Comparing Metabolic Response to Hard-Start VO2max Intervals

This week I want to look at VO2 and SmO2 response to one of my favourite VO2max workout protocol. We will see how systemic metabolism responds to hard-start VO2max intervals. The experiments were fascinating and I’ll try to give some recommendations to dial in your own individual workout prescription.

Hard-Start Intervals

The principle of the hard-start VO2max interval is to set off at a very high intensity in order to rapidly reach near VO2max (for which 90% HRmax is a good target for most people). Power can then be slowly decreased so that HR (and VO2) remain near-maximal, but the workload can be sustained for 5+ minutes without blowing up.

More specific directions:

  • Set off at extreme-intensity, ie. at a power above your VO2max workload (>PVO2max).
  • This should elicit 90% VO2max within ~90 seconds for most people.
  • 90% HRmax is a target that correlates decently well to 90% VO2max for most people.
  • Once above 90% VO2max or HRmax, begin to decrease the workload so that you remain above that 90% target, but you can sustain the full prescribed duration (6+ minute intervals at >90% VO2max should be possible).
  • Work intervals can be maintained longer, because the decreasing workload accounts for VO2 slow component and decline in gross efficiency due to fatigue. So more time can be accumulated >90% VO2max.
  • Recovery intervals can be longer than the typical 1:1 work/rest, because the hard-starts are more effective at rapidly stimulating aerobic metabolism. So we aren’t as concerned with letting VO2 drop between work intervals. This should allow longer work intervals and more repetitions, again allowing greater accumulation of time at >90% VO2max.

Systemic & Local Oxygenation Refresher

Total haemoglobin (tHb) 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 VO2master mask 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 to maximal oxygen uptake (VO2max).

With that reminder of SmO2 & VO2, let’s jump right into the metabolic data for a 3x6min VO2max workout that perfectly demonstrates the benefit (and the challenge) of a hard-start interval.

3x6min Hard-Start VO2max Intervals

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3x6min Hard-Start VO2max – Muscle oxygenation (Moxy) & VO2master data

Top chart shows:

  • Blue line is Muscle Oxygen Saturation (SmO2 in %) for¬†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:

  • Yellow line is Power
  • Red is HR,¬†highlighted above 90% HRmax
  • Blue area is Oxygen Consumption (VO2 in mL/min), highlighted above 90% VO2max
  • Note: unfortunately the VO2mask ran out of batteries before the third interval!
  • Vertical dashed lines bound the first 90sec of each 6min interval
  • Arrows indicate the hard-starts

On the first interval the athlete tried a half-hearted hard-start (indicated by the arrows). Power wasn’t high enough to stimulate the aerobic system as quickly as it could have been. As a result HR & VO2 only rose to ~80% of max in the first 90 seconds.

The athlete only reached 90% HRmax and VO2max near the end of the 6min interval (highlighted along the respective HR & VO2 lines in the bottom chart). Muscle oxygenation (SmO2 in the top chart) also gradually trended downward during the interval to a minimum at the very end of the interval.

For the second interval I took control of the athlete’s smart trainer and raised the hard-start power target significantly! This time the interval started at 355 W instead of 325 W. The athlete reached 90% HRmax & 90% VO2max rapidly, and de-saturated SmO2 to a minimum within the first 90sec. I manually adjusted resistance down as the athlete approached the target, and all metabolic values stabilized around their target values.

Average power after the hard start was the same in the first two intervals. The only difference was a harder-start on rep #2, and the athlete was able to accumulate more time >90% HRmax (1:23 vs 5:10)¬†more time >90% VO2max (1:44 vs 2:20) and more time at minimum %SmO2¬†(although I don’t have exact targets for SmO2 yet).

Average VO2 for the 6min intervals was 82% of VO2max in rep #1 compared to 86% in rep #2. Although this could still have been higher to make these intervals even more productive.

Finally, the third interval.. unfortunately the VO2 mask ran out of battery so we couldn’t collect respiratory data. But if we look at the trends in Power, HR, SmO2, and VO2 I would guess the quality of this interval was somewhere between the first and the second intervals:

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3x6min Hard-Start VO2max – Visualizing trends

Same chart as above with:

  • Pink arrows indicating SmO2 (%) trends
  • Red arrows indicating HR¬†(bpm) trends
  • Blue arrows indicating VO2¬†(mL/min) trends

Notice the third hard-start was subdued like the first interval, and again HR shows a slower rise toward 90% HRmax, instead of the rapid rise and plateau observed in the second interval.

Likewise SmO2 shows a similar trend to the first interval, with gradual de-saturation to a minimum, rather than rapid de-saturation and plateau at that minimum. I suspect we would have seen a similar trend in VO2, with a slower rise toward 90% VO2max and accumulated time >90% VO2max somewhere between that of reps #1 and #2.


  • This workout was able to demonstrate the difference between a hard-start and a… hedged-start strategy in stimulating the aerobic system up to maximal, and accumulating time at our target metabolic states of >90% HRmax,¬† >90% VO2max, or minimum %SmO2.
  • Hard-starts must be¬†hard, with intensity well above what could be sustained for the entire interval duration in order to maximize metabolic training stress of the interval.
  • We also demonstrated that for this athlete, 90% HRmax seemed to slightly over-predict 90% VO2max. As in at 90% HRmax, the athlete was slightly under 90% VO2max. They were able to sustain >90% HRmax for over 5min during the second interval while hovering just about at¬†90% VO2max, but strictly speaking only accumulated 2.5min at or above that target.

Hard-start and weak finish

The athlete above showed the typical hard-start VO2max interval, where after the first 90 seconds the interval workload more or less stabilized at a steady power output. But of course I have to share what my own intervals look like when blood flow restriction begins to affect my Left leg.

Spoiler: systemic metabolism remains elevated, but power output decreases dramatically.

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3x6min Hard-Start VO2max – Muscle oxygenation (Moxy) & VO2master data

Top chart shows:

  • Blue line is Smo2¬†(%) for¬†Right quad
  • Pink line is SmO2 for Left quad
  • We’re still ignoring hemoglobin, but it’s there if you know what you’re looking at

Bottom chart:

  • Yellow line is Power¬†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)
  • Red is HR,¬†highlighted above 90% HRmax
  • Blue area is Oxygen Consumption (VO2 in mL/min), highlighted above 90% VO2max
  • Vertical dashed lines bound the first 90sec of each 6min interval
  • Arrows indicate the hard-starts

No hedged-start for me! I have experience with exactly what intensity works for me, so I jump straight into a very hard-start. Again the first 90sec is crucial, and I am able to get HR up nearly to 90% HRmax. More importantly, VO2 reaches 90% VO2max and remains elevated for the entire remaining interval duration. Looking at muscle oxygenation, SmO2 still shows a more gradual decline through the first interval.

All three 6min intervals are very consistent in time accumulated above 90% HRmax and 90% VO2max, and for SmO2 de-saturation. One interesting comparison to the other athlete above, is that for me 90% HRmax seemed to slightly under-predict 90% VO2max. Total time accumulated >90% HRmax was 11:29, compared to 14:44 above >90% VO2max.

However, instead of power stabilizing after 90sec, my Left leg (pink power line in the bottom chart) continues to decline as blood flow and oxygen delivery becomes restricted. Meanwhile my Right leg (blue) takes over and maintains a more stable workload. The net result is that overall power output (yellow line) decreases steadily through the entire interval. My power output through the hard-start interval declines by over 25%, compared to the athlete above who stabilized with only a 15% decline.


  • Hard-starts must be¬†hard! Worth repeating.
  • Two similar workouts by two different athletes show the difference in metabolic response, especially in how well HR correlates to VO2. I would still suggest 90% HRmax is a good estimate of 90% VO2max for most people, but your individual response will vary.
  • If you are training at home with power & HR, HR should rapidly reach a peak in the first 90sec of the interval (some people may be faster) then plateau for the remainder of the interval. Power may also plateau or continue to decline after that initial 90sec.
  • I wonder how long these VO2max intervals could be maintained before either exhaustion, or VO2 cannot be sustained above 90%? Billat et al, 2013¬†found subjects were able to spend¬†over 20 minutes¬†continuously above 90%VO2max! So there may be room to push these 6min intervals further.
  • Bonus recommendation from experimenting with VO2 measurement: try to focus on full, steady breathing throughout the interval. Starting to hyperventilate (faster, shallow breaths) is a sign that anaerobic metabolism is driving systemic response, and VO2 will measurably decrease. We’ll look at this in more detail soon.