Tuesday, April 05, 2022

Aerobic Threshold or How to Estimate Metabolism with the Breath

All text and images from Alan Couzens.

The #AerobicThreshold or first rise in the lactate curve is a key training intensity. 

Primarily because it usually coincides with the highest rates of #FatBurning coupled with relatively low CHO oxidation, so the athlete can accrue a lot of work with minimal metabolic fatigue.

VE = ventillatory efficiency, or how deeply you breathe.  RR = respiration rate.

As the chart above shows, when an athlete begins a progressive metabolic test their VO2 (oxygen) requirements increase almost immediately & in fairly direct proportion to the exercise intensity. It makes sense then that we'd have to increase our breathing to match this, right?


Our body is pretty wasteful with O2 at rest. Our muscles don't take up most of the O2 in the blood & it just cycles around. Therefore, the first port of call when our body needs more O2 is simply for the muscles to take up more already circulating O2 from the blood...

This shows up as an increase in the a-VO2 difference, i.e. the *arterio* *venous* *oxygen* *difference* - the difference between the amount of O2 in the blood of our arteries going into the muscle & our veins coming out...


As we continue to increase the intensity of exercise after moving from nose breathing to quiet mouth breathing, eventually we encounter a second breakpoint in ventilation - the transition from quiet mouth breathing to loud mouth breathing...

This shows up as an increase in the a-VO2 difference, i.e. the *arterio* *venous* *oxygen* *difference* - the difference between the amount of O2 in the blood of our arteries going into the muscle & our veins coming out...

At this point, metabolic acidosis is starting to increase and the body's response to it is to "blow off" the increasing carbonic acid in the form of additional CO2...

This is directly visible in the relationship between VCO2 (the amount of carbon dioxide being produced) vs VO2 (the amount of Oxygen being consumed). At this point in the test the CO2 line takes a turn & begins approaching the O2 line.


Development of Diabetes

Everyone is on the spectrum….

Source:   https://link.springer.com/article/10.1007/s00125-021-05505-4

Tuesday, January 11, 2022

Phenology, Accumulated Growing Degree Days, and Soil Moisture

US Crop Calendar

Source: https://ipad.fas.usda.gov/countrysummary/Default.aspx?id=US

Arizona had a good year for NDVI

Source: https://glam1.gsfc.nasa.gov/

NASA SMAP data.  Data is global.

This mapped layer is delayed by 2 weeks.  I haven't found a layer that shows real-time moisture.

NPN Visualization tool can view Historical, Current, and Anomaly Accumulated Growing Degree Days. Data is only for USA.

Source: https://data.usanpn.org/vis-tool/#/explore-phenological-findings

Wednesday, December 01, 2021

Prescott Water Supply

 The Citizen Water Advocacy Group has good resources discussing this issue.  Here's a video covering frequently asked questions.  

Well water levels are declining by more than 1 foot/year in the center of the aquifer, and have declined more than 100 feet in the last 80 years.  Wells at the edge of the aquifer, such as in Williamson Valley, are declining from 2 to 7 feet per year (data not shown).

2000-2021 Drought in the Southwest


The list of Drought Impacts:

D0  Forage is limited; soil is dry

Fire risk increases

D1 Plants are stressed; hillsides are unusually brown

Stock ponds and creeks are nearly dry; some springs are dry

D2 Water and feed are inadequate for livestock

Fire danger is high; fire crews are mobilizing

Little forage remains for wildlife; pine trees are losing needles

D3 Ranching operations are affected

Fire preparedness increases; fire restrictions are implemented early

Skiing tourism is low; snowpack is extremely low

Wildlife encroach on developed areas in search of food and water

Native plants are stressed

Livestock do not have adequate water; runoff is short; conditions are dusty

D4 Fire restrictions increase; large fires occur year-round

Vegetation green-up is poor; native plants are dying

Lakes, ponds, and streams are dry

Tuesday, October 19, 2021

2021 California Wildfires and Fuel Treatments

This post and discussion on The Smokey Wire is one of the best, with in-depth reviews of the effectiveness of forest thinning to control wildfires.  


Wednesday, October 13, 2021

Overnight HRV: Comparison of PPG to ECG

Problem:  Short-duration morning HRV measurements may be susceptible to exactly when you wake, your HR, breathing rate, thinking about work etc in that moment. Different apps report widely different values of HRV.  Perhaps more importantly, morning snapshot measurements do not record HRV over a physiologically meaningful interval.  The best case would be to record HRV 24-7, but this is not feasible with current wearable technology.  However, wearables can and do report night time HRV.  At least this is measuring vagal balance over a physiologically meaningful interval, namely, the sleep recovery timeframe.

Methods: recorded overnight HRV using Polar H7 chest strap connected to the iOS app HRV Logger, for 12 nights over more than a month.  HRV Logger stopped recording on 2 nights, perhaps due to loss of connection with the H7 or other errors, so I have 10 nights of complete data.  The Polar H7 chest strap directly measures electrical activity (ECG) and is considered the gold standard for at-home HRV measurements.  

I also wore a Fitbit Inspire 2 each night and downloaded the data file from Fitbit.com at the end of the month.  Fitbit uses a green light PPB (photoplethysmography) sensor to measure blood flow changes at the wrist and infer heart rate and HRV.  

HRV Logger was set to 5 minute recording windows and RR-interval correction was set at 25%.  Fitbit also reports HRV in 5 minute recording windows.  However, the 5-minute interval windows are only approximately aligned as they are based on offset starting times.  HRV is reported as rMMSD.  

Results:  At this point, I am reporting preliminary data from 3 representative nights.  

Night of 9/19








std dev



Night of 9/21









std dev



Night of 9/28









Std dev




An examination of the time course data shows that Fitbit was able to pick up the broad changes in HRV during the night.  It appears Fitbit misses spikes in HRV that the Polar strap is able to detect.  Although it is also possible that the Polar strap is biased by movement artifacts, it seems most likely that either Fitbit is not sensitive enough to detect short-duration changes in HRV, or a Fitbit error-correcting algorithm is too aggressive at removing short changes in HRV.

In addition to being less sensitive to variations in HRV (as measured by standard deviations), Fitbit also consistently records a lower average nightly HRV than the Polar strap.

The overall R squared value is only 0.38, however this statistic probably underestimates the correlation of time series data due to the fact that the 5-minute windows are not perfectly synchronized.  Even so, R squared of 0.38 is a higher correlation that I was able to achieve using the Polar H7 with 2 other apps.   

Wednesday, October 06, 2021

No correlation between morning HRV apps

 I previously looked at Elite HRV and My Ithlete and found poor agreement between them.  I also tried HRV Logger and found even less correlation.  

This graph shows My Ithlete versus HRV Logger.  The graph of Elite HRV versus HRV Logger is similar.

All of this was using a Polar H7 chest strap with morning readings alternating between the different apps.  I believe HRV Logger is more accurate than the other apps, so I don't think this invalidates the results from HRV Logger.

Tuesday, September 28, 2021

2021 Arizona Monsoons

 It was a good monsoon in AZ this year, with some locations up 8 to 12 inches above normal (June-September 8).  But due to the uneven distribution of thunderstorms, even in this good year some areas barely received normal!

Wednesday, September 08, 2021

Fitbit Data Analysis: HRV and Temperature


Between April/May 2020 and August/September 2021....

    .... my resting HR went from average 52.4 to 55.3

    .... my deep sleep went from average 75.6 to 91.5 minutes

Nightly HRV analysis

Night of Sep 4-5th:  Morning HRV was 62 in Elite HRV, 82 in ithlete, 41 in Fitbit, and 64 in HRV logger.  

Night of Sep 5-6th: Elite HRV was 66, ithlete was 82, Fitbit was 51, HRV logger was 54.  94% of time HR was below resting.  

Sep 6th-7th (but only 6th is shown, due to not downloading Sep 7th data yet...)

Comparison of cheststrap (30 sec sample data smoothed to 5 minute window, Fitbit HRV sampled on 5 minute intervals).  Fit looks pretty good.

Recently found out Fitbit records temperature as well.  There is a diurnal rhythm, with interesting variations.  Sep 4th and 7th were both early morning outdoor exercise (jogging and hiking, respectively).  

More temperature details:

The Inspire 2 temperature sensor is at least directionally-accurate!

The nightly data ("Computed Temperature" file) has what looks like temperature in Celsius. For me, it is 31-32 Celsius at night, which is around 90 F. This is the data that is reported as Skin Temperature in Health Metrics.

Interestingly, there is also continuous 24 hour temperature data at 1 minute intervals ("Device Temperature" file). This data appears to be recorded as variation (+/-) from some baseline, as the numbers are always between +3 and -8 for me. Again, probably Celsius. I plotted my data from the last few days, and the times when I was exercising outside in the sun were consistently +2 degrees above other times that I would guess were cooler.

I don't know if the temperature measures are absolutely correct, but they do at least make sense and could potentially be used to identify unexpected temperature anomalies.

If anyone else is trying to download their data, note that you have to request it on the Fitbit account setting website, and then confirm the email.