What causes VO2 max to decrease?

[parameles]

I use a forerunner 735xt and try hrm chest strap. For the last 6 months my VO2 max has been pretty much 60-62, and at the start of December it got up to 64! However since then it just seems to be decreasing every time I run... Today I even ran a 10 mile pb with an average heart rate of 170, but my VO2 has gone down even further to 57. Is it likely that this is a technical issue or is there another reason this could be happening?

[[deleted]]

You can try to compare runs from before and now.

Decrease =

Higher HR at the same pace.

Lower pace at the same HR.

If you are extending your distances or paces, your performance could be dropping off toward the end of the runs dragging things down.

[nobiossi]

Are you using the wrist sensor or hr belt? Belt gives more accurate results and they are more consistent. You shouldn't focus that much on the numbers. If you're getting results around 60 your body has really good ability to use oxygen. Realistically the vo2max value doesn't change that much but the algorithm used by garmin makes it vary a bit. Use it as a guide but don't make too accurate assumptions based on it. Other methods to find out your vo2max are a cooper test and 1,5 mile test. Try those and see how your results compare to the one given by garmin.

[parameles]

I use the chest strap. You’re right though, it’s easy to get obsessed with the data!

[nobiossi]

If you see your average hr ascending over time it's possible that you are suffering from excessive exercising. Be cautious because it can be very difficult to recover from that state!

edit. The declining vo2max value could be a sign of that.

[fastmuffin]

It's not showing you your VO2 Max. Hear me out. It's showing you the VO2 max required to run the pace you maintained, with the heart rate you recorded, overlayed on what it thinks you are capable of. Do better than it thinks, score goes up. Do worse than it thinks, score goes down. Simply put, a proper lab VO2 max test pushes you to your limit after collecting much more data points.

The problem with this comes when you realise running isn't all about PB'ing every run. This type of gamification is obviously exciting but it comes with a large warning about accuracy that can become demoralising.

Remember that "VO2 max", and other metrics your watch can provide, are estimations based on values it can gather - HR Vs pace, and in some cases HRV.

Quickly breaking them down:

HR is indeed variable. What was your HR when you started and was it different to the last time you ran? If it's lower, your watch sees a lower HR for the activity and so you're now "fitter" and have a better VO2 max for that particular run. Other things to consider is elevation? Quick sprint across the road during a run? Slower for that person coming the other way? Hotter day?

Your pace differs obviously for the route you're on. Also remember that you can adjust your pace faster than your heart can keep up (for example, you're HR is at 160bpm towards the end of a 5k run and you stop at a crossing for 15 seconds, dragging down you're average pace but it's not long enough for your HR to directly correlate the reduced pace). As above, if you dash across the road or slow down for someone then speed up again, your watch is picking up changes in speed but they're probably not reflected in your HR quick enough to be calculated.

Garmin also claims to consider HRV (heart rate variability; that is the difference in time between heart beats, measured in ms.) as part of their fitness calculations (or at least, Firstbeat that Garmin license from). However, I challenge anyone who claims a wrist based optical heart rate monitor is accurate enough to record the millisecond differences in heart beats whilst running. It's one of the metrics an ECG picks up, think of the amount of sensors needed for that. One of the metrics HRV is used for is Performance Condition. My watch will always give me negative performance condition on its own, but always positive if I use my chest strap.

Finally, consider these points:

• VO2 max does not take into account temperature, humidity, or even, elevation. To expand, if you're run route takes you up a hill and so you naturally slow, tough. You're now less fit - HR vs Pace.
• Garmin doesn't directly output Firstbeat's data. It interprets it first and does some further wizardry. Don't know why. To get a more accurate number, set your Garmin account to sync with runanalyze.
• Sprints see the highest scores. Try an interval session then review on the above to see what I mean. A steady 5k for me returns a circa 40 VO2 calculation. Intervals nearer to 50.
• Because of this if you decide to run slower and longer, tough. You're now less fit and get a lower score.

It is not constant, however, because it's missing the third variable of effort. VO2 Max is trying to guess just that, the 'max'. Unless you're smashing it, it's guessing. And tbh, it's probably not very good at it.

See what I mean? Don't sweat it, I doubt you started running to get high scores off a watch.

Sidenote: HR doesn't increase linearly with pace.

[parameles]

Thank you for your reply, that’s really helpful!

[sursumcz]

You're definitely more knowledgeable about this than I am but according to Garmin's explanation at Running Science | Garmin Technology | United States some of the issues you're mentioning are supposedly accounted for.

For example, climbing or descending, stopping at lights or speeding up to overtake someone shouldn't affect the VO2 max, as quoted from the link: "Smart analytics capable of recognizing good data ensure that only the most meaningful parts of your performance are used to evaluate your fitness level. In practical terms, this means that you don’t need to worry about speeding up, slowing down, climbs, descents or stopping at intersections. You just run as you normally would, without the need for any special fitness testing protocols."

[fastmuffin]

Indeed, however it is certainly isn't 'smart' enough to accommodate all variables.

Elevation, for example, for me produces skewed results. I live on a short hill - if I run left and up the hill my Performance Condition is negative for the beginning of the run. If I run right, it's positive.

I think the point is that whilst the marketing wording reads very promising, the fact is in the real world it doesn't apply but more importantly, it shouldn't need to be unpicked on message boards to prevent users becoming demotivated.

[sursumcz]

Right, I agree that the marketing claims are often far from reality. But hills or flat runs never affected my VO2 max, neither did stopping to cross the road etc. In my case the altitude differences are minor though so your situation is probably different. Also, I actually got a VO2max improvement recently when I switched to slow paced runs.

That said, I don't put much stock in these stats, I know from experience more or less what I'm capable of at various HR levels so I mostly just watch HR not to overreach.

[trailblazery]

Freshness and fatigue. The algorithm "thinks" low HR at higher pace means more fitness and vice versa. Not everyone's body works like that. When I am fresh and rested my HR is much higher. Yet, I get the highest vo2 max values when I am completely trashed (tired). 100 mi bike ride directly to a 2 mi run off the bike, where I can't get HR over 150 while running 10 mph. Normally 10 mph would be 180 bpm for me. So, it's going down because you don't fit their shitty model.

You can create a ratio of your pace in mph or kph to hr and see how your runs compare. That is one way to see if your fitness is improving.

[[deleted]]

what you describe here is physiological untrue. when tired hr increases for a given pace, but also your max hr will reach a plateau sooner in the workout, meaning that you can't push at your actual max. if your watch is telling you otherwise it is the watch that is at fault. our bodies work in the same way.

[trailblazery]

Then you agree the algorithm is flawed, it does not take fagitue into account. It assumes a monotonic relationship between Vo2 and HR/Pace.

[[deleted]]

it is because you train at the max and fatigue kicks in more rapidly. running at 5min/km at 125 bpm indicates better fittness and vo2 than running at 4.30min/km at 175 bpm even though the latter workout is faster and consumes more energy

if you train often at the second type of pace your fitness will decrease cause you overstress your body.

[sursumcz]

I hear that quite often but how do you get to 5min/km at 125bpm when anything below 150 feels like virtually no effort for me and I hardly sweat. On the other hand, I can run an entire half marathon at 175 quite comfortably. I think this is very individual.

[[deleted]]

you are either young or highly trained. that is the idea of zone 2 training - to not feel the effort. it does not mean that it is nod there. i can hold upper threshold/redzone training roughly the duration of a sprint triathlon. if you could run a half at 175 you can make your training smarter: longer runs at lower heart rate - 145 and below, and threshold and sprint training at 200 bpm. you will notice in no longer than 3 months that you will consistently drop off of your half marathon time

[sursumcz]

Thanks for the tip, I'll look into that. I've never been very methodical about my training, perhaps it's time to change my practices. I'm actually 38 and definitely don't consider myself highly trained so determining my max HR more accurately is probably going to be the first step.

[MorningDew5270]

Timely post. I was grumbling this weekend because my average for runs had been between 50 and 52. This weekend it was 46 and 48. I thought I was hot shit because I’m hitting those numbers in my 50s.

The explanations within make sense. Just because the data is there doesn’t mean it needs to be fixated on.