Short, plain-English definitions of the terms that come up when you read about heart-rate variability, resonance frequency breathing, or autonomic nervous system function.
The part of your nervous system that controls involuntary processes — heart rate, digestion, pupil dilation, breathing when you're not thinking about it. Divided into the sympathetic (arousing) and parasympathetic (settling) branches. HRV is a non-invasive window on how it's balancing them.
A feedback loop in which stretch receptors in the carotid arteries and aortic arch detect blood pressure changes and signal the brainstem to adjust heart rate. Blood pressure up → heart rate down; pressure down → heart rate up. The loop's natural oscillation frequency sets your resonance frequency.
In HRV, the state where the heart-rate signal becomes narrow-band and sinusoidal — a single dominant rhythm instead of a mixture. Coherence peaks when breathing matches resonance frequency. Different methods (HeartMath, Shaffer six-criteria) quantify it differently, but they all measure how close the HRV signal is to a pure sine wave.
A direct electrical recording of the heart's activity, usually from chest electrodes. Gives the cleanest possible R-R intervals. Chest-strap heart-rate sensors (Polar H10, Garmin HRM series) are consumer-grade ECGs.
Spectral power in the HRV signal between 0.15 and 0.4 Hz. This band normally captures respiratory sinus arrhythmia at ordinary breathing rates (~9 – 24 breaths per minute). Interpreted as a relatively pure marker of vagal (parasympathetic) activity. When you breathe at 5 – 6 BPM, RSA shifts down into the LF band, which is why coherence breathing is typically scored from LF, not HF.
The fluctuation in the interval between consecutive heartbeats. Higher HRV generally reflects a more responsive autonomic nervous system. Not a single number — HRV can be measured in many ways (RMSSD, SDNN, frequency-domain power, non-linear measures) and each answers a different question.
Spectral power in the HRV signal between 0.04 and 0.15 Hz. This band reflects a mix of baroreflex activity and slower sympathetic/parasympathetic modulation. It's where the resonance peak appears during coherence breathing — 5.5 BPM corresponds to roughly 0.092 Hz.
The ratio of LF to HF power. Historically interpreted as a sympathetic/parasympathetic balance index, though that interpretation has been criticized since LF contains parasympathetic components too. Still commonly reported; treat with caution.
Rhythmic oscillations in arterial blood pressure at roughly 0.1 Hz (6 cycles per minute). Named after Siegmund Mayer who described them in 1876. They are a consequence of baroreflex feedback dynamics — essentially the resonance of the loop. Coherence breathing works by amplifying these through respiratory coupling.
NN50: the count of successive R-R intervals that differ by more than 50 milliseconds. pNN50: that count as a percentage of total intervals. Both are time-domain HRV metrics sensitive to vagal activity. Higher values indicate more beat-to-beat fluctuation.
The "rest and digest" branch of the autonomic nervous system. Carried mostly by the vagus nerve. Slows heart rate, promotes digestion, enables recovery. HRV, and especially RMSSD and HF power, are primarily parasympathetic markers.
Optical pulse detection — shining a green LED at the skin and measuring the reflected light change as blood pulses through. How wrist-based heart-rate sensors (Apple Watch, Fitbit, most Garmin watches) work. Good for average heart rate. Not accurate enough at the beat-to-beat level for serious HRV analysis.
The specific breathing rate at which your HRV amplifies into a single dominant low-frequency peak. Set by the dynamics of your cardiovascular baroreflex loop. Most adults fall between 4.5 and 6.5 breaths per minute; the individual value is stable over months but varies significantly from person to person.
Root-mean-square of successive R-R interval differences. A time-domain HRV metric dominated by beat-to-beat (short-term) variability, which in turn is dominated by vagal activity. The HRV score most consumer wearables display is usually RMSSD, computed over some window (often overnight).
The natural speeding-up of heart rate during inhalation and slowing during exhalation. Produced almost entirely by the vagus nerve modulating its brake on the sinoatrial node. The size of RSA is a direct, real-time readout of vagal tone. Coherence breathing maximizes RSA by aligning it with baroreflex resonance.
The time in milliseconds between two consecutive R peaks in the ECG — i.e., the interval between two heartbeats. HRV is calculated from sequences of R-R intervals. All of the metrics on this page are ultimately derived from these numbers.
Standard deviation of all (normal) R-R intervals in a recording window. A time-domain HRV metric that captures total variability, including slower rhythms that RMSSD misses. During coherence breathing SDNN rises substantially because the low-frequency oscillation becomes large.
Six separate measurements proposed by Fred Shaffer and Zachary Meehan (2020) as a more complete way to quantify HRV coherence than a single score. They cover: peak frequency, peak power, peak height, peak dominance, LF/total power ratio, and time-domain stability. Heart Resonance reports all six on every session.
The "fight or flight" branch of the autonomic nervous system. Speeds the heart, raises blood pressure, mobilizes energy. Acute stress, exercise, and many emotions activate it. Chronic sympathetic dominance suppresses HRV; coherence breathing helps bring the two branches back into balance.
The baseline level of activity on the vagus nerve — the main highway for parasympathetic signals from brainstem to organs. Higher vagal tone tends to track with better emotional regulation, faster recovery from stress, and higher HRV. Resonance frequency training measurably increases vagal tone over 8 – 10 weeks of daily practice.
The original procedure developed by Evgeny Vaschillo in the 1990s for identifying an individual's resonance frequency. Briefly: have the subject breathe at a series of pre-set rates (typically 4.5, 5.0, 5.5, 6.0, 6.5 BPM), measure the HRV response at each, and select the rate that produces the highest peak-to-peak oscillation. The adaptive pacer in Heart Resonance is a refinement of this protocol.