Validation of the Dmax method as a predictor of lactate threshold

INTRODUCTION The concept of anaerobic threshold has been widely used to assess the endurance performance of athletes. Although there is no universally accepted definition of anaerobic threshold, it can be functionally thought of as being the highest exercise intensity at which a physiological steady state can be maintained during prolonged exercise (Stegmann et al., 1981). A number of researchers have put forward a variety of methods for identifying such an intensity from an incremental exercise test. However, the majority of such methods have had problems in regard to their validity, objectivity, reliability and individuality. One method that shows individuality and objectivity is the Dmax method put forward by Cheng et al. in 1992. Validation of this method for cyclists has not been undertaken, therefore, the aim of this study is to determine whether the Dmax is a valid indicator of lactated threshold in response to an incremental cycling exercise. METHODS Subjects. Fourteen male volunteers, aged between 18-30 years, who had at least 12 months cycle endurance training, participated in the study. They had a mean (+/-SD) VO2peak of 63.52+/-10.2 ml/kg/min and body mass of 75.24+/-6.03 kg. Protocol. All subjects completed a series of four exercise tests, including one incremental test till exhaustion and three prolonged exercise trials with constant workloads, on a Monark cycle ergometer. The incremental exercise test was performed to determine the VO2peak and the Dmax point. The workload started from 100 watts for 3 min, then increased by 30 watts every 3 min until volitional exhaustion. In the last 30 s of each workload, venous blood was sampled via a catheter inserted in the subject’s forearm. The blood sample was analysed for whole blood lactate concentration using a YSI-1500 Automatic Lactate Analyser. Expired air was collected and analysed on-line throughout the test for gas concentrations and pulmonary ventilation. The three prolonged exercise trials were performed on separate days with at least one day rest between each trial. The exercises were performed at a constant intensity of either at the Dmax (in watts), or 15 watts below or above the Dmax, respectively, for 30 min or until volitional exhaustion. Blood samples were taken at 5 min intervals for lactate analyses. The exercise intensities of the three trials were randomly assigned to each subject. Calculation of the Dmax. The method involved fitting the subject’s Lactate-Power output profile (from the incremental exercise test) to a third order polynomial curve, then plotting a straight line from the peak lactate concentration to the first lactate point that was 0.4 mmol/L above the resting level (a modification to Cheng et al., 1992). The point with the greatest perpendicular distance from this straight line to the lactate curve was defined as the Dmax, and the corresponding exercise intensity was subsequently determined. Statistics. Lactate concentrations were compared during the prolonged exercise trials and between prolonged exercise intensities using repeated measures Analysis of Variance with Bonferonni adjustments. RESULTS The major findings were that, for the prolonged exercise trial at the intensity of 15 watts above the Dmax, the blood lactate concentration at the end of the exercise was significantly higher than that at the 10th min of the exercise (p\u3c0.05), however, no such a lactate increase was found when exercising either at or 15 watts below the Dmax intensity. The lactate concentration over the last 15 min during the tests was significantly higher in the above-Dmax trial compared with that in the below-Dmax trial (p\u3c0.05). DISCUSSION Coaches and trainers require a valid and reliable method of predicting endurance performance from a single incremental exercise test. Upon that a significant increase in lactate concentration was found during the above-Dmax intensity trial, whilst a plateau of lactate concentration was maintained during the at- or below-Dmax trials, the Dmax appears to be a valid method to predict physiological steady state during such an endurance exercise. The significant lactate concentration difference found between the below- and above-Dmax trials, with only 30 watts difference in intensity, also indicates a good sensitivity of the Dmax method. Considering its individuality, objectivity, sensitivity and validity, the Dmax method could be used as an alternative predictor of lactate threshold. REFERENCES Cheng B., Kuipers H., Snyder A.C., Keizer H.A., Jeukendrup A. & Hesselink M. (1992). A new approach to the determination of ventilatory and lactate thresholds. Int. J. Sports Med. 13(7):518-522. Stegmann H., Kinderman W. & Schnabel A. (1981). Lactate kinetics and individual anaerobic threshold. Int J. Sports Med. 2: 160-165