In previous columns I wrote about resting heart rate and heart rate recovery and more recently about the basics of heart rate variability (HRV), where we developed some basic definitions and terminology. This column looks specifically at the use of HRV in endurance training. I’ll share how and when to measure HRV and how HRV might be used to help guide your training. If you’re unsure of some of the terminology while reading below, refer back to my earlier columns for a refresher.

Measuring HRV
HRV can be measured at any time of day, over any length of time, and with any desired relationship to exercise. For endurance athletes, we might consider measurements of three sorts: “resting,” “during exercise,” or “post exercise.” Keep in mind that we’re measuring HRV as our window into the body’s autonomic nervous system, with its parasympathetic and sympathetic components, hoping this can somehow be related to training.

One important consideration is the circadian pattern of HRV. HF and rMSSD are highest in the early morning, decrease throughout the daytime, to an afternoon low, then increase until the following morning. In contrast, LF/HF follows the opposite pattern, peaking in the afternoon. This circadian pattern has implications with regard to how and when HRV should be measured. In order to eliminate the influence of this circadian effect — and to focus more solely on the influence of the autonomic nervous system — daily (or weekly, etc.) measurements ought to be made at the same time of day.

The most commonly used — and best understood — measurement is the resting HRV. The ideal time for measurement is just after waking, while still supine. A recording of the EKG to gather R-R interval measurements is made for just a few minutes. The resting HRV can also be measured in the sitting or standing positions, but there is more variability, or noise, in these measurements. Whichever choice is made for body position should probably be used for all subsequent measurements.

The use of exercise HRV is limited because the instantaneous HRV is very much related to the intensity of exercise during the measurement. The use of post-exercise HRV is limited because these measurements tend to be influenced greatly by post-exercise blood pressure regulation that, again, is dependent upon the intensity of exercise preceding the measurement. It appears that exercise and post-exercise HRV indices might well be correlated to fitness level, but difficulties with establishing constant conditions during their measurement will limit their utility for most athletes and coaches.

Another important consideration is variability from measurement to measurement. The day-to-day variation in the time domain indices (such as rMSSD) is much less than for the frequency domain indices (such as LF/HF). As an example, the coefficient of variation for rMSSD may be as high about 10-12%. As a result, some authorities advocate using a rolling several-day average for rMSSD, rather than simply a single-day measurement.

Finally, it is important from a statistical standpoint to have some idea about the smallest meaningful change for any of the indices. Again as just one example, the smallest meaningful change for rMSSD may be around 3%.

HRV and the Training Cycle
In the endurance sports, the use of HRV has been studied in two general settings: the short term and the longer term.

Short term: On any given training day, intense exercise will lead to a decrease in HRV and this effect can persist for 24 to 48 hours or so. Based on this observation, some have suggested that intense training only take place again once the HRV has returned to its baseline. Indeed, there is some evidence that training guided by this strategy might result in better performance gains over some period of time. When using this strategy, though, it’s important to remember that factors other than the ANS (such as sleep, hydration and environmental conditions) also play a role in the HRV and these factors should be kept in mind when interpreting the results. Some of the commercially available HRV devices are designed specifically for this application.

Longer term: For many endurance athletes, training comes in cycles. There are block periods of trainings followed by some sort of rest. At the end of some blocks might come tapering before an event. The use of HRV to help guide training in these various phases of training is not yet particularly well understood. I can share some generalities, though.

Thinking about a cycle of training for moderately trained, recreational endurance athletes, moderate intensity training leads to increases in aerobic fitness and a corresponding increase in HRV. Over that cycle, we would also expect a gain in fitness or performance, a decrease in the resting heart rate, and an increase in the rate of heart rate recovery after exercise. For that same group of athletes, a taper, or reduction in training load might ordinarily lead to a subsequent increase in HRV.

There is particular interest in the possibility of using HRV as a tool for identifying negative adaptation to training — to avoid the problems of overreaching or overtraining. Unfortunately, the results of studies that were designed to produce training scenarios of overtraining have produced conflicting results; some have resulted in markedly decreased HRV and others have resulted in markedly increased HRV. As a generalization, though, we might expect that accumulated fatigue would be indicated by an increase in resting heart rate together with a variable effect on HRV and that overtraining might be indicated by decreases in both resting heart rate and HRV. In general, an otherwise unexplained reduction in HRV may be an indication of fatigue. Quantities such as the natural log of the rMSSD (Ln rMSSD) have been proposed as an index of fatigue, or a marker of “readiness to perform.”

Here’s the rub, though. In elite athletes and recreational athletes with long training histories, these typical changes have been less consistent. It turns out that HRV responses to training are not only specific to an individual but also to both the recent and remote training history. The most important observation is that the relationship between HRV and fitness is simply different in well-trained athletes: there can be increases in HRV with no corresponding increase in fitness over a training cycle and there can also be decreases in HRV despite increases in fitness.

Now that we’ve covered what HRV is and how to interpret it, part 3 of this series will look at some of the hardware and software tools that are available to help you make use of HRV data.