Normalised units

Normalised units express LF 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 and HF power HF power 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 as shares of their sum, so LF nu + HF nu = 100.

Why this measure exists

Absolute band powers differ enormously between people: one person’s HF is 100 ms², another’s 3,000, and both are healthy. The 1996 Task Force introduced normalised units so that the balance between the two bands could be compared across people and conditions regardless of the size of the total. Its definition divides by total power 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 minus VLF 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 , which for a 5-minute recording is LF + HF. Shaffer and Ginsberg describe the same idea: each band’s absolute power divided by the summed absolute power of LF and HF.

The formula

$$\mathrm{LF}_{\mathrm{nu}} = \frac{\mathrm{LF}}{\mathrm{LF}+\mathrm{HF}}\times 100,\qquad \mathrm{HF}_{\mathrm{nu}} = \frac{\mathrm{HF}}{\mathrm{LF}+\mathrm{HF}}\times 100$$

Divide each band’s power by the sum of the two and turn it into a percentage. $\mathrm{LF}$ and $\mathrm{HF}$ are the absolute powers in ms². Because the two shares add to 100, LF nu, HF nu and the LF/HF ratio 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 are three ways of writing one number: $\mathrm{LF}/\mathrm{HF} = \mathrm{LF}_{\mathrm{nu}} / \mathrm{HF}_{\mathrm{nu}}$.

Worked example

Take the rhythm apart

Five minutes of the teaching recording

Power at each frequency, Hz Breathing

VLFLFHF

0 0.1 0.2 0.3 0.4 0.5

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.

Rested Stressed Drifting Ectopic beats

Breathing rate 15 breaths a minute Recording length 5 minutes

Welch (FFT) Lomb–Scargle

Detrend first (smoothness priors, λ = 500)

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 five minutes of the teaching recording give LF nu = 120 ÷ (120 + 635) × 100 = 15.9 and HF nu = 84.1. Slow the breathing to 7 a minute and LF nu climbs to 99.5, because the breathing peak crossed into the LF band. Now switch to the stressed preset: LF nu reads 39.9, a plausible-looking share, while the absolute powers underneath have fallen to a fraction of the rested values. The Task Force’s own advice is to report normalised units together with the absolute values for this reason.

What moves it

Whatever shifts power between the two bands: breathing rate 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 above all, then posture and the baroreflex 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 . Nunan and colleagues’ pooled short-term values are LF nu 52 ± 10 and HF nu 40 ± 10, each pooled from a different set of studies, which is why the pair does not sum to 100.

When it misleads

When read as a measure of total variability, which it has thrown away by design. When breathing is slow or fast, because the shares then measure breathing. And whenever two programs normalise differently: by LF + HF, by total power minus VLF, or by total power including VLF, which gives smaller numbers from the same recording.

Where you will see it

The Task Force’s names are LF norm and HF norm, in n.u. Kubios HRV prints “Normalized power” for LF and HF in n.u. beside the absolute, log and relative values; many apps that show a “balance” bar are drawing LF nu against HF nu. What do LF, HF, VLF and nu mean in my HRV app? decodes a typical app screen.