Declining cardio fitness score although resting heart rate has decreased and running regularly

[Lolsinda]

Hi all, I find my Fitbit metrics a bit surprising recently. I run almost every day, I am definitely improving, getting faster and faster-really happy and motivated. My cardio fitness rate has also been steadily improving since I got my Charge 2 three months ago  -  it went from 40 up to 46 gradually. I almost completely stopped peaking during my runs, no matter how hard I was going. My heart rate was pretty chilled always. However, I was also observing parallel to that that my resting heart rate was increasing - slowly it went from 59 up to 71 even. I was always regularly running, increasing distance, getting faster, etc. Then within a few days my resting heart rate started dropping every day - fell down to 61 (currently). I was really happy, thought it was a sign of getting fitter. And here comes the strange part -my cardio fitness score dropped down to 44! I run better and with much greater ease and speed AND I started peaking again during my runs - from zero I am now having between 10 and 15 peak minutes during each run over the past week or so... (usually run for 40-50 minutes a day). Does anyone have an explanation for this..?? 😕 😕 Big thanks in advance! 

[Greythorp]

Hi Lolsinda,

I'm puzzled by my fitness score too. Mine is 49 which is far too high! I do have a low resting BMP of 49 which I believe is not uncommon for men of my age.  The last 5k park run I did in a couple of seconds under 30 minutes - not outstanding even for some one of my age.

I suspect that the algorithm for calculating the score from the limited data available from a fitbit has to make too many assumptions to be very precise and at best it can only be used as a very rough guide to your fitness.  I'd be interested to hear from anyone who has had a more scientific measure of fitness and how it compares with their fitbit score.

[WavyDavey]

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@Lolsinda wrote:

Hi all, I find my Fitbit metrics a bit surprising recently. I run almost every day, I am definitely improving, getting faster and faster-really happy and motivated. My cardio fitness rate has also been steadily improving since I got my Charge 2 three months ago  -  it went from 40 up to 46 gradually. I almost completely stopped peaking during my runs, no matter how hard I was going. My heart rate was pretty chilled always. However, I was also observing parallel to that that my resting heart rate was increasing - slowly it went from 59 up to 71 even. I was always regularly running, increasing distance, getting faster, etc. Then within a few days my resting heart rate started dropping every day - fell down to 61 (currently). I was really happy, thought it was a sign of getting fitter. And here comes the strange part -my cardio fitness score dropped down to 44! I run better and with much greater ease and speed AND I started peaking again during my runs - from zero I am now having between 10 and 15 peak minutes during each run over the past week or so... (usually run for 40-50 minutes a day). Does anyone have an explanation for this..?? 😕 😕 Big thanks in advance! 

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Hi @Lolsinda,

Do I have this correct?

Your cardio fitness went from 40 to 46.

Your resting heart rate went from 59 to 71 (which is a significant increase)

Then resting heart rate went from 71 to 61.

Your cardio fitness score dropped from 46 to 44.

It doesn't sound like too much of an decrease in score, given the big change in resting heart rate both up and down. It might take a bit for the effect to reach the cardio fitness score. Also, a fitness score drop of 46 to 44 sounds reasonable, especially since your resting heart rate is now over by 2bpm what it was prior.
 
Do a run of 10-15 minutes, flat surface, GPS on, at moderate/high intensity. That will recalculate the score.

[bcalvanese]

Are you over training?

Any new stress related factors?

Getting enough sleep?

Have you been ill (cold, flue, etc...)?

[bbarrera]

@Lolsinda VO2max ("cardio fitness level") is your aerobic capacity, the maximum amount of oxygen your body can deliver to working muscles. VO2max is something you are born with - its genetic like your body type (pear shape, apple shape, etc), or if you are a natural sprinter or long distance runner.

In general VO2max is NOT a good indicator of YOUR current fitness level. Your VO2max can be the same, and yet you get faster as a runner because its easier to make gains on raising lactate threshold and economy of motion (fluid running, no wasted motion). 

The better articles on running and VO2max will tell you the same thing, that VO2max is the slowest to respond to training (versus lactate threshold and running economy). For the vast majority of runners, VO2max is like knowing your IQ score - interesting but of not much value. 

I would recommend not paying too much attention to your cardio fitness score. Keep in mind its really about comparing yourself to others - someone with VO2max of 65 is going to run faster than you because their body is better at delivering oxygen to working muscles.

The standard definition of resting heart rate (RHR) is your HR when first waking up, it can and will change from day to day. Mine goes up the next morning after drinking a few beers with friends, or after a hard 2 day block of cycling. 

Fitbit has its own definition of RHR, it incorporates less active times during the day and therefore can be influenced by coffee intake, stressful situations in meetings, and other factors.

Regarding HR in peak zone, you said running with greater speed and HR increases with pace so perhaps that is the reason? 

The general rule of thumb from sports science is that running or cycling almost every day is ok if 80% is at a slow pace. Sports science tells us that you get faster from overload and recovery, don't forget about recovery days to let your body rebuild and grow stronger.

[shipo]

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@Greythorp wrote:

Hi Lolsinda,

I'm puzzled by my fitness score too. Mine is 49 which is far too high! I do have a low resting BMP of 49 which I believe is not uncommon for men of my age.  The last 5k park run I did in a couple of seconds under 30 minutes - not outstanding even for some one of my age.

I suspect that the algorithm for calculating the score from the limited data available from a fitbit has to make too many assumptions to be very precise and at best it can only be used as a very rough guide to your fitness.  I'd be interested to hear from anyone who has had a more scientific measure of fitness and how it compares with their fitbit score.

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Interesting, I'm 60 and typically run a 5K in about 23 minutes (I've broken 22 a couple of times, barely), my RHR (as calculated by Fitbit) is typically a point or two either side of 43, and my sleeping heart rate bottoms out a point or two either side of 35 at night.  With the above said, my Cardio Fitness Score is 50, and I find that high.

Long story short, I think the formula needs some tweaking.  🙂

[bbarrera]

Fitting your running 5k at 23 minutes into Cooper's 12 minute test:

46.9 = 35.97*(12 / (23/3.1)) - 11.29

so yeah, a score of 47 is more realistic. It seems 50 is a "Fitbit feel good" score

[Mukluk4]

Weight plays pretty heavy in that score too.

[shipo]

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@Mukluk4 wrote:

Weight plays pretty heavy in that score too.

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I've seen authoritative sources say weight (or height and weight) is a factor, and other seemingly as authoritative sources say there is no weight element in the formula.  If weight is a factor, then that might explain my high score as I'm 5' 8" but typically weigh somewhere between 190 and 200.

[bcalvanese]

I think fitbit bases scores on regular people rather than athletes.

I also don't agree to some extent with the whole cardio respiratory endurance being based mainly on pace.

cardio (heart) respiratory (lungs) should not be based more on pace than it is the ability to keep your heart and lungs going at a certain level for a certain period of time.

pace is adding many more variables into the mix (muscles, veins/arteries, joints, etc...), which are inherited factors. Also leg length plays a role in this.

Garmin uses first beat for everything on their mid to high end devices which in my opinion is tuned more for athletes than it is for regular people like me. I always get a poor/fair fitness level on Garmin devices, where I get fair/good/excellent on every other device I have used that measures fitness level.

JMHO.

[bbarrera]

VO2max is the maximum amount of oxygen your body is capable of consuming, measured during a treadmill/cycling workout of increasing intensity. It is measured in a lab, using a metabolic cart to measure oxygen consumption.

The test starts at a casual pace, and as the intensity increases the amount of oxygen consumed by your body increases. At some point the amount of oxygen consumed levels off, even though intensity keeps increasing. That is the maximum volume of oxygen your body is capable of consuming, also referred to as "aerobic capacity." These tests are hard, read the link for what its like to do the test in a lab.

If you are healthy and in good shape, sports science has shown that aerobic capacity doesn't change much with normal changes in exercise routines. So if you normally run 3 days a week, and then increase distance by 10% a week to get ready for an upcoming 10k or half marathon, that should not cause your aerobic capacity to change much. Its somewhat like your bodies ability to metabolize alcohol, I don't drink much and two beers always makes me tipsy on a Friday night. 

What about comparing my aerobic capacity to someone else? Is it fair to compare max oxygen volume between two people? Not without normalizing by dividing by weight. Think about it, someone that is 6' tall (me) naturally consumes more oxygen than someone 5' tall (my sister).

Therefore, when we talk about VO2max it is usually normalized by taking max oxygen volume (in milliliters per minute), and dividing by your weight (in kilograms):

VO2max = max oxygen volume / weight

My current VO2max is 40 mL/min*kg, so multiplying that by my weight in kilograms gives me a max volume of oxygen of:

3818 milliliters per minute

What happens if continue exercising as normal, with no change in aerobic capacity, but my weight drops from 210lbs to 190lbs?

VO2max = 40 at 210 lbs (3818 / (210/2.2))

VO2max = 41 at 205 lbs (3818 / (200/2.2)) 

VO2max = 42 at 200 lbs (3818 / (200/2.2)) 

VO2max = 43 at 195 lbs (3818 / (190/2.2))

VO2max = 44 at 190 lbs (3818 / (190/2.2))

So that is why changes in weight affect your VO2max! Note that my actual aerobic capacity (max oxygen consumption) hasn't changed, just the normalized score has changed. 

Ok, since most of us aren't going to a lab to measure VO2max, what are easier ways to estimate it reliably? Here are a few:

1. Cooper's test involves a 12 minute run, you take the distance completed, multiply by 35.96, and subtract 11.29. This estimate is based purely on pace, I think accuracy is 90% for men.

2. Rockport fitness walking test. Walk for a mile, and then use your time, age, weight, HR at end, and gender to estimate VO2Max.

3. Firstbeat. This is patented technique, and validated to be 95% accurate. It works for both runners and cyclists. Details are available on Firstbeat website.

4. Fitbit. We don't know much about how Fitbit calculates. Fitbit describes it using the same language as Firstbeat white paper, but beyond that we aren't providing any info on accuracy or 3rd party validation.

Hope that helps.

[Corney]

Sorry to sound so stupid, but what is a cardio fitness score and where can I find it?  I have a Charge HR.

[bbarrera]

@Corney "cardio fitness score" is a Fitbit marketing term for relative VO2max.

VO2max is a measure of the maximum amount of oxygen your body can deliver to working muscles. It's somewhat like an IQ score, it allows you to compare your potential as an athlete against other people in endurance sports (running, swimming, cycling, cross country skiing). For example a score in the 70s or 80s indicates you have the potential to be a world class endurance athlete. Just like an IQ score of 140 means you will likely do better on intelligence tests versus someone with a score of 110.

If you take the time to read about it, you'll learn that VO2max is an interesting number but not terribly useful. It doesn't predict race winners in the olympics. For someone just getting active, it will improve quickly and then level off. If you've been a regular runner or cyclist, it doesn't change much.

[Mukluk4]

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@shipo wrote:

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@Mukluk4 wrote:

Weight plays pretty heavy in that score too.

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I've seen authoritative sources say weight (or height and weight) is a factor, and other seemingly as authoritative sources say there is no weight element in the formula.  If weight is a factor, then that might explain my high score as I'm 5' 8" but typically weigh somewhere between 190 and 200.

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If you set yourself a goal weight and then expand the Cardio Fitness window within the app.  You can see how (1) increased activity can impact and (2) meeting your goal weight impacts the CF score.  Just dropping my weight 15 pounds would increase my CF score about half what increased exercise would.

[bbarrera]

Weight is often the only factor in increasing/decreasing "relative VO2max" (Fitbit's cardio fitness) scores. See my math above, for someone 210 lbs it only takes losing 5 lbs for score to increase 1 point. 

The other easy way to increase score - you are coming off the couch and embark on a regular cardio exercise routine (3-4 times a week). You'll see scores go up over the first month or two before leveling off.

Increasing your "absolute VO2max" takes a lot of really hard work. I did that earlier this year, by going full throttle for 50-60 minutes at least once a week, plus at least one or two shorter VO2max interval sessions per week. That plus losing 8 pounds took me from 39 in December 2016, to 43 in May of 2017. But for various reasons I was riding faster at scores of 41 or 42, which again reinforces a higher score is not a reliable predictor of performance or sport specific fitness level.

[shipo]

@bbarrera, thanks for your comments.  One other factor which occurs to me relative to weight loss is where the loss came from.  I am getting to the point where losing weight often includes the consumption of lean muscle mass.  There are scenarios where weight loss beyond a certain point, or too much weight loss over too short a period of time will negatively impact performance.

It might could well be you hit that point last may when your score was 43.

[bbarrera]

@shipo right now, and back then, my body is carrying at least 20 lbs of fat on my waist. And I do strength training with free weights to avoid losing muscle mass. When I hit VO2max of 43, the weight loss had been very gradual over 3 months. The increasing score from 39 to 43 occurred over 4 months, the weight loss was worth just over 1 point, and the rest was gritty determination pushing myself hard for a solid 5 months of ftp and VO2 training (using 4 week periodization training cycles).

VO2max has not been shown to be a reliable predictor of performance. Here is an article you might enjoy because it talks about famous runners from our younger days: http://www.trainingbible.com/joesblog/2009/11/vo2max-and-race-performance.html

[bbarrera]

@shipo you should read all three of the "Lactate/Economy/AerobicCapacity" article series at Joe Friel's old blog:

http://www.trainingbible.com/joesblog/blog.html

[Corney]

Shipo: I didn't learn much about determining my cardio fitness score, but
I believe Joe backs up my Theory of specificity. The best way to do any
activity is to do that activity, because the body and muscles involved in
that activity are specifically trained to do that activity. There is very
little carry over when you engage in a different activity, because the
muscles are used differently. Not that I'm an avid cyclist, but I always
got a kick out of avid cyclists spending thousands of dollars on a bike
with lighter weight components, when it would be cheaper to just lose a
pound or two of weight. I recall a few years ago when a triathlete
competing in the Iron man competition finished 4th the year before but won
it the following year, when asked what changed, he answered, ( I learned
how to ride a bike). He explained that the most effective and efficient
way to ride a bike is to continually pedal between 70 and 90 RPM's, that's
why bikes have gears. I competed in a mini triathlon years ago as part of
my training for a marathon. Looking at the results later. each event time
was kept separately, I found the difference between the best and worst
swimmers was minimal, as was the times between the best and worst runners.
The race was one by the best biker. I remember passing many cyclists that
were pushing the highest gear they could, and using the 70-90 RPM theory, I
passed them all easily. So training is as important as technique in
winning a triathlon. Thanks for the information on Joe's blog, it made for
interesting reading.

[bbarrera]

@Corney the official way to determine VO2max ("cardio fitness score") is in a lab:

https://www.dcrainmaker.com/2012/01/look-at-testing-with-new-leaf-fitness.html

After that, the next best estimates come from Firstbeat technology. They have a white paper on the validation of 95% accuracy claim. Its been licensed in a bunch of devices:

https://www.firstbeat.com/en/consumer-feature/vo2max-fitness-level/

But the question is why do you care? Your aerobic capacity (VO2max, or cardio fitness score) is genetic,  and declines with age. VO2max is not used as a health metric, and it doesn't change much if you are fit. It isn't a good predictor of performance. Its primary use is to compare your aerobic endurance against others. Its like comparing IQ scores. Just because Fitbit made a big todo about it, and featured it in the app, doesn't mean its the key to understanding your aerobic fitness level. Unless you only want to compare yourself against others, and then just keep in mind that if you regularly do cardio then your score won't change much.

At Ironman championships the race is won on the run: 

https://www.trainingpeaks.com/blog/5-things-this-years-ironman-world-championship-taught-us-about-ho...

Riding at effective cadence is well known in the cycling world. It comes down to physiology - versus 50-60rpm, at 80-90rpm you are recruiting different energy systems to do the work. The aerobic system is used at higher rpms, and the energy required is easily replenished, therefore less fatigue and more endurance when spinning faster.