A hard workout lowers your HRV for a day or two, and the harder the session, the longer it takes to recover. Some athletes use that pattern to plan their week: train hard when HRV sits in its normal range, and go easy when it has dropped. Small trials suggest this works at least as well as a fixed plan.
In lesson 11 you saw that training is one of several things that move HRV. This lesson shows how to use it for training decisions, and where it stops being useful.
What a workout does to HRV
During exercise, the vagal brake from lesson 3 lifts so your heart can speed up. Afterwards it comes back on gradually, and 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 took:
| Session | Time for HRV to recover fully |
|---|---|
| Easy (low intensity) | up to 24 hours |
| Comfortably hard (around threshold) | 24 to 48 hours |
| Hard (high intensity) | at least 48 hours |
How hard the session was mattered more than how long it lasted, and fitter people recovered faster. These figures come from aerobic exercise. There is less data for strength training.
So a low reading the morning after intervals is expected while your body recovers. If you are looking at one right now, read why HRV is low after a workout.
What HRV-guided training trials found
In HRV-guided training, your morning reading decides the session. Two Finnish trials tested this against a fixed plan.
- 2007, 26 moderately fit men, four weeks (Kiviniemi and colleagues, 2007). The HRV group ran hard when their HRV was normal or rising, and easy or not at all when it fell below their own 10-day average minus one standard deviation, or kept falling for two days. Their peak oxygen uptake rose from 56 to 60 ml/kg/min, while the fixed-plan group showed no significant change. Both groups got faster on a treadmill test, and the HRV group improved more.
- 2016, 40 recreational runners, eight weeks (Vesterinen and colleagues, 2016). Runners did a moderate or hard session only when their HRV sat inside their own “smallest worthwhile change” band. The HRV group did fewer moderate and hard sessions, about 13 against about 18, yet improved their 3,000-metre time by 2.1%. The fixed-plan group’s 1.1% change was not significant. Aerobic fitness rose in both groups.
A 2021 meta-analysis pooled trials like these (Manresa-Rocamora and colleagues, 2021). HRV-guided training raised vagal HRV more than a fixed plan. For fitness and performance, the results leaned slightly towards HRV guidance, but the differences were small and not statistically significant. The authors concluded that any advantage at group level is likely small, and that HRV guidance may make poor responses less likely.
HRV guidance is at least as good as a sensible fixed plan, and it may help you avoid hard days at the wrong time. Expect small gains over a good fixed plan, if any.
Go deeper What 'smallest worthwhile change' means
The smallest worthwhile change is a band around your own baseline that is wide enough to ignore normal noise. In practice, sports scientists build it from your average over a few weeks, plus or minus a fraction of your day-to-day spread. Readings inside the band count as normal. Readings outside it count as a real change.
Studies have used different fractions, and it is not settled which works best. The Manresa-Rocamora review called for more work on which HRV measure, recording position and baseline method to use.
A simple rule you can try
This is a plain version of what the trials did. It assumes you already have a baseline Baseline Your own usual HRV level and range, built from weeks of readings taken the same way. Comparing a reading with your baseline says more than comparing it with other people. More about Baseline from lesson 10.
- Measure the same way every morning, or use your device’s overnight number. Do not switch between the two.
- Check your 7-day average first. If it sits inside your normal range, your training is going fine.
- Then check today’s reading. If it is inside your range and you feel ready, do the planned hard session.
- If today is well below your range, swap the hard session for an easy one or rest, and look at the context: sleep, alcohol, stress, yesterday’s load.
- If several days in a row are low, ease off for a few days and see whether the average comes back.
The rule never tells you to stop training because HRV is low. In the trials, a low reading meant an easy day, not a lost week. For the day-to-day call, see should I train when my HRV is low?
Where HRV stops being useful
Normal HRV does not rule out overreaching
Overreaching means training so hard that your performance drops. A 2016 review pooled 24 studies of endurance athletes (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, resting HRV was largely unaffected. So a normal-looking number can hide a training block that is going badly.
High is not always good
Plews and colleagues (2013) found studies in elite athletes where both rises and falls in HRV went with poor adaptation. In very fit people, HRV can even drop 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 keeps falling. As lesson 4 showed, a big move in either direction is worth a second look.
Your other signals still count
Keep tracking the things coaches have always used:
- how your sessions feel, and whether your times or loads are going up
- muscle soreness and motivation
- sleep quality
- resting heart rate (see HRV vs resting heart rate)
When HRV and these signals agree, act with confidence. When they disagree, lean towards caution and towards how you feel.
When to rest no matter what
Rest if you have a fever, feel ill, or have chest pain, palpitations, fainting or unusual breathlessness during exercise. Those symptoms need a doctor, not a training plan, whatever your HRV shows. For more on training and HRV, see HRV and training.
Next, in HRV, stress and breathing, you will look at the other big load on your body: stress.