Heart rate variability (HRV) monitoring has gained significant traction in competitive and recreational sports, thanks to the rise of smartphone apps and other affordable on-site tools. While the concept of HRV itself is simple, the interpretation of the data is complex. Much of the confusion about HRV interpretation stems from oversimplified guidelines for non-specialized users.
In the context of tracking athlete fatigue or training status, it is often assumed that high HRV is always good and low HRV is always bad. In addition, it is often assumed that an increase in HRV indicates increased adaptation or fitness, while a decrease indicates fatigue or “overtraining” and decreased performance. In this paper, we will delve into the interpretation of short- and long-term trends, examining when these common beliefs are valid and when they are not. We will briefly explore the origins of these ideas in the scientific literature and highlight some important exceptions.
We will focus on the log-transformed root mean square of the continuous R-R interval difference (lnRMSSD), a commonly used index of vagal-heart rate variability in popular smartphone apps. lnRMSSD is the preferred parameter of heart rate variability in athlete monitoring for the following reasons
- It can be easily calculated without specialized software.
- It reflects cardiac parasympathetic regulation.
- It is more reliable than spectral measurements (e.g. high frequency power).
- It takes only 60 seconds to measure.
- It is less affected by respiratory rate, making it more suitable for field use.
Why high HRV is seen as a positive factor and low HRV as a negative factor
HRV-guided endurance training outperforms pre-planned endurance training in improving aerobic fitness in both healthy and clinical populations. Essentially, when HRV is at or above baseline, training with higher intensity or load tends to result in better training adaptations. Therefore, a high (or baseline) HRV equates to “readiness”.
Sharp decreases in HRV have been observed after intense endurance training, resistance training, and competition. Therefore, low HRV is often interpreted as a sign of acute fatigue due to training or competition.
Why these explanations may be misleading
Athletes can also experience low HRV before a competition due to excitement or anxiety. In fact, low vagal HRV is even thought to benefit sprinters on race day. For example, data collected from a collegiate sprinter prior to a championship win showed a significant decrease in HRV on the first race day when they set a personal record. Despite the decrease in HRV, health scores did not indicate fatigue, and the swimmer had tapered off training in the weeks leading up to the meet.The decrease in HRV may have been related to anxiety or euphoria.
In addition, low HRV scores due to fatigue do not necessarily predict poor performance. For example, a small case study involving three high-level tennis players demonstrated that performance indicators such as maximal oxygen uptake (VO2 max) and jump height improved after 30 days of overtraining, despite a decrease in RMSSD (ranging from -13 to -49%). This trend was also observed in a survey of college women’s soccer teams. The investigation assessed HRV (weekly measurements of mean and coefficient of variation, or CV) and subjective health in response to fluctuations in training load over several weeks.
These findings suggest that HRV, while a useful monitoring tool, should not be viewed in isolation but should be interpreted in the broader context of an athlete’s training, performance, and overall health.
Understanding Training Load and HRV Trends
During a week of high-load training, an athlete’s health score tends to decrease, the weekly average of heart rate variability (HRV) decreases, and the HRV coefficient of variation (HRV) increases. These changes indicate a significant increase in fatigue. Despite this, athletes continue to complete training at higher intensities and loads, both in the weight room and during workouts. This suggests that even with fatigue, they were still able to demonstrate their strength and fitness.
Therefore, while heart rate variability can indicate fatigue prior to a decline in performance, it is not always associated with an immediate decline in performance. A low HRV score does not necessarily mean that performance is poor, as an athlete may perform well despite a low HRV value.
Why an upward trend in HRV is often seen as positive performance
Increased aerobic fitness levels are usually associated with increased cardiac parasympathetic activity in both individual and group athletes. Typically, athletes with increased fitness levels also have increased HRV. In contrast, athletes whose fitness levels do not increase usually have no change in HRV or even a decrease.
For example, Buchheit and colleagues showed that athletes who improved their performance on a 10-kilometer run also had a significant increase in HRV, whereas athletes who did not improve their performance had little change. There was a strong correlation between changes in HRVI and maximal aerobic speed and 10-km test performance.
In addition, our recent study evaluated the relationship between early changes in heart rate variability (HRV) and performance enhancement in team sport athletes. Halfway through a 5-week training program, athletes who showed an increase or decrease in heart rate variability (HRV) or coefficient of variation (CV) showed greater performance gains than athletes who showed the opposite trend in HRV. These findings have led to the generalization that an upward trend in heart rate variability (HRV) is always a positive indicator of training response.
For example, the athlete in Figure 2 demonstrated a gradual increase in HRV, which could be interpreted as a positive response to training due to concurrent improvements in perceived health (e.g., sleep quality, soreness, mood, and fatigue), improved performance, and maintenance of training load.
Why interpreting the trend of increasing HRV as always positive may be misleading
Although increased heart rate variability (HRV) is often associated with positive training adaptations, it does not always indicate good training. Several studies have shown a trend toward increased heart rate variability (HRV) in overtrained athletes, especially endurance athletes.
For example, a study by Le Meurr et al. found that elite endurance athletes experienced a decrease in performance along with an increase in mean weekly HRV after 3 weeks of overtraining compared with controls, who did not experience significant changes. After a gradual reduction in training volume, these athletes experienced supercompensation in performance and HRV returned to baseline levels.
Why the downward trend in HRV is usually perceived as negative
The typical response to overload training is a gradual decrease in HRV. This is an alarm response to activation of the sympathetic nervous system. As a result, resting heart rate increases and HRV decreases. If recovery time is insufficient, HRV may not fully return to baseline levels before the next training session, thus continuing the downward trend.
High-intensity training may suppress HRV for up to 72 hours after exercise. This downward trend is more common during overload training with higher volume and frequency. Essentially, HRV decreases with the stress of overload training, which may indicate that the athlete’s body needs more time to recover before high-intensity training stimuli.
Effects of overload on heart rate variability: understanding trends and performance changes
Heart rate variability (HRV) typically decreases when overload training is prolonged, indicating a build-up of fatigue.This trend was emphasized in a study by Pichot et al, which showed a significant decrease in HRV in middle-distance athletes (up to -43%) during a 3-week phase of overload training. However, once the training intensity was reduced in week 4, HRV began to recover and even exceeded baseline levels.
Case study: trends in heart rate variability in collegiate short-distance swimmers
In one case I monitored, a collegiate sprinter experienced a significant decrease in heart rate variability during the anaerobic overload phase of training, followed by an increase during the tapering phase. The decline in heart rate variability was strongly associated with fatigue, as assessed by the Daily Health Questionnaire. As the training volume was gradually reduced, not only did HRV improve, but fatigue also decreased, confirming that the decline in HRV was due to the accumulation of training stress.
When a decreasing trend in HRV is not always indicative of fatigue
Aerobic exercise usually stimulates parasympathetic activity, which is reflected in the next day’s HRV score. This is why moderate aerobic training is often used for recovery. However, the intensity of the workout has a significant impact on the HRV response. A study of Olympic rowers by Plews et al. found that high-intensity training (e.g., above the lactate threshold) suppressed HRV, whereas low-intensity training (e.g., below the lactate threshold) led to an increase in HRV.
This distinction is critical when interpreting trends in heart rate variability. Moderate-intensity aerobic activity tends to enhance HRV, whereas high-intensity training, especially without sufficient low-intensity training, usually results in decreased HRV. The lack of low-intensity training means that the parasympathetic stimulation that comes with such activity is absent, which can lead to a decrease in HRV but does not necessarily indicate increased fatigue. Therefore, a decrease in HRV during a high-intensity training phase, especially when low-intensity aerobic activity is limited, should not be immediately recognized as a sign of increased fatigue.
Examples of Heart Rate Variability Trends in College Athletes
The trend in heart rate variability in college athletes further illustrates this concept. Over a six-week period, his heart rate variability continued to increase due to an increase in aerobic exercise. However, once the amount of aerobic exercise decreased, the trend in heart rate variability began to decrease. Interestingly, the decrease in HRV was not related to fatigue or performance issues, but was simply the result of a change in training content.
Resistance Training and Heart Rate Variability Fluctuations
Another example comes from a trend I observed where daily heart rate variability fluctuated with resistance training. With the addition of daily aerobic exercise over a two-week period, heart rate variability remained stable with fewer fluctuations. After removing aerobic exercise, the trend in HRV decreased, but fatigue did not increase significantly. This suggests how aerobic exercise affects HRV trends without a direct correlation to fatigue or decreased performance.
CONCLUSION: Interpreting HRV trends in context
Monitoring heart rate variability (HRV) cannot be done using only black and white judgments (high = good, low = bad). Relying on HRV alone to assess training status can be misleading. Instead, other factors such as training load, type of training, lifestyle factors (e.g., sleep, nutrition, stress), and performance must be considered. Taken together, these factors provide a more complete picture of an athlete’s status.
To more accurately assess training progress, athletes and coaches should observe trends in heart rate variability, interpret them in the context of sport-specific demands and daily life factors, and adjust training or lifestyle accordingly to match the goals of the current training phase.
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