HRV can be used in two ways in training: as a morning reading that tells you how well you have recovered, and as a measure of intensity during the session itself.
Morning reading
A short measurement right after waking, before you get up, gives the most comparable numbers, since few other things are affecting your heart rate at that point. Measure the same way, in the same position and at the same time every day. The most widely used measure is ln RMSSD, ideally together with resting heart rate.
How to read your morning reading
- Look at the trend over a week, not at single days. Day-to-day variation is normal.
- An ln RMSSD within your usual range, together with a normal resting heart rate, suggests you have recovered.
- If ln RMSSD is clearly lower than usual for several days in a row, or your resting heart rate is higher, an easier day may be wise.
- How you feel matters at least as much as the numbers.
DFA α1 and the aerobic threshold
DFA α1 (detrended fluctuation analysis) measures the pattern of the variation, not its size. During easy activity the value is around 1 or slightly above. As intensity increases, the variation becomes more random and the value falls.
Research has shown that a value around 0.75 roughly corresponds to the aerobic threshold – the transition from easy to moderate intensity. Values around 0.5 have been proposed as an approximate marker of the anaerobic threshold, but this is less well documented.
DFA α1 requires good-quality ECG. With optical measurement, poor ECG contact or ectopic beats, the value becomes unreliable.
My app Sport HRV shows both morning readings and DFA α1 during exercise.
References
- Rogers B, Giles D, Draper N, Hoos O, Gronwald T. A new detection method defining the aerobic threshold for endurance exercise and training prescription based on fractal correlation properties of heart rate variability. Front Physiol 2021; 11: 596567.
- Rogers B, Giles D, Draper N, Mourot L, Gronwald T. Detection of the anaerobic threshold in endurance sports: validation of a new method using correlation properties of heart rate variability. J Funct Morphol Kinesiol 2021; 6: 38.