by James AJ Heathers
Ph.D. Cand., M.Sc., B.Ec.
Because low frequency power isn’t a good measure of ‘sympathetic activity’, and low/high frequency (LF/HF) power isn’t a good measure of ‘sympathovagal balance’. What we’re interested in as an index of recovery is a controlled measure of high frequency (HF) power.
Frequently with my own research, and about heart rate variability in general, people often ask me: “Do you measure LF/HF balance?” I do not. Here’s why:
Over a long enough period of time, anywhere from a few minutes upwards, heart rate isn’t stable. It exists in an environment of continual modification due to the inter-connections between spontaneously generated autonomic rhythms, the vasculature (blood vessels), breathing and any kind of imposed stress – exercise, focused attention, etc. This provides us with two dominant frequencies (two speeds of regular fluctuation in beat-to-beat variability), and because one is faster than the other, they’re called high and low frequency.
HF rhythms are mediated primarily by the vagus nerve’s innvervation of the sinoatrial node on the top of the heart. This reflects respiratory sinus arrhythmia, the process where breathing in increases heart rate, and breathing out decreases heart rate. And as the vagus nerve contains the primary parasympathetic outflow to the heart, this allows us to measure HF rhythm as a measure of parasympathetic dominance – the resting state of the autonomic nervous system.
LF rhythms are less well understood. Original research from the 80s proposed that that LF, proportional to HF, measured the state of autonomic balance in the heart. However, this research has been persistently misunderstood. The original papers used this in the context of a tilt-table test, not a normal standing or lying HR measurement. That is, to get their ‘sympathetic’ changes the researchers tilted the experimental participant on a flat table to various angles, and measured the relative HRV present.
This provides various degrees of postural stress, which is carefully compensated by the activity of the baroreflex – the body’s system for regulating blood pressure. Research since strongly supports the fact that LF power is best understood as measuring the outflow of the baroreflex response to regular changes in blood pressure. While this definitely involves the sympathetic nervous system – which controls both the size of the blood vessels and heart rate – it isn’t by any means a straightforward relationship where we can measure sympathetic activity at the heart by measuring LF HRV power.
Now, while LF power is useful for research and an interesting phenomenon, it isn’t much good for making determinations about your training state for several reasons.
Firstly, it’s poorly understood and there is a lot of debate surrounding the mechanisms involved. Also, at rest we are more concerned with the absence of parasympathetic activity than the presence of sympathetic activity. If you are overtrained, your primary concern is the absence of parasympathetic activity. While the autonomic nervous system is usually either dominated by one or the other, there are a variety of situations where this isn’t the case. Lastly, we can control the parasympathetic signal we measure by the simple task of breathing at a certain rate – this gives us a consistent situation in which to measure our HF activity over time.
As a consequence, the most informative and replicable measure of HRV in the short term is a paced breathing HF task. And that is what we use.
About the Author
James is doing a Ph.D in cardiovascular psychophysiology, improving HRV methods and applications. In his spare time, he lifts things up and puts them down.
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