Reproducibility of the blood lactate threshold, 4 mmol·l ^-1 marker, heart rate and ratings of perceived exertion during incremental treadmill exercise in humans

The aim of this study was to investigate the reproducibility of blood lactate measurements, heart rate (HR) and ratings of perceived exertion (RPE) during treadmill exercise at speeds corresponding to the lactate threshold (ν_Th,1a-) and a fixed blood lactate concentration of 4 mmol·l^-1(ν_1a-,4). Possible differences in reproducibility related to fitness levels were also investigated. A group of 20 men [mean (SD)] [age 20.5 (1.4) years] and 16 women [age 21.2 (0.9) years] took part in the study. The subjects performed two identical incremental exercise tests consisting of at least six 4 min stages. Blood lactate concentrations, HR and RPE were recorded at the end of each stage. Limits of agreement (LoA), correlation coefficients and 95% confidence intervals for the mean difference between tests were employed to investigate the level of agreement and reproducibility of blood lactate concentration, HR and RPE. For the group as a whole, the sample correlation coefficient for speed at ν_Th,1a- was r = 0.88, and was r = 0.92 for the speed at ν_1a-,4. At ν_Th,1a-, the correlation coefficients for the moderately fit and unfit were r = 0.94 and r = 0.36, respectively, and at ν_1a-,4 r = 0.93 and r = 0.68, respectively. The LoA for the moderately fit group indicated that a change of 1.62 km·h^-1 in ν_Th,1a-would be necessary to be considered a change in training status. For HR and RPE, relationships between the tests were generally poor. The LoA suggested that changes in scores must be unacceptably large. These findings cast doubt on the sensitivity of testing for change of blood lactate concentration, HR and RPE in this population.