HRV and Recovery Status
A number of research studies have highlighted a reduction in HRV following intense training sessions. One, in particular, observed a reduction in HRV 24-hours post-workout following a high-intensity strength training session. HRV and weightlifting performance returned to baseline (i.e. pre-workout values) after 72-hours of recovery, indicating a relationship between HRV and recovery (34). Another study also found a relationship between training load and HRV before and during rowing World championships in national rowers (35).
They reported that when the athletes’ were subjected to high training loads leading up to competition, their HRV dropped significantly. Once training loads were substantially lowered during competition, their HRV then increased and returned to baseline. Another study which monitored athletes over a long period of time (multiple months) found that HRV increased during an intense training period, but then stagnated and lowered during an overload training phase. But after a 2-week recovery period, HRV rebounded and even led to an increase in HRV (36). This also indicates that intense training regimes may even improve HRV.
HRV and Overreaching
Though HRV has been reported to be a reliable marker of overreaching, two recent reviews may not support this allegation. A 2016 systematic review and meta-analysis has suggested that resting HRV is largely unaffected by overreaching – i.e. HRV cannot identify overreaching – but they also suggested that this may be the result of methodological issues that require further investigation (6). This study agrees with a previous review which also reported that resting HRV may not necessarily correlate with overreaching (33).
HRV and Performance
When discussing HRV and its effects on performance, the concept of HRV-guided training versus pre-planned training is often analysed. HRV-guided training simply refers to prescribing each training session based on the athlete’s HRV score. For example, if the athlete’s HRV is normal or higher than normal, then they will be prescribed with an intense training session. Alternatively, when the athlete’s HRV is below normal, then they are prescribed with an easier, lower-intensity session. Pre-planned training simply refers to a programme that has already been designed and does not cater for daily changes in HRV.
A 2007 study aimed to compare the effects of HRV-guided training against pre-planned training. The authors reported that the HRV-guided training group improved running performance (maximal running velocity) more than the pre-planned group (37). In support of this study, another more recent investigation also found HRV-guided training in males to be more effective than pre-planned training (38). Interestingly however, they did not see the same improvements in females. But, the female HRV-guided training group who performed fewer high-intensity training sessions, still experienced parallel performance gains to the pre-planned group. Furthermore, other research found a relationship between athletes who had high HRV scores and improved V02max, compared to those who had low HRV scores and showed deteriorations in V02max performances (39).
All in all, this suggests that HRV-guided training may be more beneficial for improving aerobic performance when compared to pre-planned training regimes. It also suggests that athletes who have a higher HRV score may be more sensitive to performance gains than those with a lower HRV score. Important to note, there is no research to our knowledge which demonstrates that HRV-guided training is more effective for strength development than pre-planned training – the research only supports aerobic development.
HRV and Illness or Injury
To date, there is little research identifying any relationships between HRV and illness or injury. In fact, to the best of our knowledge, there is only one study to date which has displayed a link between HRV and illness in elite swimmers (i.e. upper respiratory tract and pulmonary infections) (40).
In regards to HRV being a predictor of injury, one unpublished study conducted on horses – another mammal – displayed a relationship between HRV and both injury and illness (31). Another study which is currently under the data collection phase is attempting to identify a relationship between HRV and overuse injuries in humans (32). The authors believe that HRV monitoring has the potential to reveal the influence of external stimuli such as fatigue, nutrition, and stress on the recovery and protection of damaged somatic tissue.