Recovery

What is HRV? Heart rate variability for training

Heart rate variability (HRV) is the small variation in the time between heartbeats. For training, what matters is not your number but how your recent average compares with your own usual.

Updated October 24, 2026

What HRV is

Your heartbeat isn't a metronome. The gap between beats changes a little from one beat to the next, and HRV puts a number on that variation, in milliseconds. Wearables usually measure it while you sleep or sit still.

Two statistics are common. SDNN reflects the overall spread of the gaps across the measurement window. RMSSD focuses on the changes from one beat to the next. They are calculated differently, so they aren't interchangeable: 45 ms in one is not the same reading as 45 ms in the other.

Why your own baseline matters

HRV differs a lot between people. Age, genetics, fitness, the device and the statistic all shift it, so a number that is high for one person can be ordinary for another. Comparing yours with a friend's, or with a population chart, tells you little. Comparing it with your own history tells you much more.

That is how Fortior uses it. It never judges an HRV value against a norm. It asks whether your recent average sits above, near or below your own typical range.

Day-to-day noise

A single night of HRV is noisy. Sleep timing, a late meal or drinks, a hard session, travel, feeling unwell, or just how the sensor sat can all move one reading. That is why most training tools, Fortior included, look at a short rolling average instead of one night.

How Fortior reads your HRV

Fortior compares your last seven days of HRV with your own previous 28 days. A few rules keep the comparison honest:

• One source at a time. Each provider and statistic is its own series. SDNN and RMSSD are never mixed, and Fortior won't borrow readings from another source to fill a gap. If two sources are connected, each is compared with its own history and the two results are averaged equally.
• A log scale. Fortior averages the logarithm of each night's value, which keeps a few unusually high nights from skewing the picture.
• One value per day. Duplicate readings for a day collapse into one, so a repeated import can't count twice.
• Enough data first. It needs at least seven days of history in the 28-day window, at least three readings in the last seven days, and a newest reading no older than about 36 hours. Otherwise HRV stays out of the picture instead of guessing.

The result is how far your 7-day average sits from your baseline, measured in your own usual day-to-day spread (standard deviations).

A worked example

Say your typical nightly HRV is around 60 ms and nights usually swing about 15% either way, which is 0.15 on the log scale. Your last seven nights averaged 52 ms. The comparison is (ln 52 − ln 60) ÷ 0.15 = (3.951 − 4.094) ÷ 0.15 ≈ −0.95. That puts you just under one standard deviation below your usual: a modest dip worth noting beside how you feel, not a dramatic reading. Fortior treats it that way, passing it to readiness as −0.95 on a scale from −1 to +1.

HRV counts for 30% of Fortior's readiness blend when it is available. It is also one of several signals behind a deload suggestion: a 7-day average more than 1.5 standard deviations below baseline counts as one signal, and HRV alone doesn't trigger a suggestion.

Where Fortior gets HRV

Connect them in Settings under Integrations. If you use more than one, Fortior keeps each in its own lane, and its HRV chart shows SDNN and RMSSD as separate series.

Using HRV sensibly

• Look at the trend over a week or more, not one morning.
• Pair it with how you feel and how training went. HRV that dips while you feel fine is information, not an order.
• Measure the same way. A wearable worn the same way each night gives a cleaner trend than switching devices.
• Use it as context for training choices only. A low reading isn't a diagnosis of anything, and Fortior doesn't draw health conclusions from HRV.

HRV figures are educational and based on wearable data. They are not medical advice, and Fortior does not use them to diagnose or monitor any health condition.