HRV Hardware: Don’t They All Measure the Same Thing?
IIn our special series HRV Insight, Dr Alfred Lohninger, CEO and Medical Director of Autonom Health GesundheitsbildungsGmbH, explores and documents various HRV topics, including current research areas.
This time, we’re examining the differences between smartwatches and electrode-based sensors. Through this analysis of HRV hardware, we aim to provide you with arguments for positioning our products. This will clearly demonstrate the benefits and added value our products offer compared to competitors, and most importantly, their unique selling propositions. This information will help you market them effectively.
High HRV indicates strong adaptability, whilst low HRV suggests stress.
Heart Rate Variability (HRV) measurements provide insights into the autonomic nervous system, revealing the interplay between parasympathetic and sympathetic activity.
As HRV becomes increasingly recognised and intriguing, recent years have witnessed a significant rise in HRV providers, both in hardware and software sectors.
Today, every other person wears a smartwatch claiming to measure HRV. But is this accurate? And what about data quality? In this article, we’ll explore the differences between smartwatches, chest straps, and adhesive electrode sensors.
What exactly does a smartwatch measure?
Depending on model and type, wearables use various sensors to collect, analyse and record data for comparison purposes. Smartwatches are wearables that combine mobile phone and fitness tracker functions, such as displaying messages, managing appointments, tracking workouts, and measuring fitness, heart rate and sleep quality.
Thanks to advanced technology, including gyroscope sensors for detecting rotational movements, altimeters, and positioning systems like GPS, GLONASS, or GALILEO, smartwatches can determine whether users are moving or stationary. Some models even measure blood pressure, whilst others monitor blood oxygen levels and heart rate. Many also feature sleep analysis capabilities, distinguishing between wakefulness, light sleep, and deep sleep.
The following features are common in many smartwatches:
- Breathing Rate: By measuring how frequently the wearer breathes per minute, the device can determine activity and stress levels.
- Distance: Most contemporary wearables display the distance travelled, utilising GPS tracking and motion sensors.
- ECG: Select watches can record ECGs (electrocardiograms) to evaluate heart health.
- Recovery Need: Some wearables employ an index to indicate whether the body would benefit more from training or rest.
- Women’s Health: This functionality tracks menstrual cycles and provides insights about energy levels on specific days, general wellbeing, and overall health status.
- Heart Rate: This metric helps users maintain optimal training zones during exercise.
- Elevation: Many sports watches incorporate a barometer that detects air pressure variations, enabling the device to determine whether the wearer is climbing inclines or stairs. This allows the wearable to calculate elevation gain.
- Intensity: A smartwatch can gauge workout intensity through the combined use of motion and GPS sensors.
- Calories: The smartwatch measures both calories burned and consumed based on activity level, workout duration, and intensity.
- Position/Compass/Navigation: Various satellite navigation systems, including GPS (Global Positioning System), GLONASS (Global Navigation Satellite System), and Galileo, assist in determining location. Working in conjunction with smartphones, these systems can pinpoint the wearer’s current position or track movement.
- Blood Oxygen Saturation: SpO2, expressed as a percentage, indicates the estimated oxygen saturation in the blood.
- Sleep Quality: Wearables analyse sleep phases and overall sleep quality by monitoring heart rate, breathing patterns, and movement throughout the night.
- Stress/Heart Rate Variability: The Fitbit “Sense” employs an electrodermal activity (EDA) sensor to evaluate stress indicators by measuring electrical changes on the skin’s surface.
- Training Progress: When a fitness watch records data such as movement, exercise, repetitions, distance, and speed over days, weeks, and months, these metrics are presented through graphical displays.
- Training Duration: The time spent engaged in an activity serves as a crucial factor in calculating calorie expenditure, exercise intensity, and overall training advancement.
How Does a Smartwatch Measure?
Most devices now utilise optical pulse measurement at the wrist using light sensors or transmit heartbeat data to the watch via a chest strap. For direct pulse measurement, two or more LED lights and an optical sensor on the underside of the casing work in tandem.
And here’s what makes it particularly intriguing for HRV professionals:
- Smartwatch values are displayed only as trends and
- they don’t provide precise results, as measurements are taken indirectly through software.
Verdict: Smartwatches cannot match the accuracy of sensors that receive electrical signals, such as chest straps or recorders with electrodes. And here’s another rather significant side note: Light sensors additionally require a steady body position like lying down or sitting. They become prone to measurement errors as soon as there’s significant movement!
Can I Conduct Long-term HRV Measurements with a Smartwatch?
Generally, smartwatches and wrist sensors are engineered to measure pulse rather than R-R intervals. And it’s precisely these R-R intervals – the time spans between heartbeats measured in milliseconds – that enable heart rate variability measurement!
According to a February 2021 study (link to study), there are currently no smartwatches capable of 24-hour continuous recordings:
“Heart rate variability (HRV) measurements provide insights into the autonomic nervous system and the balance between parasympathetic and sympathetic activity. High HRV can be advantageous, reflecting the adaptability of the autonomic nervous system, whilst low HRV may indicate fatigue, overtraining, or health concerns. There has been a surge in wearable devices claiming to measure HRV. Some offer spot measurements, whilst others record only during rest and/or sleep phases. Very few can measure HRV continuously (≥24 h). We conducted a narrative literature review to determine which currently available wearable devices are capable of continuous, precise HRV measurements. The review also aims to assess which devices would be suitable in a field environment specific to military populations. The Polar H10 appears to be the most accurate wearable device when measured against criteria and seems to even supersede traditional methods during exercise. However, at present, the H10 must be paired with a watch to extract raw data for HRV analysis when users need to avoid using an app (for security or data ownership reasons), which incurs additional costs.”
WHOOP, for instance, is among the providers marketing long-term measurements. This Boston-based technology company utilises a wristband that can be worn continuously. However, WHOOP doesn’t display recognised HRV values, instead working with proprietary values whose calculation method remains undisclosed.
Why Long-term Measurements?
“Because long-term measurements allow us to understand correlations much more comprehensively!” This was the spontaneous response from our team member Lorenz Pühringer to this question. He immediately provided several self-explanatory examples:
An Example
If I go running in the morning and overexert myself, my body becomes overtaxed and struggles to cope with work-related stress. Consequently, I feel exhausted and lethargic throughout the day. However, if I were to take a 5-minute measurement before running in the morning – as many smartwatch manufacturers recommend taking measurements after waking up – the watch might indicate that I’m fit. In contrast, with long-term measurement, I can track the progression of various events and understand their relationships.
Example Sleep Measurement and Sleep Quality
Many watches measure sleep quality. When I experience poor sleep, the watch provides certain values but cannot explain why they are suboptimal. However, with a 24-hour measurement, this becomes entirely possible. Furthermore, I can precisely observe in the spectrogram when my sleep was good and when it was rather poor.
It extends further: Heavy, unhealthy food consumed in the evening, particularly after 6:00 PM, typically sits heavily in the stomach. The body must work extensively and intensively to digest this food. If I go to bed whilst still in this digestive process, my sleep quality will undoubtedly be poorer compared to eating something more easily digestible or dining earlier.
Coaching and Support
Another crucial aspect is the feedback and guidance based on measurement data.
At Autonom Health, we are one of the few companies offering coaching services for HRV measurements. This is particularly significant because HRV is a multifaceted field, and extracting the full potential from HRV measurements requires extensive experience and expertise. Most users find this process too complex and time-consuming. This is precisely where the advantage of coaching becomes apparent.
This tremendous added value is exclusively provided by HRV measurement with a chest strap and professional analysis!
Conclusion
Smartwatches are less accurate due to their sensor technology and cannot perform long-term measurements. However, long-term measurements are precisely what makes HRV significant, as they provide fundamental insights into the measured person’s health status!
Nevertheless, smartwatches certainly have their place! They are particularly suitable for individuals who wish to familiarize themselves with their health in a playful manner without investing significant time.
However, when precise insights and lifestyle changes are needed, for instance due to burnout, it’s advisable to opt for an accurate sensor and sophisticated software.
When is a Smartwatch Appropriate, and When Should You Choose 24-Hour HRV Measurement?
The current fitness movement has driven rapid development in smartwatches, life trackers, and wearables. These devices can capture numerous vital signs, providing users with extensive analytical data.
However, the key questions are: How significant, how straightforward, and how reliable is the utility of this data, and most importantly, how valid are the results?
Limitations of Smartwatches
The measurement of vital signs using wearables or smartphones exhibits considerable fluctuations. Most wearables detect heart rate and other parameters by directing a light beam onto the wrist and measuring the amount of light absorbed. Higher light absorption indicates a greater blood volume flowing through the veins beneath the skin. However, this method demonstrates significant variability, influenced by factors ranging from current activity levels to skin thickness and pigmentation.
Furthermore, there is a substantial deficit in interoperability and standardisation. Manufacturers often define their own analytical values with minimal standardisation. Standardised and open interfaces are essential features for wearables in mHealth applications. However, this requirement conflicts with the interests of most device manufacturers. Additionally, many smartwatches raise concerns regarding data protection and security.
What happens to the collected data? Where is it stored? Who has access to it? What everyone should know: Data protection and security remain a significant blind spot for wearables. Many manufacturers even utilise the collected data for advertising purposes without consulting their customers.
HRV by Autonom Health
HRV measurement serves as a reliable and highly sensitive screening instrument for assessing an individual’s general health condition. Autonom Health’s HRV analyses comply with all required HRV standards and meet the guidelines established in 1996 by the Task Force of the European Society of Cardiology (ESC) and the North American Society of Pacing and Electrophysiology (NASPE) for conducting and analysing HRV measurements. In terms of data security, we fulfil IT security guidelines that meet the requirements of both EU data protection regulations and network and information security directives.
The most crucial health parameters are precisely measured and analysed:
- Health Status: Describes the functional efficiency of the entire organism, including current constitution, performance capability, and regenerative capacity.
- Current Biological Age: Heart rate variability correlates with health and age. The functional current biological age therefore measures one’s immediate adaptability to changing external and internal conditions.
- Performance Potential: The level and dynamic progression of cardiac performance and HRV data, compared to one’s age and gender group, reflect the current capacity for physical and mental performance.
- Stress Management: Vegetative resilience describes the ability to handle stress and pressure productively whilst maintaining effective regeneration.
- Burnout Resistance: This value measures stress resilience. Lower biological age, more appropriate physiological patterns during activation and rest periods, better physical and mental performance, and improved sleep all contribute to higher burnout resistance.
- Sleep: Precise sleep quality (exhaustion sleep, suspected snoring, poor, moderate, good, excellent) is calculated in 5-minute intervals in one’s own bed.
- Recovery: Number, timing, duration and quality (reduction in heart rate and sympathetic activity, increase in parasympathetic tone) of recovery phases are evaluated.
- Mental Performance: Complex algorithms calculate the degree of focus, physical tension state and efficiency (from exhaustion or fatigue to flow) during mental activity.
- Physical Performance: Cardiac and HRV performance data calculate the target range, effectiveness during and recovery after physical exertion.
- Eating Behavior: Number and duration of meals and eating breaks, in conjunction with individual energy levels during and after eating, enable current reflection on personal eating habits.
- Peer Group Comparisons: All results, cardiac performance and HRV data presented graphically and numerically, benchmarked against age and gender cohorts.
- Recommendations: Tailored individual guidance based on current measurement outcomes for targeted health optimization.
