1.1 Camera acquisition
Current generation phones include both a camera and a light emitting diode, which can be used for reflection based bio-optical imaging.
The technique is called photoplethysmography (PPG for short) and consists in detecting changes in blood volume during a cardiac cycle, by illuminating the skin and measuring changes in light absorption.
HRV4Training uses the phone camera to extract PPG and then determine markers of the autonomous nervous system activity, in particular parasympathetic activity. PPG has been used for a long time in clinical settings, and has been validated multiple times, proving to be a reliable measurement of HRV, as good as standard electrocardiograms with sticky gel electrodes. More details on the accuracy with respect to a chest strap can be found here (this technique works on iPhones only, iPads do need a chest strap).
2. HRV-based Daily Advice
HRV4Training provides advice based on objective assessment of physical condition and your subjective scores. HRV over the last week is used to determine your baseline (at least 4 days of measurements are required), while the past 2 months of data are used to determine what your normal HRV values are.
The daily advice aims at helping you in making small daily adjustments to your training program, by keeping in consideration not only how your score changed from yesterday's, but also with respect to your past 7 days moving average (i.e. the baseline), your normal values, and factoring in your subjective scores and recent trends (when enough data is present).
You can learn more about the daily advice at this link.
2.1 History and Training Annotations
Once you've got some data, use the history tab to browse through it and look at the different features that were extracted during the test. You'll also be able to see the impact of some of your tags (e.g. travel, alcohol intake, injuries, etc.) on your physiological stress level.
The Baseline page helps you in going beyond short daily variability, and get a better overview of your physical condition.
The blue line is a 7 days moving average, which captures the global trend of your HRV, without being too affected by daily swings. At the bottom you can see annotated trainings as well.
2.2 HRV4T Recovery Points
HRV4T Recovery Points provide a single, straightforward metric to analyze your physical condition. They are based on the single most used metric to quantify recovery based on HRV, which is rMSSD. Additionally, HRV4T Recovery Points adapt the score based on your anthropometrics. HRV4T Recovery Points are used to compute your baseline. Daily deviations from this baseline are shown in the color-coded Advice bar, in your home screen.
2.3 Population comparisons
In this page you can see how you compare with respect to other HRV4T users. You'll be able to pick different physiological parameters (heart rate, rMSSD, Recovery Points) as well as stratify by age and gender.
You can learn more about HR and HRV population values on this blog post.