HRV (Heart Rate Variability)

Heart rate variability (HRV) is the variation in the time interval between one heartbeat and the next. A resting heart rate of 60 bpm doesn't mean a beat every exact second — the gaps are 0.94s, 1.05s, 0.97s, and so on. HRV measures how much those gaps vary.

That variation is controlled by the autonomic nervous system — specifically the balance between its "fight or flight" (sympathetic) and "rest and digest" (parasympathetic) branches. More variability generally reflects a nervous system that's rested and able to respond flexibly; less variability generally reflects a body under stress, whether from hard training, poor sleep, illness, alcohol, or life stress unrelated to sport.

What HRV is not

HRV is not a fitness score. A very fit athlete mid-block, sleep-deprived and fighting off a cold, can post a low HRV reading; someone far less fit but well-rested can post a high one. It's a recovery and readiness signal layered on top of fitness, not a replacement for it.

It's also not comparable between people. Normal HRV ranges vary hugely by individual — genetics, age, and measurement method all matter. A reading of "45ms" means nothing on its own; what matters is how today's reading compares to your own rolling baseline.

How it's actually measured

Most consumer wearables (Garmin, Whoop, Oura, Apple Watch) estimate HRV using a metric called RMSSD (root mean square of successive differences between heartbeats), usually captured during a stable window — typically overnight sleep or a short guided reading first thing in the morning. Because HRV fluctuates naturally within seconds, single-point readings are noisy; a week-long rolling average is far more informative than any one morning's number.

Using it in a training plan

The useful pattern isn't "HRV was low, so skip today's session" — a single low reading is common noise. What matters is the trend against your own baseline:

• In your normal range: proceed with the planned session.
• Meaningfully below your rolling average, for multiple days: treat it as one input (alongside sleep, soreness, and how the last few sessions actually felt) suggesting the plan should ease off — not an automatic override.
• A single unusually low morning: worth noting, rarely worth panicking over on its own.

This is exactly why an adaptive plan that only reacts to one input (HRV alone, or pace alone) tends to overcorrect. Triathlify's plan adaptation weighs readiness signals like HRV alongside what you actually completed and how sessions felt, rather than triggering a rebuild off a single morning reading.