Usually, and modestly. Pooling 13 training studies with 322 people, Sandercock, Bromley and Brodie (2005) found that exercise training raised high-frequency HRV, a vagal measure, by a moderate effect size (d = 0.48) and lengthened the resting RR interval RR interval The time from one heartbeat to the next, measured in milliseconds between the R peaks of an ECG. HRV is calculated from a series of these intervals. More about RR interval by more (d = 0.75). The heart slows more reliably than HRV rises, the response is smaller after 60, and very fit athletes can reach a ceiling.
What training studies show
The 2005 meta-analysis is still the clearest summary. Across 13 studies of high-frequency 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 (322 participants) the pooled effect was d = 0.48 (95% CI 0.26 to 0.70); across 12 studies of the RR interval (298 participants) it was d = 0.75 (0.51 to 0.96). A moderate effect of this size means a typical participant moved from the middle of the untrained group to around its 70th percentile, with plenty of overlap. Subgroup analysis found a smaller RR interval response in older participants (at the authors’ P < 0.1 threshold), and the HRV response showed the same trend. The authors suggest that training slows the heart through more than one mechanism, not only more vagal activity Vagal tone The ongoing influence of the vagus nerve on the heart, which slows it down. It is usually estimated from HRV measures such as RMSSD and HF power. More about Vagal tone , which is one reason the two measures do not move together.
For older adults, Raffin and colleagues (2019) found that endurance training raised SDNN SDNN The standard deviation of all NN intervals in a recording, in milliseconds. It measures overall variability, and its value depends strongly on how long the recording lasts. More about SDNN in people around 69, with gains that grew with sessions per week; can HRV improve after 60 has the numbers. A review by Grässler and colleagues (2021) found that endurance, coordination and mixed programmes improved autonomic control in older adults, while resistance training alone did not.
Fitness or training load?
Buchheit and Gindre (2006) separated the two in 55 adults of around 31 years. Vagal HRV at rest correlated with fitness (peak oxygen uptake, VO2max) but not with how much the person trained each week; heart rate recovery after exercise did the opposite, tracking training load but not fitness. The fit groups had higher vagal HRV than the unfit groups whatever their training; the moderately trained groups differed from the low-trained groups in heart rate recovery whatever their fitness. So resting HRV reflects the fitness you have built more than the week you have just done, which is why it moves slowly.
How fast
The studies do not give a clean number of weeks. The pooled training programmes ran for months, and the authors of the older-adult meta-analysis found that frequency, not just duration, drove the gain. A fair expectation is a slow rise over a few months of regular endurance training, with 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 often moving first, and wide differences between people: is HRV genetic explains why two people on the same programme can differ.
The ceiling in very fit people
In endurance athletes HRV can stop rising, or fall, as the heart slows further. Plews and colleagues (2017) followed four world-champion rowers day by day and saw consistent, substantial falls in the ratio of log RMSSD lnRMSSD The natural logarithm of RMSSD, with no unit. Taking the log evens out the spread of RMSSD values, which makes day-to-day changes easier to compare and analyse. More about lnRMSSD to the RR interval during heavy training. They write that reductions in log RMSSD that come together with reductions in this ratio indicate parasympathetic saturation Parasympathetic saturation When the vagal brake is so strong, usually in very fit people with very slow resting heart rates, that the beat-to-beat swing shrinks. HRV then falls while vagal activity is rising. More about Parasympathetic saturation : the vagal brake is already fully on, so the usual beat-to-beat swing shrinks even though the athlete is coping. A fall in HRV in a very fit person therefore does not always mean lost fitness or fatigue. Reading HRV next to resting heart rate is the practical check; HRV vs resting heart rate shows how, and can HRV detect overtraining covers the opposite case.
What to do with your own data
- Track resting heart rate and HRV together over months. The heart rate trend usually shows training effects first.
- Judge the 7-day average, not single days. The baseline lesson shows how.
- Prefer regular endurance sessions if raising HRV is the goal; the training page covers session types.
- If you are already very fit and HRV drifts down while you feel strong and your heart rate is low, consider saturation before fatigue.
When to talk to a doctor
HRV cannot measure fitness or diagnose overtraining. Talk to a doctor if training brings chest pain, fainting, breathlessness out of proportion to the effort, or a racing or irregular heartbeat.