaerobic metabolism (1980) Res Q Exerc Sport., 51, pp. 234-248. , http://dx.doi.org/10.1080/02701367.1980.10609285Binder, R.K., Wonisch, M., Corra, U., Cohen-Solal, A., Vanhees, L., Saner, H., Methodological approach to the first and second lactate threshold in incremental cardiopulmonary exercise testing (2008) Eur J Cardiovasc Prev Rehabil., 15, pp. 726-734. , http://dx.doi.org/10.1097/HJR.0b013e328304fed4, PMid:19050438Beaver, W.L., Wasserman, K., Whipp, B.J., A new method for detecting anaerobic threshold by gas exchange (1986) J Appl Physiol., 60 (6), pp. 2020-2027. , PMid:3087938Wasserman, K., The anaerobic threshold measurement to evaluate exercise performance (1984) Am Rev Respir Dis., 129, pp. S35-S40. , PMid:6421216Soler, A.M., Folledo, M., Martins, L.E.B., Lima-Filho, E.C., Gallo Jr., L., Anaerobic threshold estimation by statistical modelling (1989) Braz J Med Biol Res., 22, pp. 795-797. , PMid:2620195Crescêncio, J.C., Martins, L.E., Murta, L.O., Antloga, C.M., Kozuki, R.T., Santos, M.D., Measurement of anaerobic threshold during dynamic exercise in healthy subjects: Comparison among visual analysis and mathematical models (2003) Comput Cardiol., 30, pp. 801-804Higa, M.N., Silva, E., Neves, V.F., Catai, A.M., Gallo Jr., L., Silva de Sá, M.F., Comparison of anaerobic threshold determined by visual and mathematical methods in healthy women (2007) Braz J Med Biol Res., 40 (4), pp. 501-508. , http://dx.doi.org/10.1590/S0100-879X2007000400008, PMid:17401493Hopker, J.G., Jobson, S.A., Pandit, J.J., Controversies in the physiological basis of the 'anaerobic threshold' and their implications for clinical cardiopulmonary exercise testing (2011) Anaesthesia., 66, pp. 111-123. , http://dx.doi.org/10.1111/j.1365-2044.2010.06604.x, PMid:21254986Conconi, F., Ferrari, M., Ziglio, P.G., Droghetti, P., Codeca, L., Determination of the anaerobic threshold by a noninvasive field test in runners (1982) J Appl Physiol., 52, pp. 869-873. , PMid:7085420Hofmann, P., Bunc, V., Leitner, H., Pokan, R., Gaisl, G., Heart rate threshold related to lactate turn point and steady-state exercise on a cycle ergometer (1994) Eur J Appl Physiol Occup Physiol., 69, pp. 132-139. , http://dx.doi.org/10.1007/BF00609405, PMid:7805667Hofmann, P., Von Duvillard, S.P., Seibert, F.J., Pokan, R., Wonisch, M., Lemura, L.M., %HRmax target heart rate is dependent on heart rate performance curve deflection (2001) Med Sci Sports Exerc., 33 (10), pp. 1726-1731. , http://dx.doi.org/10.1097/00005768-200110000-00017, PMid:11581558Hofmann, P., Wonisch, M., Pokan, R., Schwaberger, G., Smekal, G., Von Duvillard, S.P., Beta1-Adrenoceptor Mediated Origin of the Heart Rate Performance Curve Deflection (2005) Med Sci Sports Exerc., 37 (10), pp. 1704-1709. , http://dx.doi.org/10.1249/01.mss.0000176308.70316.cc, PMid:16260969Jürimäe, J., Von Duvillard, S.P., Mäestu, J., Cicchella, A., Purge, P., Ruosi, S., Aerobic-anaerobic transition intensity measured via EMG signals in athletes with different physical activity patterns (2007) Eur J Appl Physiol Occup Physiol., 101, pp. 341-346. , http://dx.doi.org/10.1007/s00421-007-0509-5, PMid:17624542Candotti, C.T., Loss, J.F., Melo, M.O., La Torre, M., Pasini, M., Dutra, L.A., Comparing the lactate and EMG thresholds of recreational cyclists during incremental pedaling exercise (2008) Can J Physiol Pharmacol., 86 (5), pp. 272-278. , http://dx.doi.org/10.1139/Y08-020, PMid:18432288Graef, J.L., Smith, A.E., Kendall, K.L., Walter, A.A., Moon, J.R., Lockwood, C.M., The relationships among endurance performance measures as estimated from VO2PEAK, ventilatory threshold, and electromyographic fatigue threshold: A relationship design (2008) Dyn Med., 7 (15), pp. 1-5. , http://dx.doi.org/10.1186/1476-5918-7-15Karlsson, J., Jacobs, I., Onset of blood lactate accumulation during muscular exercise as a threshold concept (1982) Int J Sports Med., 3, pp. 190-201. , http://dx.doi.org/10.1055/s-2008-1026087, PMid:6759424Al-Mulla, M.R., Sepulveda, F., Colley, M., A review of non-invasive techniques to detect and predict localized muscle fatigue (2011) Sensors., 11 (4), pp. 3545-3594. , http://dx.doi.org/10.3390/s110403545, PMid:22163810 PMCid:PMC3231314Hinkley, D.V., Inference about the intersection in two-phase regression (1969) Biometrika., 56 (3), pp. 495-504. , http://dx.doi.org/10.1093/biomet/56.3.495Silva, E., Catai, A.M., Trevelin, L.C., Guimarães, J.O., Silva Jr., L.P., Silva, L.M.P., Design of a computerized system to evaluate the cardiac function during dynamic exercise (1994) Phys Med Biol., 33, p. 409Basmajian, J.V., De Luca, C.J., (1985) Muscles alive: Their functions revealed by electromyography, , Baltimore: Williams & WilkinsJammes, Y., Caquelard, F., Badier, M., Correlation between surface electromyogram, oxygen uptake and blood lactate concentration during dynamic leg exercises (1998) Respir Physiol., 112, pp. 167-174. , http://dx.doi.org/10.1016/S0034-5687(98)00023-1Bland, J.M., Altman, D.G., Statistical methods for assessing agreement between two methods of clinical measurement (1986) Lancet., 1, pp. 307-310. , http://dx.doi.org/10.1016/S0140-6736(86)90837-8Lamarra, N., Variables, constants, and parameters: Clarifying the system structure (1990) Med Sci Sports Exerc., 22, pp. 88-95. , http://dx.doi.org/10.1249/00005768-199002000-00014, PMid:2304410Rosic, G., Pantovic, S., Niciforovic, J., Colovic, V., Rankovic, V., Obradovic, Z., Mathematical analysis of the heart rate performance curve during incremental exercise testing (2011) Acta Physiol Hung., 98 (1), pp. 59-70. , http://dx.doi.org/10.1556/APhysiol.98.2011.1.8, PMid:21388932Bunc, V., Hofmann, P., Leitner, H., Gaisl, G., Verification of the heart rate threshold (1995) Eur J Appl Physiol., 70, pp. 263-269. , http://dx.doi.org/10.1007/BF00238574Lucía, A., Vaquero, A.F., Pérez, M., Sánchez, V., Gómez, M.A., Chicharro, J.L., Electromyographic response to exercise in cardiac transplant patients (1997) Chest., 111 (6), pp. 1571-1576. , http://dx.doi.org/10.1378/chest.111.6.1571, PMid:9187176Glass, S.C., Knowlton, R.G., Sanjabi, P.B., Sullivan, J.J., Identifying the integrated electromyographic threshold using different muscles during incremental cycling exercise (1998) J Sports Med Phys Fitness., 38 (1), pp. 47-52. , PMid:9638032Bearden, S.E., Moffatt, R.J., Leg electromyography and the VO2-power relationship during bycicle ergometry (2001) Med Sci Sports Exerc., 33 (7), pp. 1241-1245. , http://dx.doi.org/10.1097/00005768-200107000-00025, PMid:11445775