Experiment III: Embodied Resonance – HRV Metrics and meaning

Heart-rate variability, or HRV, is the tiny, natural wobble in the time gap from one heartbeat to the next. It exists because two automatic “pedals” are always tugging at the heart. One pedal is the sympathetic system, the same chemistry that makes your pulse race when you are startled. The other pedal is the vagus-driven parasympathetic system, the brake that slows the heart each time you breathe out or settle into a chair. The more freely these pedals can trade places, the more variable those beat-to-beat spacings become. HRV is therefore a quick, non-invasive way to listen to how relaxed, alert, or exhausted the body is.

When we measure HRV we usually pull out a few headline numbers.

SDNN is the overall statistical spread of beat intervals during a slice of time, for example one minute. A wide spread means the heart is flexible and ready to react. A very narrow spread means the system is locked in one gear, as happens in chronic stress or heart failure.

RMSSD zooms in on the jump from one beat to the very next, averages those jumps, and reflects how strongly the vagus brake is speaking. During slow, deep breathing RMSSD grows larger; during mental tension or sleep deprivation it falls.

Frequency-domain measures treat the heartbeat trace like a piece of music and ask how loud each note is. Very-low-frequency power, or VLF, comes from extremely slow body rhythms such as hormone cycles and temperature regulation. Low-frequency power, or LF, sits in the middle and rises when the sympathetic pedal is pressed, for example in the first minute of exercise or during mental arithmetic. High-frequency power, or HF, sits exactly at breathing speed and is almost pure vagus activity: it swells during calm, diaphragmatic breathing and shrinks when breathing is shallow or hurried. A simple way to summarise the tug-of-war is the LF-to-HF ratio. When the sympathetic pedal dominates the ratio climbs; when the vagus brake dominates the ratio slides downward.

In a healthy, rested adult who is quietly seated the heart rate is steady but not rigid. SDNN and RMSSD show a modest but clear jitter, HF power pulses in step with the breath, LF is similar in size to HF, and the LF/HF ratio hovers around one or two. If the same person begins brisk walking heart rate rises, HF power fades, LF power grows, and the LF/HF ratio can shoot above five. During slow breathing meditation RMSSD and HF surge while LF/HF drops below one. In someone with chronic anxiety or PTSD the resting pattern is different: SDNN and RMSSD are low, HF is thin, LF/HF is already high before any task, and it climbs even higher during mild stress. The pattern can be even flatter in advanced heart disease, where both pedals are weak and total HRV is minimal.

Put simply, HRV lets us watch the nervous system’s soundtrack: fast notes reflect breathing and relaxation, mid-notes reflect alertness, and the overall volume tells us how much capacity the system still has in reserve.

The raw material for every HRV metric is the NN-interval sequence:
NNi is the time in seconds between two consecutive normal (sinus) beats.

SDNN is the standard deviation of that sequence.

SDNN = √[ Σ (NNi – NN̄)² / (N – 1) ]
Units are milliseconds because the intervals are expressed in ms. A resting, healthy adult who sits quietly will usually show an SDNN between roughly 30 ms and 50 ms. Endurance athletes can sit in the 60–90 ms range, while chronically stressed or cardiac patients may drift below 20 ms.

RMSSD focuses on the beat-to-beat jump and is dominated by parasympathetic (vagal) tone.

RMSSD = √[ Σ ( NNi – NNi-1 )² / (N – 1) ]
Again the unit is milliseconds. Typical resting values in a calm, healthy adult are about 25–40 ms. Slow breathing, a nap, or meditation can push it up toward 60 ms, whereas sustained mental effort, anxiety, sleep deprivation, or PTSD often pull it down below 15 ms.

Frequency-domain indices start from the same NN series but first convert it into a power-spectrum, most accurately with a Lomb–Scargle periodogram when the points are unevenly spaced:

P(f) = (1/2σ²) { [ Σ NNi cos ωi ]² / Σ cos² ωi + [ Σ NNi sin ωi ]² / Σ sin² ωi }
where ω = 2πf and f is scanned from 0.003 Hz upward.

Power is then integrated over preset bands and reported in ms² because it represents variance of the interval series per hertz.

Very-low-frequency power VLF integrates P(f) from 0.003 Hz to 0.04 Hz. In a healthy resting adult VLF is often 500–1500 ms². Because the mechanisms behind VLF (thermoregulation, hormones, renin-angiotensin cycle) change only slowly, values can drift greatly between individuals and between days.

Low-frequency power LF integrates P(f) from 0.04 Hz to 0.15 Hz. A quiet, healthy adult usually sits near 300–1200 ms². LF rises when the sympathetic accelerator is pressed, for example during the first few minutes of exercise or a stressful mental task.

High-frequency power HF integrates P(f) from 0.15 Hz to 0.40 Hz, exactly the normal breathing range. Calm diaphragmatic breathing drives HF toward 400–1200 ms², whereas rapid or shallow breathing in anxiety or hard exercise cuts HF sharply, sometimes below 100 ms².

LF/HF is the simple ratio LF ÷ HF. At rest a ratio near 1–2 suggests a balanced tug-of-war. If the ratio soars above 5 the sympathetic branch is clearly on top; if it falls below 0.5 the vagus brake is dominating (seen in deep meditation or in some fainting-prone individuals).

All of these numbers rise and fall in real time as the two branches of the autonomic nervous system jostle for control, so plotting them across the exercise-rest protocol lets us see how quickly and how strongly each person’s physiology reacts and recovers.