endurance athlete

Heart Rate Variability: Application in the Endurance Sports

June 30, 2017 By Larry Creswell, MD Leave a Comment

This past week, an excellent article by Charles Wallace, entitled “What’s Your Heart-Rate Variability? It May Be Time to Find Out,” was published in the Wall Street Journal. My good friend, exercise physiologist and coach, Alan Couzens, contributed on the practical aspects of using heart rate variability (HRV) in the training of endurance athletes generally, and his coached athlete, Inaki de la Parra, specifically.

Since then, several readers inquired about previous columns I’d written for Endurance Corner on the topic of heart rate variability, noting that links at my blog to those columns were no longer working. My apologies! I’d need my own IT person to keep track of the many old links.

Here’s part 2 of the original Endurance Corner columns….

Part 2: Application in the Endurance Sports

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. Today’s column looks specifically at the use of HRV in endurance training. We’ll talk about how and when to measure HRV; how HRV might be used to help guide your training; and about some of the hardware and software tools that are available to help you make use of HRV.

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 3 sorts: “resting,” “during exercise,” or “post exercise.” And 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 (eg, rMSSD) is much less than for the frequency domain indices (eg, 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 2 general settings—the short term and the longer term.

In the short term. On any given training day, intense exercise will lead to a decrease in HRV and this effect can persist for 24-48 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 (eg, sleep, hydration, 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.

In the 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.

Hardware and Software

A variety of hardware and software tools are available for athletes and coaches who are interested in using HRV to help guide their training plans.

Omegawave. In triathlon circles, Omegawave is probably the most familiar name in HRV technology. Their system for individual athletes includes a heart rate monitor/chest strap that communicates by Bluetooth with a subscription-based mobile software app. The device is used to make a 2-minute recording of the resting HRV. Then, using proprietary algorithms (invisible to the user), the software calculates an index of Cardiac Readiness along with Cardiac Readiness Elements that include “stress,” “recovery pattern,” and “adaptation reserves.” The software also generates a table of appropriate training zones based on heart rate and an index of aerobic readiness. Omegawave touts the utility of their system in helping athletes determine their “readiness to train.” I’ve used the Omegawave system and found it very easy to use. The down side, of course, is that it’s a bit of a black box. Athletes just don’t know exactly what’s being measured or reported.

BioForce HRV. Like the Omegawave system, the BioForce system includes a mobile app together with web-based software that are designed to work with a hear rate monitor (eg, Polar). An index of HRV, again not explicitly defined, can be measured during a 3-minute rest period and stored for comparison with succeeding days. Included with the system is a book, “The Ultimate Guide to HRV Training,” where training recommendations are based primarily on the day-to-day changes in HRV. Like the Omegawave system, the user is blinded to what exactly is being calculated or derived for the HRV index.

Ithlete. Another similar product is the ithlete HRV system which uses a proprietary heart rate monitor or finger probe, together with a mobile app, to calculate an index of HRV. ithlete offers the advice that a large drop in HRV from one day to the next should prompt the athlete to back off from training. Like the Omegawave and BioForce systems, the user is blinded to what exactly is being calculated.

Heart Rate Monitors. Some heart rate monitors (eg, Polar, Suunto) include a feature that allows for data collection and reporting on R-R intervals that serve as the basis for any HRV calculations.

Kubios HRV software. Made available for free download by the Biosignal Analysis and Medical Imaging Group at the University of Finland, and intended originally for use by scientific investigators, Kubios HRV software allows for calculation of the most common time and frequency domain measures of HRV. Inputs can come from an ASCII file of R-R interval data or from some standard heart rate monitor data files (eg, Polar, Suunto). This software is probably the best (and cheapest) tool for athletes who might want to derive particular measures of HRV and relate them to their training. The Kubios user’s guide includes not only instruction on the software but also general information about the underpinnings of the various HRV indices.

Physionet software. Another option for free, open-source software comes in the form of a HRV Toolkit from the Division of Interdisciplinary Medicine and Biotechnology at Beth Israel Hospital/Harvard Medical School. These tools do not have a graphical user interface like Kubios, but do allow for calculation of many of the relevant HRV indices and graphical representation of the results.

Some Thoughts and Recommendations

HRV technology might well be most useful for dedicated amateur and elite endurance athletes who are looking for additional ways to monitor their training, make day-to-day adjustments to their training patterns, and avoid the negative adaptations of overreaching or overtraining. But from what we know from the rather limited studies of elite endurance athletes, HRV may not have the same, predictable relationships to a training cycle that have been observed in less-trained recreational athletes and non-athletes.

In thinking about the hardware and software tools that are currently available, the Omegawave, Bioforce, and ithlete systems might be best suited for athletes who want to use HRV monitoring for the “short term” application I described above. A Kubios-based approach might be more suitable for athletes who want to use HRV monitoring during and through various training blocks. There seems to be a real opportunity for the heart rate manufacturers and the training data analysis/repository vendors (eg, TrainingPeaks) to offer some easy-to-use, mathematically transparent tools for everyday athletes.

Realize that none of this is particularly simple, at least not yet. The serious endurance athlete who wants to make use of HRV monitoring might do well to use a Kubios-based approach to track some indices for a season and to simply gain familiarity with the process. In so doing, you’d become aware of how various HRV indices related specifically to each phase of your training. You’d become aware of both positive and negative trends in that regard. You’d then be in a position to see how best to make use of HRV in conjunction with other markers like fatigue, performance, resting heart rate, exercise heart rate, and heart rate recovery.

There’s no doubt in my mind that the use of HRV technology will become more widespread in the endurance sports, particularly as we learn more about the real-world experiences of well-trained recreational athletes. Stay tuned.

Heart Rate Variability: The Basics

In the Medical News: Does Heart Function Suffer in Long-term Endurance Athletes?

April 17, 2016 By Larry Creswell, MD 19 Comments

Background

Moderate amounts of exercise produce a multitude of health benefits. Both the American Heart Association (AHA) and World Health Association (WHO) now recommend 150 min per week of moderate intensity exercise for adults.

Some recent observations, though, have raised the question: when it comes to exercise, can there be too much of a good thing? And, more specifically, can too much exercise somehow be harmful to the heart. With the increased popularity of adult recreational and competitive sports–particularly in cycling, running, and triathlon–there is a growing number of adults who are pursuing exercise far in excess of the AHA and WHO recommendations. The questions surrounding the issue of “too much exercise” are very pertinent.

A few recent articles in the popular press summarize some of the findings and frame the debate:

Can Too Much Extreme Exercise Damage Your Heart?–at Cleveland Clinic online
Can Too Much Exercise Harm the Heart?–by Gretchen Reynolds at NY Times Well Blog
Is Too Much Exercise BAD for the Heart?–by Anna Hodgekiss at The Daily Mail
Extreme Exercise and the Heart–by Lisa Rosenbaum at The New Yorker

The worrisome observations have generally been made in small numbers of symptomatic athletes, in groups of athletes whose athletic history–or “dose” of exercise is poorly characterized, or in studies that were designed for some purpose other than determining the effects of “too much exercise” on the heart.

I’ve said previously that these observations deserve our attention, but that we really need more targeted investigation into this issue. In a study just released online (ahead of print) in the medical journal, Circulation, Philipp Bohm and colleagues from the Institute of Sports and Preventive Medicine at Saarland University in Germany bring us an important new look at “extreme” exercisers.

The Study

This new study focuses on a group of 33 healthy, male, Caucasian competitive elite master endurance athletes. This group of athletes was selected so that it included only athletes with a 10+ year continuous training history of 10+ hours per week; the average training was 16.7 hours per week and the average training history was 29 +/- 8 years. These athletes had an average age of 47 years (range, 30-60 years). This group of athletes included:

Sixteen were former elite professional athletes
One Ironman world champion and several 2nd and 3rd place finishers
The world record holder at the long distance triathlon
A 2nd ranked cyclist of the Vuelta a Espana (Tour of Spain)
Six Olympic athletes in the sports of triathlon and rowing
A former winner of the Munich Marathon.

A control group consisted of 33 healthy Caucasian men who were pair-matched for age, height, and weight. This control group was selected to include only individuals who exercised <3 hours per week.

All of the subjects underwent a comprehensive evaluation that included:

History and physical examination (to exclude any athlete with a history of overt heart disease, high blood pressure, smoking history, or other risk factor for heart disease)
Resting EKG
Cardiopulmonary exercise testing
Echocardiography, including tissue-Doppler imaging and speckle tracking
Contrast-enhanced cardiovascular magnetic resonance imaging (CMR).

Interestingly, none of the athletes presented with, or reported a history of, atrial fibrillation.

There were several unsurprising, and expected differences between the athletes and the controls. First, the resting heart rate (HR) for the athletes (48 +/- 7 beats per minute) was slower than for the controls (65 +/- 11 beats per minute). Second, the size of the athletes’ hearts was significantly greater. The left ventricular (LV) mass for the athletes’ hearts (188 +/- 26 g) was significantly greater than the controls (124 +/- 23 g). Similarly, the right ventricular (RV) mass for the athletes’ hearts (70 +/- 13 g) was significantly greater than the controls (49 +/- 11 g). Among the 33 athletes, 22 met a traditional definition of “athlete’s heart,” with a heart volume of 13+ mL/kg of body weight. As expected, the VO2 max of the athletes (60 +/-5 ml/kg/min) was significantly greater than controls (37 +/- 6 ml/kg/min).

The important results of the study were those that showed no difference between the athletes and the controls. With echocardiography, there was no difference between athletes and controls in LV longitudinal strain or RV longitudinal strain–measures of the strength of contraction. Using CMR, there was no difference between athletes and controls in LV ejection fraction (EF) or RV EF–again, measures of the strength of contraction. One athlete (3%) had a LV EF slightly less than normal, at 45%. No athletes or control subjects had abnormalities of the RV that could be suggestive of the potentially life-threatening problem of arrhythmogenic right ventricular cardiomyopathy (ARVC). One athlete (3%) had late gadolinium enhancement (LGE) on CMR that suggested previous, asymptomatic inflammation of the pericardium, the sac in which the heart sits. LGE analysis showed no evidence of unusual fibrosis or scarring in either athletes or controls.

My Thoughts

Kudos to the investigators here. The study is apparently self-funded. It’s expensive to perform this kind of testing; in the United States, the costs of this study would easily run into the many hundreds of thousands of dollars. Kudos, too, to the editors at Circulation. There is a tremendous bias against publishing so-called “negative” studies, where no important differences are found between study and control groups. Many “negative” studies are left on the editing room floor–and we never hear about them.

This is an important study because it is the first to gather and study a group of long-term endurance athletes with a substantial, and defined, training load over an extended period of time. The results deserve our attention. At nearly 17 hours per week of exercise or training, these athletes obviously far exceeded the contemporary recommendations for 150 minutes of moderate exercise per week. Just doing some quick math, the average cumulative exercise “dose” is more than 25,000 hours. As I’ve said many times before, it’s worth asking the question if such an exercise pattern can be harmful to the heart over the long term. This is a terrific group of athletes to study in order to help answer that question.

We must keep in mind that, with just 33 athletes, this is a small study. With only 33 athletes, it’s obviously possible to miss something that would be found in the 34th athlete. We must also keep in mind that the study only involves male athletes. Female athletes are not immune from heart problems and deserve study, too.

It is a striking finding that no athlete was found to have atrial fibrillation–either now, or in the past. Moreover, no athletes experienced arrhythmias during the cardiopulmonary exercise test. A number of previous studies have reported a 2- to 5-fold increase in atrial arrhythmias among long-term endurance athletes. Like the current study, all of those previous studies have involved small number of athletes. None, though, have focused on athletes like these, with such extensive exercise and training histories. In my opinion, endurance athletes broadly can take some comfort from the findings of this new study with regard to the potential risk of atrial arrhythmias.

It’s noteworthy that the LV and RV function of the athletes was no different than the controls. One athlete had mildly depressed LV function, for reasons that are not clear. In short, though, the study found no evidence of cardiac damage–at least, in terms of the pumping function–that accrued over the long term. We know that there is some depression of LV and RV function immediately after an intense bout of exercise (eg, marathon, long-distance triathlon, long-distance cycling event), but we also know that these changes resolve within days to weeks afterward. The current study argues against the hypothesis that repeated episodes of intense exercise (ie, many marathons or triathlons over a lifetime) might result in a decrease in LV or RV function. Again, this is encouraging news for endurance athletes.

Finally, the CMR and LGE results are important. Aside from the 1 athlete with possible previous pericarditis, there were no worrisome findings of fibrosis or scarring that might be attributable to injury from repeated episodes of intense, strenuous exercise. These LGE findings are at odds with some observations of unexplained fibrosis in other cohorts of long-time runners, even if the consequences of such findings remain uncertain. This area of investigation deserves further attention. For now, I’d say that long-time participation in endurance sports does not necessarily result in unexplained fibrosis in the heart.

Related Posts:

Thoughts on the Recent VeloNews Article

September 1, 2015 By Larry Creswell, MD 3 Comments

I enjoyed reading a recent article in VeloNews by Chris Case, entitled “Cycling to Extremes: Are endurance athletes hurting their hearts by repeatedly pushing beyond what is normal?” The article is good reading.

First, I give a lot of credit to Chris Case and the editors at VeloNews. It’s great that a publication with such a broad audience would devote time and space to the issue of heart health and endurance sport. In recent months, they’ve also brought attention to the heart problems of pro cyclists, Robert Gesink, Olivier Kaisen, and Eddy Merckx. I wish that other writers and publications would do the same.

I also thank Lennard Zinn and Mike Endicott for sharing their personal stories with their heart problems. It would be very easy to keep quiet. I very much enjoy reading personal accounts such as these. Their stories are real and also familiar. This is how we learn.

Since the article was published, I’ve gotten a bunch of inquiries asking my opinion about the article in general or about specific information that was presented. Let me share a few thoughts that may be helpful to readers here at the blog….

Don’t be scared (too much)! For most people, cycling is a healthy pursuit. In general, exercise is healthy and provides a myriad of benefits. So don’t stop cycling! It’s important to keep in mind that the stories of Zinn and Endicott are not the norm, even among veteran endurance athletes. Zinn’s multifocal atrial tachycardia (MAT) is one of the least common atrial arrhythmias and Endicott’s sudden cardiac death is rare. As you absorb their stories, focus not on the particular arrhythmias but rather on the possibility that an arrhythmia–any arrhythmia–can cause significant problems or be an indication that things are amiss with the heart. In that sense, their stories should cause you to put on your thinking cap.

My favorite quote from the article? “But fit for racing doesn’t necessarily equal healthy.” Readers here at the blog will know that I’ve said this repeatedly. It’s easy for seemingly healthy endurance athletes, particularly men, to believe that fitness is the same thing as healthiness. This isn’t necessarily true. To dispel the myth, I’ve shared the stories of many elite endurance athletes who’ve struggled with heart problems of various sorts. We can add Zinn and Endicott to these lists. Heart problems are common….and athletes aren’t exempt. This is the most important take-home message from the article.

Arrhythmias are common–in athletes and non-athletes, alike. All athletes experience arrhythmias. Infrequent premature beats, originating in either the atrium (premature atrial contractions, or PAC’s) or in the ventricles (premature ventricular contractions, or PVC’s) most likely have no consequence. Sustained arrhythmias, on the other hand, deserve attention and evaluation. There are far too many varieties of arrhythmias to consider here, other than to mention some of their names: SVT, or supraventricular tachycardia; WPW, or Wolff-Parkinson-White syndrome; atrioventricular (AV) nodal re-entry tachycardia; atrial or ventricular bigeminy; sick sinus syndrome; sinus bradycardia; atrial fibrillation (AF); atrial tachycardia; MAT; ventricular tachycardia (VT); and ventricular fibrillation (VF).

The last part of the VeloNews article alludes to AF. Other than sinus bradycardia (simply a heart rate slower than 60 beats per minute, which may be very healthy in athletes) or innocuous premature beats, AF is probably the most common arrhythmia in athletes. We know from longitudinal studies that the lifetime risk of having AF is approximately 25% in the general population. The question of whether athletes–and endurance athletes, specifically–are more prone to AF is a current controversy, with important implications for long-term endurance athletes. I’ll try to finish up a separate blog post that summarizes the accumulated evidence on this issue. For men, there may be an association with long-term exercise and the prevalence of AF, but there is certainly no consensus among experts. For women, the evidence does not suggest an association between long-term exercise and AF.

Pay attention to warning signs. I particularly like the last section of the article, written by Dr. John Mandrola. He’s a cardiologist who specializes in arrhythmias and who is also a (former?) triathlete and current avid cyclist. He provides good advice in the Q&A. I like to talk about 5 important warning signs of possible heart disease: chest pain or discomfort, especially during exercise; unexplained shortness of breath; light-headedness or blacking out (syncope), especially during exercise; unexplained fatigue; and palpitations–the sense of a rapid or irregular heartbeat. Any of these warning signs may be due to an arrhythmia. All deserve investigation. Dr. John makes the apt point that, very often, heart rhythm problems start off small and get worse with time. Not surprisingly, it’s best to get things sorted out earlier rather than later.

Less may be more. Lastly, I would encourage athletes with identified arrhythmias to be open to the idea that less exercise may be helpful. In fact, this may be the most appropriate prescription. For the long-term endurance athlete, this can be difficult to accept. In this regard, the stories of Zinn and Endicott are particularly poignant.

2. Atrial Fibrillation in Athletes (in a Nutshell)

3. Too Much Exercise, Revisited

4. Don’t Stop Running Yet!

In the News: Atrial Fibrillation in Cross Country Skiers

February 17, 2010 By Larry Creswell, MD 2 Comments

We have talked previously here at the blog about arrhythmias….and specifically about atrial arrhthmias–those that arise in the upper chambers of the heart.

In an interesting study summarized in an article last week at ScienceDaily.com, Norwegian investigators reported on a longitudinal study of cross country skiers, focusing on the development of atrial fibrillation. Starting in 1976, a group of 122 athletes have been followed, with monitoring for the development of arrhythmias. The study is remarkable because of the length (30+ years) of follow-up.

It turns out that, among participants who were alive for the entire period, the prevalence of “lone atrial fibrillation” (that is, without other heart disease) was 12.8%. And this compares to a prevalence of ~0.5% in the general population. Among the athletes with atrial fibrillation, there was also a higher frequency of enlargement of the left atrium and bradycardia (a heart rate
Other studies have also shown an increased prevalence of atrial fibrillation among endurance athletes. It’s not entirely clear yet what the long-term implications might be. And it may well be the case that no specific treatment is needed for athletes who have “lone atrial fibrillation.”

I imagine that more information from this study (and commentary, as well) will become available, and I will share anything else I learn.