Technique for determining the lactate thresholds
Many commercially available computer programs are available for evaluating lactate curves. The power output is almost always used as the X-axis and the lactate concentration and heart rate as the Y-axis. You can choose between various lactate threshold concepts defined in the literature. However, the differences between the various concepts are very small and in view of the accuracy of lactate determination, the heated discussions about which concept is the best are somewhat exaggerated!
Definition of the thresholds used in the following:
Since lactate measurement has a certain error, and lactate concentration can even decrease at the beginning of exercise, the first lactate threshold (LT1) is defined as the heart rate at which lactate has increased by 0.2 mmol/l above the lowest measured lactate concentration (known as the Freiburg model). In the following, LT2 is defined, based on Dickhut, as the heart rate at which the blood lactate concentration is 1.5 mmol lactate/l higher than the lactate at LT1.
The figure above shows the intersection of the lactate thresholds (LT1 here in green, LT2 in red) with the watts vs. lactate curve, indicating the power output, which can be expressed as watts or speed, at which the lactate thresholds lie. The heart rate is first displayed on a secondary Y-axis. Then the heart rate that was reached at the lactate thresholds is sought out. This procedure makes particular sense if you primarily want to know how high the performance of a test person is at the lactate thresholds.
This makes sense for competitive athletes. For recreational or “health athletes”, however, the lactate test is primarily used for training planning. I consider training control via the heart rate to be much more effective than specifying what performance should be aimed for during training. That’s why I prefer to interpret the lactate curves using the heart rate as the x-axis! In this case, however, the lactate thresholds are no longer given as power (watts or running speed) but as the heart rate in beats/min (beats/min) at which the 1st or 2nd lactate threshold is reached.
As I have always found “dark calculations behind the lid of a computer” less than transparent, and we have always been interested in the heart rate at the lactate thresholds rather than the performance achieved, we have been evaluating the lactate tests ourselves in practice for decades. We only used Excel for this. A short guide is available here:
Practical approach:
Plot heart rate on the X-axis and lactate concentration on the Y-axis. Read the heart rate at the intersection points of the lactate thresholds with the lactate curve. The lactate thresholds are now ready in the form of heart rates at LT1 and LT2.
To display the lactate threshold concentrations in the diagram, the lactate concentration at LT1 and LT2 were each entered in a separate line at the lowest and highest heart rate in the table and inserted into the diagram as LT1 (green line) and LT2 (red line). It is advisable to insert the power (watts) on a secondary axis at the same time.
As you can see in the figure below, you have to interpolate the heart rates at the defined lactate concentrations from the figure; drop the perpendicular from the intersection point onto the x-axis. Since dropping a perpendicular onto the X-axis is sometimes not so easy in Excel (always the slightly slanted lines!), it is advisable to insert a rectangular square whose sides go through the intersection points of the lactate curve with the thresholds and whose intersection points with the X-axis result in LT1 and LT2 in beats/min.
Of course, it is also interesting to determine the performance at the time of the 2nd lactate threshold, LT2. To do this, a square is again inserted into the Excel figure, the left side of which meets the intersection of the lactate curve with LT2 (see below). If you now drop the perpendicular from the intersection of this side with the power curve onto the 2nd Y-axis, you can easily read off the power (e.g. in watts) at LT2. You can proceed in the same way if you are interested in the power at LT1.
