Blood lactate during constant-load exercise at aerobic and anaerobic thresholds

Venous blood lactate concentrations [la_b] were measured every 30 s in five athletes performing prolonged exercise at three constant intensities: the aerobic threshold (Th_aer), the anaerobic threshold (Th_an) and at a work rate (IWR) intermediate between (Th_aer and Th_an. Measurements of oxygen consumption \(\left( {\dot V_{O_2 } } \right)\) and heart rate (HR) were made every min. Most of the subjects maintained constant intensity exercise for 45 min at Th_aer and IWR, but at Th_an none could exercise for more than 30 min. Relationships between variations in [la_b] and concomitant changes in \(\dot V_{O_2 } \) or HR were not statistically significant. Depending on the exercise intensity (Th_aer, IWR, or Th_an) several different patterns of change in [la_b] have been identified. Subjects did not necessarily show the same pattern at comparable exercise intensities. Averaging [la_b] as a function of relative exercise intensity masked spatial and temporal characteristics of individual curves so that a common pattern could not be discerned at any of the three exercise levels studied. The differences among the subjects are better described on individual [la_b] curves when sampling has been made at time intervals sufficiently small to resolve individual characteristics.