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Understanding the Heart Rate Variability
Everything you need to know about Heart Rate and Cardiovascular Health.
HRV, while not a novel concept and relatively straightforward to utilize effectively, faces challenges due to a lack of standardization and methodological variances. These issues often complicate individuals’ ability to leverage the technology efficiently or to interpret findings reported in scientific research accurately. HRV is a term that refers to ways to summarize in a number the variability between heartbeats.
Alright, but why do we care?
Simply put, HRV represents the most accessible, non-invasive, and cost-effective method to gauge the functioning of our autonomic nervous system (or at least a segment of it). Our bodies are in a constant state of adjustment aiming for equilibrium, known as homeostasis. Various physiological aspects—such as heart rate, blood pressure, glucose levels, and hormone secretion—respond to the challenges we encounter, including stress. The autonomic nervous system plays a pivotal role in maintaining this balance, ensuring we remain in peak condition (for instance, preventing chronic ailments or boosting our performance).
At rest, our heart rhythm, and by extension, HRV, is predominantly governed by the parasympathetic branch of the autonomic nervous system, responsible for rest and recuperation. As such, HRV serves as a window into how our body reacts to various stressors, be it from training, lifestyle choices, and more.
A decrease in specific HRV metrics (such as rMSSD) generally indicates a dip in parasympathetic activity, suggesting either incomplete recovery or heightened stress levels, hindering our ability to swiftly return to a state of normalcy. However, these insights are predicated on the assumption that the data collection follows validated methodologies and best practices, which will be discussed in subsequent sections.
How to measure heart rate Variability
It is strongly recommended to use a Chest Strap for measuring Heart Rate and Maximum Heart Rate in particular, as chest straps are known to be the most precise tools to get real numbers, while devices that use Optical Sensors are lagging and show heart rate up to approximately 85% accuracy, depending on many factors: wearing contact, day light, movements, sensor itself, etc.
Once you’ve secured a method for obtaining precise data, measuring your HRV first thing in the morning or throughout the entire night are the two recognized and dependable approaches. It’s important to note that our focus is on identifying levels of chronic physiological stress, which results from both intense short-term stressors (such as rigorous exercise or jet lag from intercontinental flights) and persistent chronic stressors (like ongoing work-related concerns). By assessing how these stressors affect our baseline physiological state, we can implement targeted changes that significantly enhance our overall health and performance (details to follow).
There are some important differences between morning and night measurements, but the data is very similar, as long as you follow the most important best practices:
- Morning Measurement: Upon waking, reach for your phone and begin your HRV measurement using either the phone’s camera, a chest strap, or a dependable optical sensor (like the ones previously discussed). This morning routine typically takes 1 to 2 minutes, although extending the duration isn’t necessary to obtain a valid HRV reading. During this time, minimize any movement and maintain natural breathing patterns without consciously controlling your breaths. Should you need to use the restroom, it’s best to do so prior to measuring. I suggest conducting the measurement while seated to introduce a minor orthostatic challenge, enhancing the data’s responsiveness to stress factors. This is particularly beneficial if you exhibit a lower heart rate or engage in endurance sports.
- Night Measurement: Throughout the night, you’ll wear a device designed to track your HRV as you sleep. This device periodically records your HRV for several minutes at a time, calculating an average HRV value from these intervals to determine your overall night HRV. It’s important to recognize that measurements taken briefly across the night can be influenced by your circadian rhythm and different sleep phases, along with potential misidentification of these sleep stages. Such factors may impact the data’s accuracy, a concern thoroughly examined in this article. Regardless of which sensor you opt for, you have the option to input your data into the some apps as part of your morning routine, to understand and decode the data. This allows for a nuanced analysis and understanding of your measurements.
Which measurement to choose: Morning or Night Measurements?
Both morning and night measurements are credible techniques that have been extensively applied in both research settings and everyday life to assess physiological stress. Consequently, I suggest choosing the method that best aligns with your personal preferences and routine.
Nonetheless, distinctions do exist between the two. For instance, various stressors like alcohol consumption, exercise, and dietary choices acutely affect your resting physiology. Consequently, stressors occurring later in the day might more directly influence your night HRV data since the measurement occurs sooner. By the next morning, your physiology may have returned to its baseline, assuming the stressor wasn’t exceptionally intense. Morning measurements might offer more clarity if your evenings are typically characterized by late meals, alcohol, or physical activity. Conversely, if your nightly routine is relatively stable with minimal late-day stressors, your night data should closely mirror your morning readings.
Moreover, morning sessions allow for orthostatic measurements (taking readings while seated or standing), introducing a slight physiological challenge that enhances the measurement’s sensitivity to stress. Thus, morning measurements can more accurately reflect your readiness for the day, whereas night measurements may predominantly capture the effects of the previous evening’s activities.
With that in mind, both morning and night measurements serve as reliable methods for assessing baseline physiological stress. Thus, if wearing a device overnight aligns with your preference, opt for a gadget designed for nocturnal use. Conversely, if you’d rather not wear a device while sleeping and prefer a morning check-in, then that approach is equally valid. For those on the fence or seeking a cost-effective entry point into daily physiological monitoring, utilizing your smartphone’s camera and a minimal investment of around $10 can provide accurate daily insights.
HRV4Training is compatible with all mainstream sensors (including chest straps, the Scosche Rhythm24, and the Apple Watch) and even the Oura ring. Moreover, it allows for manual entry of night HRV data from any device. Regardless of your chosen method, you’ll gain access to comprehensive analytics and insights. The act of measuring is merely the beginning.
Consistency is the key
The array of factors that can influence HRV analysis is virtually limitless. Why is this the case? As outlined in the introductory theory section of this post, HRV serves as our body’s reflex to stress—and virtually anything can act as a stressor.
This also dispels the notion that HRV holds no relevance to specific sports. Regardless of whether you’re engaged in a particular sport or lead a sedentary lifestyle, stress is a universal aspect of human existence. Thus, HRV emerges as a valuable tool for gauging our stress response and guiding us towards necessary adjustments.
Moreover, this underscores the importance of adhering to a consistent routine, be it measurements taken in the morning or derived from nighttime data. Establishing and maintaining a consistent protocol allows for the accumulation of reliable, longitudinal data reflective of your stress response. In essence, the process is straightforward: maintain a relaxed state, breathe naturally, and you’re on your way to gathering meaningful insights into your physiological stress levels.
What does the Heart rate variability reading mean?
HRV is calculated by analyzing specific metrics derived from a sequence of RR intervals, or the time differences between consecutive heartbeats, as illustrated in the diagrams at the start of this post.
Unlike heart rate, which can be viewed as an almost immediate measure, HRV necessitates the collection of a substantial dataset for accurate calculation. Over the past decade or so, the sports science community has converged on the rMSSD metric as the most significant and practical HRV figure for both academic research and everyday application.
Why focus on rMSSD?
The rationale lies in our body’s physiological mechanisms. Specifically, the parasympathetic system, represented by the vagus nerve, influences heart rate variability on a beat-to-beat basis through receptors in the heart, while the sympathetic system’s influence is mediated through slower pathways. Therefore, the beat-to-beat variations measured by rMSSD directly reflect parasympathetic activity, a mathematical outcome of how this metric is calculated. Although HF (high-frequency power) is another commonly used HRV metric, rMSSD is generally preferred because it’s less affected by breathing patterns, making it a more stable indicator of parasympathetic activity.
To put it simply, parasympathetic activity is indicative of our body’s rest and recovery mechanisms. In the context of HRV, an external stressor might prompt a decrease in parasympathetic activity, manifesting as a lower HRV. This reduction in HRV is a direct result of the nervous system’s regulation of heart rhythm in response to stress, offering a window into our physiological stress response.
important bit to remember, for now, is simply that HRV data is highly individual and has inherently high day-to-day variability. This means that it is not meaningful to compare to others and that in your own data there can be large fluctuations between consecutive days. Similarly, HRV should not be interpreted as “higher is better”, a common issue with most other apps and tools out there.
What are the implications? To make effective use of the data, we need to be able to determine what changes are trivial, or just part of normal day-to-day fluctuations (what we call your normal range), and what changes do matter and might require more attention or simply truly represent a positive (or negative) adaptation to training and other stressors
What to do with measured data?
As you begin to gather HRV data, the crucial aspect to remember is to always contextualize your HRV readings against your own historical records. Comparing your HRV with someone else’s is not productive because everyone’s “normal” varies—what’s standard for you may differ significantly from someone else’s, and that’s perfectly normal. These variances in what we term as the average baseline HRV are often rooted in genetics and other less changeable factors (except in instances where lifestyle or health improvements can enhance your baseline). However, the most insightful application of your HRV data lies in monitoring your trends over time.
Understanding that our focus should be on the relative shifts observed in our data over time, against our past measurements, becomes pivotal in interpreting HRV data accurately. Now, let’s delve deeper into an essential aspect of data analysis: recognizing and understanding the significance of day-to-day variations in HRV.
Fluctuations of Heart Rate variability from day to day
HRV is known for its significant variability from one day to the next, exhibiting larger swings than you might see in more stable metrics like heart rate or body weight.
What does this mean for interpreting your data? To harness HRV insights effectively, it’s crucial to differentiate between minor shifts that fall within the realm of everyday variability and those significant alterations that signal meaningful changes, possibly due to training or other stress factors. Essentially, understanding your typical HRV range is key to identifying when a deviation from this norm is noteworthy. In the upcoming sections, we’ll delve deeper into distinguishing between these variations and utilizing HRV data to its fullest potential.
Find your “Normal range”
Analyzing HRV necessitates reevaluating common perceptions about health metrics. Instead of adhering to a “higher is always better” philosophy, it’s crucial to embrace the concept that “normal is optimal.” Stability in physiological conditions often indicates good health.
The natural fluctuation of HRV measurements presents a challenge that your chosen application or software must adeptly manage. This challenge has been a focal point in the development of HRV4Training, particularly in formulating the app’s daily recommendations.
Software that interprets increases in HRV uniformly as positive and decreases as negative overlooks the essential fact that physiological variations within a normal range are to be expected. It’s deviations from this norm that may warrant further investigation or could be seen as significant changes.
In HRV4Training, we emphasize whether your daily score falls within your typical variability or if it diverges significantly. For instance, a notably low HRV reading could indicate increased bodily stress, such as from travel. Conversely, resting heart rate might not exhibit the same sensitivity to such stressors, a topic we’ll explore in greater depth. This approach ensures users receive a nuanced understanding of their physiological state, beyond mere numerical values.
Monitor the heart rate variability from day to day
I suggest initially focusing on simply gathering data over a span of several weeks, or even months. This approach allows you to observe your personal response to different stressors and see how your decisions, encompassing both training and lifestyle, are mirrored in the data. Additionally, it provides insight into how these objective measures correlate with your subjective feelings. Without immediately adjusting your habits, you can start to analyze your data using two primary methods:
- Acute Fluctuations: Observing day-to-day changes in response to significant stressors.
- Medium to Long-term Patterns: Monitoring responses and adjustments to ongoing or major stress factors over time.
Let’s dive deeper into these analytical approaches.
What are the stressors and what happens after?
What constitutes an Acute Stressor? Acute stressors are incidents that momentarily impact your physiological state. Examples include a rigorous exercise session, a long-haul flight, menstrual cycle phases, illness, a night of excessive drinking, or consuming a lot of caffeine. These are influences that temporarily alter your physiology, typically lasting from a few minutes to around 24–48 hours.
Identifying and understanding acute stressors is relatively straightforward, and analyzing your data with these events in mind can offer insights into your body’s reactions. Examining immediate physiological changes in response to such stressors can also bolster your trust in the monitoring tools at your disposal, as these variations should be consistently detectable and replicable.
However, it’s crucial to acknowledge the complexity of human physiology. While the HRV shifts triggered by acute stressors are generally consistent and comprehensible, they don’t exist in a vacuum. Numerous other elements related to our lifestyle, training, and health are always at play. This interconnectedness underscores the value of HRV monitoring as a comprehensive tool to oversee and adjust for stress impacts, irrespective of the sports or activities you engage in. Stress is a universal constant, and HRV serves as a pivotal resource for managing it effectively. For example traveling or alcohol intake are a big stressors and impact the HRV directly.
heart rate variability and Training
Exploring training from an acute-response perspective is equally insightful. The principle of using Heart Rate or Heart Rate Variability to track recovery stems from the understanding that intensive training nudges the autonomic nervous system towards sympathetic dominance. This shift is observable as an increase in HR and a decrease in HRV, typically manifesting within the first 24 to 48 hours post-exercise.
To examine the link between training load and physiological response, one approach is to calculate the day-to-day fluctuations in an individual’s resting HRV. Following this, we can scrutinize how HRV changes in the days succeeding workouts of varying intensity.
As an illustration, we contributed to a study that demonstrated how morning measurements, taken in natural, everyday environments (outside the confines of a laboratory), effectively reflect the varying recovery needs triggered by different training intensities. This research, encompassing nearly 30,000 participants, highlights the capacity of unsupervised HRV tracking to indicate the diverse recuperation demands across a wide demographic.
Both – heart rate and HRV are indicative of recovery requirements post-training sessions of varying intensities. However, HRV stands out for its heightened sensitivity, making it a more valuable tool for monitoring recovery (the percentage variation in HRV is more significant and consistent across various age groups, unlike the variability seen with heart rate). Relation between HR, HRV, and training load split into two categories, analyzed on the entire dataset and grouped by age group. HR is consistently increased on days following higher intensity training load, while rMSSD is consistently reduced. Relative changes in rMSSD are larger, highlighting how HRV can be more discriminating for training intensity. Additionally, percentage changes in heart rate reduced with age while remaining constant for rMSSD. Error bars indicate the standard error.
For those intrigued by the study, the paper also methodically explores the effects of various factors on HRV, including training intensity, menstrual cycles, illness, and alcohol consumption.
The data clearly demonstrates a marked correlation between high-intensity exercise and decreased HRV the next day. This phenomenon represents a classic acute stressor, where the dip in HRV serves as an indicator for recovery needs, potentially signaling the necessity for a rest day.
Yet, this correlation doesn’t shed light on the long-term physiological adaptations to a training regimen or how one adjusts to other lifestyle stressors. It’s through the analysis of medium to long-term patterns that we gain insight into these broader trends.
An important consideration to add: Viewing training solely as a cause for decreased HRV oversimplifies the matter. HRV essentially reflects our physiological response to stress—or more accurately, our adaptation to and management of stress. While a decrease in HRV often points to heightened stress, effective adaptation to stress (such as from an intense training cycle) should lead to HRV stability or even an increase. It’s only when faced with excessive stimuli (like too much intensity or volume) or external stressors unrelated to training that a decrease in HRV would typically be observed.
Now, let’s examine some data from another perspective to further elucidate these concepts.
Long term adaptation to stress and heart rate variability
Analyzing HRV trends over extended periods, such as weeks to months, presents a fascinating and intricate challenge. While short-term, day-to-day fluctuations can accurately indicate the training load from previous days—thereby serving as a basis for using HRV in assessing recovery needs—the scenario becomes significantly more complex over the long haul. Nonetheless, delving into these long-term patterns offers profound insights into how we adapt to training, helping us decide when it might be beneficial to adjust our training plans.
The past few years have seen a surge in research facilitated by the advent of more user-friendly tools, yielding consistent findings that apply broadly across various sports and athlete demographics. This wealth of data has greatly enhanced our understanding of long-term HRV trends and their implications for athletic training and overall physical conditioning.
Exploring HRV trends over extended periods, such as weeks to months, unveils some of the most fascinating yet intricate patterns for analysis. While short-term, day-to-day fluctuations can effectively mirror the training load preceding the measurement—underpinning the rationale for utilizing HRV in gauging recovery needs—the narrative becomes more complex over the long haul. Nevertheless, examining these prolonged trends offers profound insights into how we adapt to training and when it might be necessary to adjust our plans.
Recent advancements in accessible tools have spurred extensive research in this area, revealing several key insights consistent across various sports and athlete demographics.
Example
This section will zoom in on a crucial metric: the HRV baseline (a 7-day moving average), and how it compares to normal values derived from historical data. Consider the following example: Various stress factors, each with its own effect on our capacity to manage additional stress, thereby influencing our health and performance. The accompanying illustration showcases how events like running a marathon or unusual activities around New Year’s Eve can lead to a drop in baseline HRV below normal levels—a definitive indicator of heightened stress and challenges in coping.
To gain a clearer insight into our physiological state, it’s essential to utilize statistical analyses that offer a more definitive perspective. Our software presents your historical data as a band, derived from the last 60 days of data, indicating the expected range of your data in the absence of significant disturbances. Typically, the baseline, or 7-day average, falls within this band, signaling that everything is proceeding according to plan. However, when daily scores or the baseline dip below this band, it suggests considerable stress, indicating a deviation from what’s considered normal for your physiology.
The graphical representations are invaluable for understanding the broader context, beyond just immediate reactions. For instance, in the midst of a demanding training period, a well-adapted response should not necessarily result in decreased HRV over the medium to long term. Instead, positive adaptation might manifest as sustained or even elevated HRV levels over several weeks.
HRV4Training integrates data from multiple parameters to offer a comprehensive view of your overall condition. By examining shifts in your HRV baseline, heart rate, and HRV’s coefficient of variation, the app can discern whether recent trends represent minor fluctuations or significant shifts warranting attention, all based on your past data. After analyzing these trends, HRV4Training categorizes your physiological response to training into one of several states: stable condition, effective coping with training, maladaptation to the current regimen, or cumulative fatigue.
Ultimately, the goal is to achieve stability. However, stability doesn’t imply an absence of stress—it represents the body’s recuperative response measured in a restful state, ideally in the morning, well after any training or stressor events. A rapid recovery in our HRV indicates a robust ability to manage and adapt to stress, signifying an optimal response in most scenarios.
Life is happening
Training isn’t the sole source of stress, regardless of whether you’re a seasoned professional athlete or someone who just embarked on their first run. Life is filled with various stressors, including work, family obligations, and personal expectations, all of which have a physiological impact beyond just the effects of physical exercise.
The essence of HRV monitoring underscores a crucial insight: it’s overly simplistic to consider training as the sole factor of importance. Regardless of the sport you’re involved in—or even if you’re not active in any sport—tracking how you respond to various stressors can be instrumental in guiding your path to recovery and balance.
Viewing your HRV data in isolation misses the mark. HRV serves as a gauge of your body’s reaction to both training and everyday life stressors. It’s vital to interpret the shifts in your HRV in relation to these influencing factors.
How is your overall feeling
Your subjective experience plays a crucial role in adding context to your measurements. At times, how you feel may align perfectly with your data (for example, your HRV is within its normal range, and you’re feeling great). However, there may be occasions where the data reveals stressors that haven’t yet become perceptible (such as a drop in HRV preceding illness, or the gradual onset of chronic stress). It’s important to recognize that subjective feelings and HRV data aren’t expected to consistently mirror each other or converge over time.
Contrary to the view that pits subjective experiences against objective data, it’s clear that both subjective insights and the ability to gauge our feelings hold significant value. This is the rationale behind incorporating a questionnaire following each measurement, offering a moment to reflect and subjectively evaluate how you’re feeling—an integral step in the process.
Leveraging the data can enhance this reflective practice. Over time, as you accumulate more data, you’ll become adept at refining your subjective assessments with insights drawn from your objective responses to stressors.
A discerning coach, educator, or athlete recognizes the importance of combining training load, HRV, and subjective insights. These elements collectively inform daily decisions, optimizing the path forward.
Ultimately, there’s no competition between objective and subjective measures; each fulfills its unique function, contributing to a comprehensive understanding of well-being and performance readiness.
Analyzing and adjustments by heart rate variability data
Until now, we’ve explored the utility of HRV data in shedding light on our responses and adjustments to a range of stressors. This understanding is crucial for enhancing health and performance, yet it marks only the beginning, with actual modifications yet to be applied.
From my perspective, HRV ought to serve as an ongoing feedback mechanism. The ultimate aim, over time, is to boost health and performance by strategically applying stressors when appropriate, as evidenced by HRV-guided training research. The focus should be on enhancing these aspects rather than improving HRV scores themselves. This approach contrasts with metrics like resting heart rate (or, more precisely, submaximal heart rate during physical activity), which directly correlates with cardiorespiratory fitness improvements and decreases as fitness levels rise. Consequently, HRV does not serve as an effective fitness indicator, which is why we do not incorporate it into our VO2max estimations.
Revisiting the Fundamentals: HRV serves as a reflection of parasympathetic activity, essentially gauging our physiological stress levels. By tracking how our bodies react to various stressors—including those stemming from everyday life, such as work and family pressures—we gain valuable, objective feedback. This insight enables us to implement practical adjustments in response.
Often, the most straightforward adjustments involve a greater level of self-honesty and the willingness to slow down occasionally, particularly when we’re experiencing high levels of stress. As described earlier, the connection between increased stress (indicated by a drop in HRV) and the need for moderation is clear. Lower HRV can signal the necessity to ease up, thereby preventing overtraining and facilitating quicker recovery, which are crucial for enhancing performance.
When your HRV falls within normal ranges—a “green light” scenario—it should reassure you that your training and lifestyle are being well-managed. However, adherence to your training schedule remains paramount. If it’s a rest day, take it; if it’s time for a low-intensity workout, proceed as planned. It’s crucial to recognize that HRV and other physiological metrics are designed to augment your understanding of your body’s reactions to your regimen, not to overhaul your training strategy entirely.
