Exercise scientists determine the lactate threshold heart rate of individual runners by having them run at incrementally increasing speeds on a treadmill until the blood lactate concentration is 4 mmol/L. The heart rate at this point is by definition the runner’s current lactate threshold heart rate. It’s not quite as precise as it sounds, though. A runner who goes through this test five days in a row might get a slightly different result each time, because the body’s physiological state changes from day to day.
