VO2 Master Pro & Moxy – Revealing Metabolic Limitation – Part 1

I’ve been using a few new sensors in my training and coaching practice recently and learning a ton about central and peripheral limiters to performance. The sensors are the VO2 Master Pro wearable VO2 analyzerĀ and Moxy muscle oxygen monitor. Together these sensors have given me a better view of systemic oxygen consumption (VO2), and local muscle oxygen saturationĀ (SmO2).

Huge thanks to Peter at VO2 Master Pro for allowing me to test their device for a generous trial period. And to Assaf at DRKHORSE Coaching for lending me the Moxy monitors for my own experimentation.

Systemic & Local Oxygenation

Total haemoglobin (tHb)Ā and Muscle Oxygen Saturation (SmO2) reported by the Moxy monitors tell us the total blood oxygen carrying capacity within the local tissue (tHb in grams per deciliter, g/dL) and the percent saturation of that capacity (SmO2 in %). The absolute and relative values begin to build a picture of local muscle aerobic capacity, ie. how well the muscle can extract and utilize the available O2 to produce power.

Volume of Oxygen Consumption (VO2) should be well familiar by now, but read here for a refresher. The VO2 Master Pro analyses how much oxygen is getting in, and how much is coming back out. This reflects net systemic O2 consumption and can be used to determine how hard the body is working relative toĀ maximal oxygen uptake (VO2max).

The blood flow insufficiency issue with my leg is still ongoing and further investigations are underway, but at least this gives me the opportunity to study my performance and systemic response to two very different local tissue metabolic environments. Namely, my Right leg with sufficient blood flow & oxygen delivery, and my Left leg with a confirmed blood flow limitation and possibly also metabolic insufficiency.

Today I want to start looking at a couple of tests I’ve done on myself, what the new data revealed, and how these devices might be used to diagnose potential limiters to performance.

Sprint Interval Training – Incomplete Recovery Intervals

The first part of this workout was repeated all-out 30sec efforts, with 90sec active recoveries. The recovery intervals were short enough that the workout should (and did) quickly take me to failure.

All-out 30/90sec to failure – Performance data
  • Yellow line is PowerĀ measured by dual sided crank-based 4iiii power meter
  • Blue line shows Right leg Power (good leg)
  • Pink line shows Left leg Power (bad leg)
  • White line is L/R Power BalanceĀ highlighted where it diverges from 50/50
  • Red is HR, highlighted above 90% HRmax

Note how R & L balance began to diverge already (grey area) after the second 30sec interval, but only during the recovery intervals. My power during the 30sec work intervals remained balanced, but the clear decline in net power through the workout suggests that the Left leg was the limiter in the standing sprints. The area highlighted in blue shows the growing gap between R and L leg power.

By the second interval I was feeling the typical burning pain in my quad associated with blood flow restriction, and by the 5th interval the pain and cramp was so bad that I couldn’t even spin the left leg up, hence the false start and final short lived attempt.

Let’s take a look at systemic metabolism via the VO2 Master Pro, showing us oxygen consumption (VO2).

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All-out 30/90sec to failure – Systemic metabolism (VO2 Master Pro) data
  • Chart is similar to above, with Performance data, plus:
  • Blue area is Oxygen Consumption (VO2 in mL/min), highlighted in pink when VO2 reaches 90% of my previously tested VO2max (5600 mL/min)

VO2 just barely touches 90% for the first three reps, indicating just how important aerobic metabolism still is for these predominantly anaerobic efforts. But after the third interval VO2Ā during both the work and recovery intervals beginĀ to show a decline, along with peak HR.

This could be caused by fatigue limiting my aerobic capacity upstream and decreasing supply of O2 to the working muscles. Or it could be reflecting a diminished response to reducedĀ demand for O2 by the working muscles downstream.

If all we could measure was systemic VO2, we would be left with the question of whether this decline in efficiency and net power output is the result ofĀ supply-side orĀ demand-side limitation. But with the Moxy muscle oxygen monitors we can look closer at oxygen extraction and re-synthesis at the local working muscle. Let’s take a look:

nirs_vo2_combined_li (3)
All-out 30/90sec to failure – Muscle oxygenation (Moxy) & VO2 Master Pro data

Top chart shows:

  • Blue line is Muscle Oxygen Saturation (SmO2 in %) forĀ Right quadriceps
  • Pink line is SmO2 for Left quad
  • Dotted pink & blue lines show Total Hemoglobin (tHb in g/dL) for each leg respectively (we won’t focus on this yet)

Bottom chart is the same as above, with:

  • Performance data
  • VO2Ā (mL/min)

Let’s focus on SmO2 response during the all-out 30/90sec workout. Muscle Oxygen saturation of the L leg (pink) is actually higher at baseline. I’m very curious what this means, but I’m going to leave this question for another time.

Both legs begin the first interval at roughly the same SmO2, giving us a good comparison of what happens during the work and recovery intervals.Ā The work intervals show thatĀ both legs almost entirely deplete the available oxygen supply. SmO2 de-saturates from a high of ~60% to under 5%.

This demonstrates that both muscles are equally able to utilize and convert oxygen into powerĀ during the work intervals. Possibly even indicating a supply-side limitation. If my cardiovascular system could deliver more oxygen, could the muscles produce even more power output?

However even the first recovery interval shows that the L leg is slower to recover and unable to re-saturate muscle oxygen to the same capacity as the R leg, despite the R leg actually working harder by 30-40 W. The blue highlighted areasĀ in the top chart show the growing gap in SmO2 re-saturation between R & L legs as the intervals continue.

If the R leg is unaffected, the limitation must be somewhere downstream. And because it appears that O2 re-synthesis during recovery is more affected than O2 consumption during work intervals, I interpret this to be consistent with the hypothesis thatĀ tissue metabolic extraction of oxygen isĀ sufficient, while O2 delivery to the tissueĀ isĀ the limiting factor.


  • I’ve been able to play around with two very cool training devices: theĀ VO2 Master Pro wearable VO2 analyzer, and the Moxy muscle oxygen monitor.
  • These devices are extremely helping to investigate metabolic efficiency for supply- and demand-side limiters to performance, for instance:
    • Full depletion of SmO2 before attainment of VO2max may indicate supply-side (O2 delivery) limitation.
    • Limited depletion of SmO2 combined with other respiratory metrics (FeO2) may indicate demand-side (muscle O2 utilization) limitation.
  • As for my own pathological blood flow limitation, oxygen utilization capacity seems to be equal between legs, while oxygen re-synthesis of L leg appears limited.
  • Decreased ability of L leg to recover and re-saturate muscle oxygen at the same rate as R leg during recovery intervals suggest O2 delivery limitation somewhere downstream of the descending aorta bifurcation.

Final Thoughts

So far I’ve only discussed the first half of this workout.

Sprint Interval Training. 300 TSS trainer workout!.. needed a week to recover from this

After the all-out 30/90sec intervals I allowed the leg to recover, got some aerobic volume in, then repeated the 30sec work intervals in a fatigued state. This time however I gave myself 4min complete recovery intervals between efforts to compensate for the decreased rate of SmO2 re-saturation in the L leg.

The comparison between these two interval protocol within the same workout revealed even more about how the L leg limitation interacts with changes in oxygen supply & demand during work & recovery intervals.