How is the Lactate Threshold Measured?
The lactate threshold is typically measured during a VO2 max test, with blood samples taken from a finger prick or from a catheter placed into a vein in the arm. The lactate threshold is defined as the speed (or VO2) at which the blood lactate concentration begins to increase exponentially (see Figure 1). In lieu of taking blood samples during the test, the ventilatory threshold, which is determined from changes in ventilation and respiratory gas samples, is often used to indicate the lactate threshold since there is a close relationship between the two thresholds—as the speed increases, the greater reliance on oxygen-independent metabolism increases the amount of carbon dioxide that is produced, which stimulates ventilation to expire the carbon dioxide.
More: A Lesson in Oxygen Intake and VO2 Max
Alternatively, the lactate threshold can be measured with a series of steady-state runs at increasing speeds, with blood samples taken at regular intervals during each run. At the slower speeds, lactate will increase slightly and plateau. When lactate threshold pace is reached, however, lactate keeps increasing rather than leveling off.
Lactate is measured in millimoles of lactate per liter of blood (mmol/L). The lactate threshold can be reported as a speed or pace or as a percentage of VO2 max. The higher the percentage the lactate threshold occurs in relation to VO2 max, the better the runner's endurance.
Figure 1 - The lactate threshold is the speed at which the blood-lactate
concentration begins to increase rapidly. A shift in the lactate curve to the
right, depicted by the dotted curve, represents an increase in the lactate threshold
(because it now occurs at a faster speed) and an improvement in endurance.
Knowing your VO2 max is not be enough to determine your fitness. For example, two runners may have similar VO2 max values but differ in their lactate thresholds. If Runner A and Runner B both have a VO2 max of 60 ml/kg/min, but Runner A's lactate threshold is 70 percent and Runner B's lactate threshold is 80 percent of VO2 max, Runner B can sustain a higher intensity and will beat Runner A. Also, two runners may have different VO2 max values but perform similarly due to differences in their lactate thresholds. If Runner X has a VO2 max of 60 ml/kg/min and a lactate threshold that is 67 percent of her VO2 max, and Runner Y has a VO2 max of 50 ml/kg/min and a lactate threshold that is 80 percent of her VO2 max, both Runner X and Runner Y will be able to sustain a similar intensity, assuming their running economy is equal (67 percent of 60 = 40 ml/kg/min vs. 80 percent of 50 = 40 ml/kg/min).
More: What Pace Should Runners Run Lactate-Threshold Workouts?
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