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Does HRV represent sympathetic nervous system activity?

This is an excerpt from Heart Rate Variability by Sylvain Laborde,Marco Altini,Emma Mosley,Dan Plews.

The misconception that HRV can directly reflect the activity of the sympathetic nervous system is widespread—and has found its way into many popular HRV apps, devices, and software platforms—as well as scientific literature still published today. Although you will read more about these tools later in the book (see chapter 4), this is an important point to highlight early on. One widely used platform, Kubios (Tarvainen et al., 2014), has played a major role in advancing HRV analysis in both scientific research and applied settings. However, one feature that warrants caution is the inclusion of a so-called sympathetic nervous system index, which attempts to estimate sympathetic activity using HRV-derived indices—specifically, mean heart rate, Baevsky’s stress index, and Poincaré plot SD2 (the standard deviation of the long-term or beat-to-beat RR interval variability along the line of identity in the Poincaré plot; the opposite of SD1, which reflects short-term variability) in normalized units.

Current scientific evidence does not support the idea that these—or any—HRV indices can accurately or uniquely reflect cardiac sympathetic activation (see Myth 1 for a detailed discussion and references to pharmacological blockade studies). This concern is not specific to Kubios; many other commercial platforms make similar claims that go beyond what HRV science currently supports. Kubios also presents a parasympathetic nervous system index that combines mean RR intervals, RMSSD, and SD1, the standard deviation of points perpendicular to the line of identity in the Poincaré plot (reflecting short-term variability, as opposed to SD2, representing long-term variability, see above). Although RMSSD is a well-established index of cardiac vagal activity, and SD1 essentially reflects the same information—both quantify short-term, beat-to-beat variability in RR intervals in slightly different mathematical forms (Ciccone et al., 2017)—it is confusing to see mean RR intervals placed in the parasympathetic nervous system index and mean heart rate in the sympathetic nervous system index. After all, heart rate (HR) and mean RR intervals are simply mathematical inverses of one another, linked by the formula HR = 60,000 / RR (with RR in milliseconds). This means that they carry exactly identical information, just expressed in different units, and that both reflect a mix of influences (Gordan et al., 2015) rather than uniquely sympathetic or parasympathetic activity. To be clear, we hold Kubios in high regard for its robust HRV signal processing and use it extensively in our own research. Our critique—which is relevant to any platform that attempts to infer sympathetic nervous activity from HRV alone—centers on two key issues: the overinterpretation of HRV as a marker of sympathetic activity and the use of composite indices (e.g., “stress score,” “health score,” “readiness score,” “recovery score,” “well-being score,” “sleep score”) whose physiological validity is questionable and may mislead users by implying a level of diagnostic precision that current scientific evidence does not justify (Doherty et al., 2025).

One of the key messages of this book is to favor well-established, physiologically grounded HRV indices, such as RMSSD, which reliably reflects cardiac vagal activity as indexed by pharmacological blockade studies (Laborde et al., 2017; Quigley et al., 2024). When it comes to assessing sympathetic function, more direct measures—such as impedance cardiography with pre-ejection period analysis—are required for valid interpretation.

This issue of misinterpreting HRV as a marker of sympathetic activity connects directly to two common myths we address in the following sections: first, the mistaken belief that HRV is a direct measure of stress; and second, the oversimplified notion that HRV reflects a quantifiable “sympathovagal balance.” Understanding these limitations is essential if we want to use HRV in a meaningful way—not by overinterpreting what it cannot tell us but by focusing on what it does well: giving us insight into how the body recovers, adapts, and responds to stress.

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