Tuesday, September 25, 2018

VO2 max/peak estimation

As physical effort rises, the amount of fuel used by the active muscle increases.  Part of this energy is dependent on oxygen, and the amount of O2 used at a given time is referred to as the VO2.  This will rise as muscular work increases, up to an eventual maximal value.  The VO2 was discussed in some detail in the last post looking at the Fast Start strategy.  

Why is the VO2 max such a popular subject?
Many sports scientists believe that the maximal VO2 attainable is predictive of success in many endurance sports.  In addition, many of the interval protocols in the literature are adjusted to a percent of VO2max.  The usual way to measure the VO2 max is with a metabolic cart looking at accurate gas exchange and power parameters.  

Since most of us don't have access to this type of testing, I would like to discuss some non metabolic cart estimations of VO2 max power, in particular the VO2 peak cycling power.  There are other terms used to describe this maximal usage of O2.  According to this paper, several functional limits can be measured:

"The V̇O2max, mode-specific V̇O2max and V̇O2peak provide estimates of functional limits: V̇O2max represents the upper functional limit; mode-specific V̇O2max represents an upper functional limit reached during a specific exercise mode; and V̇O2peak represents an upper functional limit during a single test."

What we will be discussing here will be more of a mode specific VO2 peak, since it is cycling specific and based on single test (or multiple single tests).

Interestingly, even if we were able to be tested with a full metabolic cart approach, the VO2/power result from ramp type exercise is different than a constant steady state pace (or perhaps a Fast Start).  Dr Murias and his group published a great paper both explaining the issues with ramp testing, as well as providing some actual examples. 
Here is an illustration of the variation in power measured at the peak VO2 depending on the ramp type as well as constant intensity pacing.
Although the VO2 max (gas exchange) is the same, the corresponding power is quite different.  Training zones and race pacing would be dramatically affected by this.

Here is another look at constant power intervals (from 20 watts) labeled in blue.  At the lower intensities, the VO2(not max) stabilizes by about 90 sec and is flat for 10 minutes.  At the higher power zones, VO2 max is achieved eventually, but quicker at the higher power (Fast start interval like).  The 360 watt line is short, presumably since the power was so high, the rider was unable to continue.

The bottom line here is that it may be possible to approximate VO2 max (or peak VO2 cycling power) by observing the power curve of a given individual, particularly at the 3-6 minute interval length.

Another study looked at a 4 minute maximal run compared to the traditional graded exercise VO2 max:
The first trial was circled as an example, and indeed the VO2 is very close to the standard test result.  One of the points made in the discussion was that the speed during this 4 min test could be used as a field measure of speed at VO2 max:

An interesting paper on estimating the MAP (maximal aerobic power similar to VO2 peak) in elite cyclists used their own maximal power historical records for a range of time intervals on a log axis.  It is a very informative paper and I recommend taking a look.
According to the authors, the shift in the power vs log time plot tracing could be used for MAP (VO2 max cycling power).

In addition, the plot can be extended to estimate power for longer time spans

Although I have done many near max 3, 4, 5, 7 and 10 min intervals, I don't do longer time spans.  Based on my last years numbers, I made a log plot and tried to emulate what the study did above.

Although it is certainly not definitive, but it does give me some guidelines for MAP, VO2 peak power.  According to this tracing, my MAP is about 350 watts at the 4+ minute mark which agrees with the study.

Ventilation response to VO2 max testing
Since we are able to measure ventilation parameters in the field with the Hexoskin shirt, can these metrics be helpful in verifying VO2 peak power territory?  Although that was not the question asked in this study, a look at their data does support the concept.  This paper addressed the potential differences in respiratory response in treadmill vs cycle VO2 testing in different populations.  

The red grouping is the ventilation rate that increases steadily up to VO2 max, with a reasonable stratification below 100%.  HR in orange also behaves this way.
Interestingly, the breathing rate did increase between 90 and 100%, but the tidal volume remained the same:
In summary, at the VO2 power peak I would expect a maximal breathing rate and ventilation volume.

Does looking at the 4 min interval data indicate that a peak VO2 value should be present? 

Since VO2 depends on cardiac output (stroke volume x heart rate) we should see a near max heart rate.  The other part of the equation is the A-V O2 difference.  If flow in the measured area is near maximal, the NIRS O2 desaturation should correlate with oxygen extraction.  Therefore we would expect the desat to be near max.  Let's look at the interval tracings:

  • The heart rate by mid interval was near max, and by the end was at max.
  • The ventilation by mid interval was near max, by end interval was close to max.
  • The breathing rate was high throughout and spiked at the end
  • The RF desat of 51% at end interval was the same as the end of the 1 min max interval later in the ride (530 watt avg).
Therefore: HR, ventilation and O2 extraction were at maximal values by mid to end interval, which is good supporting evidence for reaching VO2 peak in a time zone consistent with that found in the literature.

Conversion Formulas to traditional VO2 values
Although there are many equations based on the ACSM field guidelines, the VO2 max ml/kg/min is relatively meaningless number in the field (not so in a lab setting).  But if you are interested, here is a commonly used formula and relationship of watts to VO2:

 Summary points:
  • A practical approach to estimating your VO2 peak cycling power can be valuable for monitoring your training, fitness, pacing and choosing interval protocols.
  • The power average during a relatively stable 4 minute maximal interval may be a indicator of maximal aerobic power/VO2 peak power.
  • The end interval HR, breathing rate, O2 desaturation and ventilation rate should be at near maximal levels

Other VO2 max related posts

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