“A healthy heart is not a metronome. The oscillations of a healthy heart are complex and constantly changing, which allows the cardiovascular system to rapidly adjust to sudden physical and psychological challenges to homeostasis.” -Shaffer&Ginsberg, 2017
The development of wearable fitness and health devices over the last decade has been fast and furious. The first wearable wire-free heart rate monitor came out in the early 80s, but it wasn’t until the late 2000s that the technology really made large improvements.
Fitbit didn’t release a wrist-worn tracker until 2013 and it took another two years for Apple to release the first Apple Watch.
Since that first-generation of wrist technology, the wearable device industry has transformed. The average device can now track a myriad of individual biometrics – steps, distance travelled, floors climbed, minutes active, calories burned, exercise modality, sleep time/quality, heart rate, fitness score, and more.
Although the fitness industry pioneered the device market, wearable technology is quickly transitioning its focus toward general health outcomes. Continuous monitoring of biometrics with these devices allows individuals to track their health in a novel way. Going beyond heart rate or step count, devices can now evaluate health status to a greater degree with measurements such as heart rate variability (HRV).
What is heart rate variability?
Heart rate variability is the measurement of change in time between heartbeats. More specifically the time between R-R points on an ECG reading (the highest wave point). Tracking heart rate variability can help someone understand their heart health, stress and anxiety levels, and training readiness.
HRV changes and responds to autonomic nervous system activity. The ANS is a regulatory system that acts without our control (auto) to alter bodily functions like heart rate, digestion, respiratory rate, and sexual arousal.
We can further divide this system into two subcomponents, the sympathetic nervous system (fight or flight) and the parasympathetic nervous system (rest and digest). These systems operate in response to the environment—if the body is under stress the sympathetic system responds by increasing heart rate, dilating airways and pupils, and reducing digestive activity. On the flip side, if the body is in a more relaxed, parasympathetic state heart rate and blood pressure will decrease—among other things. For instance, in medical school we remembered the function of the parasympathetic system in sexual health by memorizing “parasympathetic points.” In other words, it is the system responsible for an erection in men. Which is why performance anxiety is a big deal! But I digress…
Heart rate variability changes in response to which system is the most active.
A low HRV shows an autonomic imbalance. Autonomic imbalance is characterized by the sympathetic branch of the autonomic nervous system dominating over the parasympathetic branch. If the sympathetic system is dominating, it will result in a decrease of heart rate variability. Chronically low HRV is associated with symptoms of worsening depression and anxiety, and increased risk of death and cardiovascular disease. Ok, that’s not so good.
That being said, it’s important to understand the activities or environments that trigger a sympathetic response.
Factors from our lifestyle that upregulate the sympathetic system include:
- Poor sleep
- Unhealthy diet
- Dysfunctional relationships
- Isolation or solitude
- Lack of exercise or, conversely, too much exercise (overtraining syndrome)
This is why tracking heart rate variability can be useful. Understanding HRV and how it changes throughout the day can help highlight behaviors or situations that negatively influence the activity of the autonomic nervous system.
While HRV can provide pathological and physiological information, there are limitations to its use.
Since heart rate variability is not a direct assessment of cardiac autonomic activity, any relationship between HRV and cardiac autonomic regulation is qualitative rather than quantitative. The information doesn’t provide quantifiable information of the actual nerve firing rate.
Regarding training – using HRV alone as a guide is not enough to determine an athlete’s readiness, or lack thereof, to train. Using heart rate variability alone to prescribe training would suggest that there’s a direct relationship between HRV and fitness level. Unfortunately, it’s not that simple. We should use heart rate variability to influence training decisions, not make them entirely.
Further, the reliability of commercially available devices needs to be approached with some skepticism. While we consider most devices generally accurate, with only insignificant differences when compared to the gold standard measures, the possibility of error is still present.
Lastly, HRV only assesses the autonomic nervous system which is only one component of our entire nervous system. This nervous system includes the brain, spinal cord, enteric (gut nervous system), and many smaller subsystems.
Applied to Training
While the devices that read heart rate variability might have small amounts of error, the information provided can still be very useful. A small amount of error won’t hide the general trends found in an individual’s HRV and can be used effectively to help plan workouts and training.
By tracking HRV an individual can more easily recognize when their body is well rested and when it needs time to recover.
The beautiful thing about using heart rate variability is that it takes many types of stress into consideration—not just the stress from a workout. Changes in HRV result from the total stress on the body (what a good friend of mine who is our lead human performance director calls “allostatic load.” Stress from work, relationships, partying, poor nutrition, exercise—anything and everything that upregulates sympathetic nervous system activity.
If an individual had a stressful day at work and was planning to go for a run in the evening, tracking HRV can give them a sign as to how that work stress has impacted their body and whether or not they should adjust their training to a shorter distance, slower pace, etc.
This probably sounds at least somewhat complicated and confusing.
How are you to interpret changes to your HRV? High variability is good and low is bad, but how much should it change from day to day and how should you change your exercise routine in response?
That’s where technology comes in. Living in the twenty-first century is fantastic—not only do devices track heart rate variability, they also help you understand the information.
Many devices on the market today provide analysis, tips, and suggestions from the data collected.
Current HRV Tech
The best and clinically accepted way of evaluating HRV is through analysis of a 12-lead ECG reading. Luckily, technology has improved to where wired devices are no longer needed to get reliable readings.
Today, wearable devices use one of two methods to calculate heart rate variability. Electrocardiogram (ECG) or Photoplethysmography (PPG). Typically, devices that are worn on the wrist use PPG while devices worn across the chest or on the fingertip are equipped with ECG technology. PPG is widely considered to be more susceptible to error and could produce less accurate readings.
Although the accuracy of these devices has been questioned, more advanced and accurate models are always being created.
Different types of HRV equipment include chest, wrist, finger, and ankle sensors, along with a variety of apps that can connect to different wearable devices. Below are some of the best products on the market. While there are many other options available, unfortunately there isn’t room to mention everyone.
The world’s first ECG monitor designed to provide continuous medical grade data while not interfering with your life. This monitor does not require shaving, patches or adhesives. It fits like a normal heart rate strap and uploads data automatically to the Qardio app which can then be easily shared with your doctor. From the website this product doesn’t seem to provide fitness related measures or analysis. All the same – this is a great option for an individual tracking heart health.
They design this product with athletic performance as priority number one. Designed by an HRV expert, strength and conditioning coach Joel Jamieson, the BioForce comes with a standard chest strap and an additional finger sensor, app access, and a 180-page guide on using HRV. BioForce tracks and compiles your day to day HRV, helping you to identify trends and determine your readiness to train. This seems to be the most HRV-centric product available.
WHOOP is a wrist-worn device designed to track and analyze a wide range of biometrics. The WHOOP strap collects five different metrics 100x per second for as long as you keep the strap on. It delivers analysis of your recovery, strain, and sleep to you daily via their app. By tracking data 24/7, WHOOP claims to optimize training and boost performance.
Biostrap is another wrist worn device designed to track a myriad of biometrics – but this product is unique in several ways. For one, they equipped the wrist strap with a pulse oximeter for continuous oxygen saturation readings. Most notably, a “shoepod” is also included with purchase. This shoepod tracks cadence, velocity, and reps – among other things.
This app provides an extremely in-depth analysis of heart rate variability. You’ll need to purchase a separate heart rate monitor to link to the app – and you might have to pay a monthly fee. They offer three different membership options – $0, $8 and $30 per month. For an individual serious about their training, this is an inexpensive option to level up your training.
A more complex and elegant system for HRV is OmegaWave. Aptly named since it combines HRV measurement with direct current potential of the brain (DC Potential). Brain biopotentials within a frequence range that is just shy of the EEG range is referred to as the Omega potential.
“Traditional periodization places the training volume and intensity as the point of control, regardless of the athlete’s functional state. In contrast, Omegawave’s approach situates the athlete as the object of control, adjusting their workout to accommodate their daily functional state. Due to the vast genetic and environmental variability between athletes, it is crucial that the training process remain fluid rather than fixed.”
DC potential is a more comprehensive marker of the functional state since it represents the cumulative activity of all functional systems in the central nervous system. When it comes to skills acquisition, learning and memory, psychological responses to stress and adaptive capacity, the measurement of DC potential can help tells us better when to train, how intensely to train, etc. and thereby improve recovery while reducing the risk of injury.
Three states are assessed with the Omegawave device.
- Current state of the Cardiac System (HRV) – assess “internal load” and is a fuel gauge for adaptation.
- Current state of the Regulatory Mechanism – DC Potential estimates the state of the CNS (brain and spinal cord), Gas exchange system (lungs), Detoxification system (liver and kidneys), and Hormonal system. These are the engines that drive adaptation and represent the horsepower of the engine.
- Current state of energy supply systems – A metabolic reaction index is used to reflect the overall
The Omegawave is elegant, comprehensive, and expensive. It’s what we use where I work and it is hoped that, with time, this will filter into the commercial sector at a better price tag.
Heart rate variability is a useful and clinically relevant metric to recognize and understand. We can use HRV to help identify autonomic imbalance, evaluate heart health, and aid in recovery efforts. Most importantly it can keep us from training intensely when we should spend more time recovering. Using heart rate variability to support programming can improve recovery, fatigue management, and overall fitness outcomes.
Wearable devices are now at the point in their development where accuracy and reliability are not major concerns – especially when using an ECG equipped device.
While the utility of using HRV in training is just beginning to be understood, it doesn’t appear to be harmful and, at the very least, will provide valuable insight around stress levels and general heart health.
To your health and performance,
Lanny Littlejohn, MD and Andrew Nixon
Andrew Nixon is a premier health and fitness writer www.nixonhealthfitness.com
- Shaffer, Fred, and J P Ginsberg. “An Overview of Heart Rate Variability Metrics and Norms.” Frontiers in Public Health. Frontiers Media S.A., 28 Sept. 2017. Web. 15 Mar. 2019.
- Billman, George E. “Heart rate variability – a historical perspective” Frontiers in physiology 2 86. 29 Nov. 2011, doi:10.3389/fphys.2011.00086