Blog post by Marco Altini
Quite a few users and elite teams asked about the relation between resting physiology and temperature changes as well as with respect to altitude (think elite athletes training at altitude for certain periods of time).
A few questions that come to mind: can altitude adaptations be captured through resting measurements? (for example how well we adapt). What kind of changes should we expect? What's behind individual differences in altitude adaptations?
Additionally, some of the insights we provide might be affected by other variables, for example what's the relation between resting physiology and temperature (basically seasonality of these measures)? How are other metrics estimated by the app potentially effected by temperature or altitude? (e.g. VO2max).
We have some insights on these relationships based on published literature (see very short and not comprehensive summary below), but we believe we'll understand much more now that these parameters can be tracked in large populations for much longer periods of time (e.g. years), so hopefully there will be more to say or understand in a couple of months from now, especially on individual differences.
What do you get?
With this feature, we also improved the basic location tracking present in the app to automatically fill in your location instead of asking you to do so manually as well as include these parameters in the Correlation analysis under Insights. The traveling Tag will also be updated automatically when location changes, and all data will be available to your coach in the coach app. In particular, the new feature will enable the following functionalities:
We hope you'll enjoy the new feature and find it valuable to better understand how your body responds to different environmental conditions. Please read below for more details.
Short literature summary
This is by no means an exhaustive literature review, but simply an short overview of some of the work that has been done in the field.
Buchheit et al.  analyzed HRV responses to acute hypoxia both at rest and during exercise back in 2003. As Buchheit puts it, physiological adaptations are more and more relevant today as not only athletes but more people in general travel and get exposed to different environmental conditions. What the authors reported at simulated altitude (4800m) was a decrease in vagal-related HRV indexes (RMSSD and absolute HF power), suggesting a reduced vagal control of the heart.
Few studies have analyzed HRV at rest and under hypoxia and sometimes with inconsistencies, possibly due simply to individual variability and the differences in subjects populations involved, which are typically limited to a small number of individuals, with hardly any woman. For example, while a certain reduction in parasympathetic activity has been shown multiple times, Bernardi et al.  put through simulated altitude different groups of subjects, from controls to yoga teachers, and showed how in yoga teachers all changes were blunted.
Kanai et al  also showed reductions in HRV indices representative of parasympathetic activity at two different simulated elevations (2700 and 3700m). In particular, reductions were greater at higher altitude, showing a progressive reduction in parasympathetic activity with more altitude. Some researchers have proposed that a decrease in HRV indicates a reduction in the autonomic nervous system responsiveness and an inability to adapt the body to challenging conditions.
Liu et al  brought more evidence to the reduced rMSSD during hypoxia hypothesis.
A very interesting study from Huang et al.  followed longitudinally trekkers from seal level and then at altitude, showing how trekkers with acute mountain sickness despite having similar HRV at sea level, had a more pronounced reduction at altitude, which might be a way to identify poor adaptation or individuals at higher risk. The authors also showed how a greater reduction at moderate altitude (1317m) was predictive of an even greater reduction at and acute mountain sickness at higher altitude (3440m).
Most of these studies involve only male subjects and are limited to a handful of participants, limiting our understanding of how altitude affects physiology in different subpopulation and making it hard to understand how we can adapt what we learn from this research and apply it to the individual (e.g. ourselves or the athletes you might be coaching). What do we expect given an athlete characteristics and background? Does a different response mean there’s something we can do? We hope that by collecting more data on a more diverse population we will be able to shed some light in the future.
What do you have to do to enable this feature
On iPhone: Make sure to go to Settings / Configure TAGS and disable the Location tag, then re-enable it so that the app will ask for authorization. At that point the app will read your location only right before the measurement in the morning, and pre-populate your tags. You can still change the location name without affecting altitude and weather data. Make sure to be connected to the internet as weather data is retrieved through third party APIs and this procedure requires an internet connection.
On Android: Make sure to go to Settings / Configure TAGS and disable the Location tag, then re-enable it. Wait a few seconds on this screen as the app needs to get a first location to enable this feature. At that point the app will read your location only right before the measurement in the morning, and pre-populate your tags. You can still change the location name without affecting altitude and weather data. Make sure to be connected to the internet and also have GPS enabled as weather data is retrieved through third party APIs and this procedure requires an internet connection. Make sure also to authorize HRV4Training to read your location, you might need to check the specific permissions granted to HRV4Training from Settings / Applications on your Android phones, as having location on but no permission won't do the trick.
We hope you'll enjoy the new feature and find it valuable to better understand how your body responds to different environmental conditions.
 Buchheit, M., et al. "Effect of acute hypoxia on heart rate variability at rest and during exercise." International journal of sports medicine 25.04 (2004): 264-269.
 Bernardi, Luciano, et al. "Breathing patterns and cardiovascular autonomic modulation during hypoxia induced by simulated altitude." Journal of hypertension 19.5 (2001): 947-958.
 Kanai, Masaki, et al. "Alterations in autonomic nervous control of heart rate among tourists at 2700 and 3700m above sea level." Wilderness & environmental medicine 12.1 (2001): 8-12.
 Liu, X. X., et al. "Analysis of heart rate variability during acute exposure to hypoxia." Hang tian yi xue yu yi xue gong cheng= Space medicine & medical engineering 14.5 (2001): 328-331.
 Achten, Juul, and Asker E. Jeukendrup. "Heart rate monitoring." Sports medicine 33.7 (2003): 517-538.
 Huang, Hsien-Hao, et al. "Alternations of heart rate variability at lower altitude in the predication of trekkers with acute mountain sickness at high altitude." Clinical Journal of Sport Medicine 20.1 (2010): 58-63.
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This blog is curated by
Marco Altini, founder of HRV4Training
The Ultimate Guide to HRV
1: Measurement setup
2: Interpreting your data
3: Case studies and practical examples
1. Intro to HRV
2. How to use HRV, the basics
3. HRV guided training
4. HRV and training load
5. HRV, strength & power
6. Overview in HRV4Training Pro
7. HRV in team sports
1. Context & Time of the Day
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4. Orthostatic Test
5. Slides HRV overview
6. Normal values and historical data
1a. Acute Changes in HRV
1b. Acute Changes in HRV (population level)
1c. Acute Changes in HRV & measurement consistency
1d. Acute Changes in HRV in endurance and power sports
2a. Interpreting HRV Trends
2b. HRV Baseline Trends & CV
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4. Ectopic beats & motion artifacts
5. HRV4Training Insights
6. HRV4Training & Sports Science
7. HRV & fitness / training load
8. HRV & performance
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10. Repeated HRV measurements
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12. HR, HRV and performance
13. Training intensity & performance
14. Publication: VO2max & running performance
15. Estimating running performance
16. Coefficient of Variation
17. More on CV and the big picture
18. Case study marathon training
19. Case study injury and lifestyle stress
20. HRV and menstrual cycle
21. Cardiac decoupling
22. FTP, lactate threshold, half and full marathon time estimates
23. Training Monotony
Camera & Sensors
1. ECG vs Polar & Mio Alpha
2a. Camera vs Polar
2b. Camera vs Polar iOS10
2c. iPhone 7+ vs Polar
2d. Comparison of PPG sensors
3. Camera measurement guidelines
4. Validation paper
5. Android camera vs Chest strap
6. Scosche Rhythm24
7. Apple Watch
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1. Features and Recovery Points
2. Daily advice
3. HRV4Training insights
4. Sleep tracking
5. Training load analysis
6a. Integration with Strava
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6c. Integration with SportTracks
6d. Integration with Genetrainer
6e. Integration with Apple Health
6f. Integration with Todays Plan
7. Acute HRV changes by sport
8. Remote tags in HRV4T Coach
9. VO2max Estimation
10. Acute stressors analysis
11. Training Polarization
12. Lactate Threshold Estimation
13. Functional Threshold Power(FTP) Estimation for cyclists
14. Aerobic Endurance analysis
15. Intervals Analysis
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1. HRV normal values
2. HRV normalization by HR
3. HRV 101