LF LF power The variance of the heartbeat intervals that falls in the 0.04 to 0.15 Hz band, in ms²: rhythms repeating every 7 to 25 seconds. Once read as sympathetic activity, which is now disputed. More about LF power , HF HF power The variance of the heartbeat intervals that falls in the 0.15 to 0.40 Hz band, in ms²: the range of normal breathing. It mainly reflects vagal activity when breathing is 9 to 24 breaths a minute. More about HF power and VLF VLF power The variance of the heartbeat intervals in the 0.003 to 0.04 Hz band, in ms²: rhythms repeating every 25 seconds to 5 minutes. It needs at least a 5-minute recording and is best read over 24 hours. More about VLF power are three slices of the HRV spectrum: how much of the variation in your heartbeat intervals repeats slowly (LF, every 7 to 25 seconds), at breathing speed (HF, every 2.5 to 7 seconds) or very slowly (VLF, every 25 seconds to 5 minutes), each in ms². The nu Normalised units LF or HF power as a percentage of their sum (LF + HF), rather than in ms². The two always add up to 100, so they carry one piece of information between them. More about Normalised units figures are LF and HF written as percentages of their sum. HF is the one with a clear meaning: it is your breathing rhythm, driven by the vagus nerve Vagus nerve The main nerve of the parasympathetic nervous system, running from the brainstem to the heart, lungs and gut. Its fast signals to the heart drive most beat-to-beat HRV. More about Vagus nerve . LF mixes blood-pressure control with breathing, and the LF/HF ratio LF/HF ratio LF power divided by HF power, with no unit. It was proposed as a gauge of balance between the sympathetic and parasympathetic systems, but research shows it cannot measure that reliably. More about LF/HF ratio does not measure “balance”.
The screen, decoded
A typical frequency panel shows five or six numbers. Here is what each one is, using five minutes of this site’s teaching recording. That recording is synthetic and HF-dominant by design, almost all breathing swing, so its LF/HF of 0.19 sits far below the 1.5 to 2.0 the Task Force listed for real resting recordings. Read the column for what each number means, not as a norm.
| Label | What it is | Teaching recording | Read it as |
|---|---|---|---|
| VLF | Power from 0.003 to 0.04 Hz, ms² | 3 | Mostly drift in a short reading; needs 5 min at least |
| LF | Power from 0.04 to 0.15 Hz, ms² | 120 | The ~0.1 Hz blood-pressure wave, plus breathing if it is slow |
| HF | Power from 0.15 to 0.40 Hz, ms² | 635 | The breathing rhythm; vagal activity Vagal tone The ongoing influence of the vagus nerve on the heart, which slows it down. It is usually estimated from HRV measures such as RMSSD and HF power. More about Vagal tone at normal breathing rates |
| Total power Total power The sum of the power in every band of the spectrum, in ms²: the variance of the heartbeat intervals. For a 5-minute recording it is VLF + LF + HF; over 24 hours ULF is added. More about Total power | VLF + LF + HF, ms² | 758 | The variance of the intervals, SDNN SDNN The standard deviation of all NN intervals in a recording, in milliseconds. It measures overall variability, and its value depends strongly on how long the recording lasts. More about SDNN squared more or less |
| LF nu, HF nu | Each as a share of LF + HF, % | 15.9, 84.1 | One number written twice; they add to 100 |
| LF/HF | LF divided by HF | 0.19 | The same number a third way; not sympathetic Sympathetic nervous system The "fight or flight" branch of the autonomic nervous system. It speeds up the heart and makes it pump harder, but it acts too slowly to drive beat-to-beat HRV. More about Sympathetic nervous system balance |
| Peak | Frequency of the tallest peak, Hz | 0.250 | Your breathing rate Breathing rate How many breaths you take a minute. It moves HRV on its own: slower, deeper breathing makes the beat-to-beat swing larger, and below about 9 breaths a minute it shifts from the HF band into LF. More about Breathing rate ÷ 60 |
The Task Force fixed the band edges in 1996. Power is a variance, which is why the unit is milliseconds squared, and why the numbers vary so much between people: healthy HF values pooled by Nunan and colleagues average 657 ms² with a standard deviation of 777.
Try it: slow your breathing
Take the rhythm apart
Five minutes of the teaching recording
- ULF
- needs 24 h
- VLF
- 3 ms²
- LF
- 120 ms²
- HF
- 635 ms²
- Total
- 758 ms²
- LF nu
- 15.9
- HF nu
- 84.1
- LF/HF
- 0.19
- Peak
- 0.250 Hz, HF
Rested, 15 breaths a minute (0.250 Hz), 5 minutes, Welch's method after detrending. The breathing peak sits at 0.250 Hz in the HF band, with the ~0.1 Hz blood-pressure wave as a smaller LF peak. LF/HF is 0.19.
The controls work when JavaScript is on.
A line chart of power against frequency from 0 to 0.5 Hz with the VLF (0.003–0.04 Hz), LF (0.04–0.15 Hz) and HF (0.15–0.4 Hz) bands shaded and a marker at the breathing frequency. Readouts give ULF, VLF, LF, HF and total power in ms squared, LF and HF in normalised units, the LF/HF ratio and the peak frequency, followed by a sentence explaining what changed. Controls: a breathing-rate slider from 5 to 20 breaths a minute, a recording-length slider, a detrending checkbox, Welch or Lomb–Scargle buttons, and four preset buttons for a rested, stressed, drifting or ectopic recording.
The teaching recording breathes 15 times a minute, so its tall peak sits at 0.250 Hz, inside the HF band, and HF nu is 84.1. Drag the breathing rate to 7 a minute. The same breathing rhythm now sits at 0.116 Hz, inside the LF band: HF falls from 635 to 19 ms², LF rises to 3462 ms², and LF/HF goes from 0.19 to 186.8. The size of that jump belongs to the synthetic recording, which has almost nothing left in HF once breathing leaves the band; the direction is the point. Nothing about the nervous system changed. This is the single most common reason an app shows a “stressed” spectrum in a calm person: slow breathing, which many people do during a reading on purpose.
What each band means, and does not
HF is respiratory sinus arrhythmia Respiratory sinus arrhythmia The normal rise in heart rate as you breathe in and fall as you breathe out. It is driven by the vagus nerve and makes up much of resting HRV. More about Respiratory sinus arrhythmia , the speed-up on each in-breath and slow-down on each out-breath, which the vagus nerve produces. The Task Force and later reviews treat it as the frequency-domain index of vagal activity, valid while you breathe between 9 and 24 times a minute. Deeper breaths make it bigger without any change in vagal tone, so the number is a mix of physiology and breathing habit. It needs about 1 minute of recording. RMSSD RMSSD Root mean square of successive differences: an HRV metric built from how much each heartbeat interval differs from the next, in milliseconds. It mainly reflects vagal activity. More about RMSSD tracks the same activity in the time domain; it also changes with breathing, but it does not depend on where the breathing peak falls relative to a band edge, and it is the number most wearables report.
LF was read for years as sympathetic (“fight or flight”) activity. That reading did not survive testing. At rest LF reflects the baroreflex Baroreflex The reflex that steadies blood pressure from beat to beat by changing heart rate. Its built-in delay makes heart rate rise and fall about every 10 seconds, the 0.1 Hz rhythm in LF power. More about Baroreflex , the loop that steadies blood pressure, and both branches of the nervous system feed it; Billman’s 2013 review found LF’s relationship to sympathetic nerve activity exceedingly poor. It needs about 2 minutes of recording.
VLF holds very slow waves whose origin is still debated: thermoregulation, hormones and the heart’s own nervous system are the candidates. In 24-hour recordings it predicts outcomes after a heart attack better than LF or HF; in a 5-minute recording it is mostly how much your heart rate drifted. From anything shorter it is noise.
nu stands for normalised units. Dividing LF and HF by their sum removes the huge person-to-person differences in absolute power and leaves the balance between the two bands, which is why apps like it for a gauge. But the two shares always add to 100, so LF nu, HF nu and LF/HF are one piece of information written three ways, and the Task Force’s own advice is to show them with the absolute values, never alone.
LF/HF was meant to be sympathovagal balance. Billman’s verdict is that the ratio cannot quantify balance in health or disease, and the breathing experiment above shows one reason: the ratio responds to breathing rate more than to anything else.
Why two apps disagree
Each program resamples the intervals (usually at 4 Hz), removes a trend, cuts the series into windows and transforms them, and every one of those choices changes the band powers. Kubios HRV uses Welch’s method with the smoothness-priors detrend Detrending Removing the slow drift from a series of beat intervals before a spectrum is calculated, so the drift does not pile up as false power in the lowest frequencies. More about Detrending , and offers an autoregressive model and the Lomb–Scargle periodogram as alternatives; Elite HRV uses Welch’s method and asks for 5 minutes. Elite HRV’s own help page warns that small differences in setup and artifact Artifact An error in a heartbeat recording, such as a missed, extra or mistimed beat, often caused by movement or poor sensor contact. Artifacts can badly distort HRV numbers. More about Artifact handling make large differences in frequency results, and two programs can disagree on the same beats without either being wrong. Compare frequency numbers only within one app, one recording length and one breathing pattern.
What to do with them
Look at HF, or better RMSSD, and note your breathing rate. If an app’s “balance” gauge leans to the stressed side after a slow-breathing session, it is reporting your breathing. For the time-domain numbers most apps show, the lesson RMSSD, SDNN and lnRMSSD explained is the place to start; the glossary pages on LF power, HF power, normalised units and VLF power go deeper on each.