A hard workout lowers your heart rate variability (HRV) for a day or two, and harder sessions keep it down for longer. Over weeks of training, HRV that holds steady or rises tends to go with good adaptation, which is why some athletes use a morning reading to decide when to push and when to go easy.
What a workout does to HRV
During exercise, your nervous system pulls back the parasympathetic Parasympathetic nervous system The "rest and digest" branch of the autonomic nervous system. It slows the heart, mainly through the vagus nerve, and drives most of the beat-to-beat variation in HRV. More about Parasympathetic nervous system brake on your heart so it can speed up. After you stop, that brake comes back on gradually. HRV shows how far it has returned.
A 2013 review pulled together studies of athletes and healthy people after aerobic exercise (Stanley and colleagues, 2013). Full recovery of the heart’s parasympathetic control took:
| Session | Time for HRV to recover |
|---|---|
| Low intensity | up to 24 hours |
| Around threshold (comfortably hard) | 24 to 48 hours |
| High intensity | at least 48 hours |
How long the session lasted seemed to matter less than how hard it was, and fitter people recovered faster. These figures come from aerobic exercise. There is less data on strength training.
If you are looking at a low number the morning after a hard session, see why HRV is low after a workout.
HRV-guided training
In HRV-guided training, you do the hard session when your HRV is in its normal range, and go easy or rest when it has dropped.
Two trials tested this against a fixed plan:
- Finland, 2007. 26 moderately fit men trained for four weeks (Kiviniemi and colleagues, 2007). One group followed a set plan; another chose hard or easy days from a morning HRV reading. Peak oxygen uptake, a measure of aerobic fitness, rose from 56 to 60 ml/kg/min in the HRV group, while the fixed-plan group showed no significant change. Both groups got faster on a treadmill test, and the HRV group improved more.
- Finland, 2016. 40 recreational runners trained for eight weeks (Vesterinen and colleagues, 2016). The HRV-guided group did fewer moderate and hard sessions (about 13 against 18) yet improved their 3,000-metre time by 2.1%, while the fixed-plan group’s 1.1% change was not significant. Aerobic fitness rose in both groups.
Both trials were small and short, and they used recreational or moderately fit people. They suggest HRV-guided training works at least as well as a fixed plan, and in the 2016 trial it did so with fewer hard days. They do not prove it is better for everyone.
Go deeper How the trials decided what counts as low
Each trial compared that morning’s reading with the person’s own recent values rather than with a fixed number.
In the 2007 trial, a reading below the 10-day average minus one standard deviation, or a falling trend over two days, meant an easy day or rest. Otherwise, the runner trained hard. In the 2016 trial, each runner had an individual “smallest worthwhile change” band. A reading inside the band allowed a moderate or hard session, and a reading outside it meant easy training.
Both rules only work if you measure the same way every morning.
What HRV can and can’t tell you over weeks
A 2016 review that pooled 24 studies looked at how HRV changes in endurance athletes over a training block (Bellenger and colleagues, 2016). When training made athletes faster, resting RMSSD RMSSD Root mean square of successive differences: an HRV metric built from how much each heartbeat interval differs from the next. It mainly reflects vagal activity. More about RMSSD tended to rise a little. When athletes overreached, meaning they trained so hard their performance dropped, resting HRV barely changed. So a normal-looking HRV does not rule out overreaching.
In elite athletes the picture is even less tidy. A review by Plews and colleagues found studies where both rises and falls in HRV went with poor adaptation (Plews and colleagues, 2013). In very fit people, HRV can drop even as resting heart rate Resting heart rate How many times your heart beats per minute while you are fully at rest. For adults it is usually 60 to 100 beats per minute, and lower in well-trained athletes. More about Resting heart rate falls. The authors recommend averaging readings rather than reacting to single days, and following each athlete’s own pattern over months.
How to use HRV in your own training
- Measure the same way every day. Same time, same position, before coffee and training. Many people use an overnight reading from a wearable or a short reading on waking.
- Watch the average, not the single day. Compare a several-day average with your own normal range. The lesson on building your HRV baseline shows how to set one.
- Treat a low day as a question. Did you sleep badly, drink, travel or train hard yesterday? One low reading after a hard session is expected. Several low days in a row are worth an easier day or two and a look at what changed.
- Look at resting heart rate too. After drinking or at the start of an illness, resting heart rate often rises as HRV falls, so seeing both move together tells you more than one number. See HRV vs resting heart rate.
- Keep your other signals. Performance, motivation, soreness and sleep still count. HRV is one more signal alongside them.
For the day-to-day decision, read should I train when my HRV is low?