Reliability of using the D-max method to define physiological responses to incremental exercise testing

The purpose of this study was to examine the reliability of using a mathematical method, D-max, to define blood lactate kinetics in response to an incremental exercise test, and to compare the physiological responses corresponding to the workload at D-max with those at the traditional 4 mmol l-1 lactate threshold and ventilatory thresholds. Ten male endurance trained athletes, with an average (+/- SD) age of 25.6 +/- 8.2 years and maximal oxygen consumption of 64.0 +/- 1.7 ml kg-1 min-1, performed an incremental cycling test on two occasions separated by four weeks. The expired gas was analysed on-line and plasma lactate concentration was analysed for each workload and at exhaustion. The lactate response to exercise was represented by a third-order polynomial regression curve. The D-max was defined as the point on the regression curve that yields the maximal distance to the straight line formed by the two end points of the curve. The results demonstrated a high test-retest reliability (intraclass correlation coefficients 0.77-0.93, p < 0.01) in oxygen consumption, heart rate and exercise intensity at both D-max point and exhaustion. No significant differences were found in the mean values of the variables between the two tests. It is concluded that the D-max appears to be a reliable method for defining the individual physiological responses to exercise tests, with the advantage of objectivity. However, there is no evidence to support the theory that the exercise intensity defined by the D-max method is superior to that defined by other methods to prescribe training intensity or predict aerobic performance for athletes. Further investigations are warranted to examine the validity of using this method in exercise prescription.