Metabolic Steady State – Inside vs Outside

A few weeks ago I performed a series of steady-state submaximal 30min intervals, looking at VO2 & ventilaton with the VO2 Master Pro and muscle oxygenation (SmO2) with Moxy. The first trial was inside on the trainer, followed a few days later by the second trial outside on the road.

The purpose was to observe how systemic and local metabolic signals respond to a constant workload. This was part of a preliminary experiment related to my left leg blood flow limitation… but I’ll get more into that another time (experiments are ongoing!).

Steady-State under Anaerobic Threshold

I had previously established my Anaerobic Threshold (AnT), which is an estimate of the highest workload at which I should be able to maintain a stable metabolic equilibrium between anaerobic and aerobic energy production. ie. the highest power at which relevant physiological measurements (blood lactate, VO2, ventilation, SmO2, etc.) are able to reach a steady-state, or stable plateau without continuously increasing. (Billat et al, 2003; Svedahl & Macintosh, 2003; Faude et al, 2008; Hauser et al, 2014, lots more possible references)

Protocol matters! when talking about thresholds, zones, steady-states, and physiological equilibriums. Forgive me for glazing over how I have defined and experimentally determined anaerobic threshold. I will return to these important details at another time.

The first workout trial I confirmed a low estimate for AnT at 260 W, where the physiological markers I was measuring were clearly at an elevated, but stable equilibrium. I then performed a 30min steady-state effort at 260 W inside on the trainer. A few days later I repeated the 30min effort at 260 W, this time outside on a fairly uninterrupted stretch of road.

I was interested in comparing how these physiological signals might be different on the trainer in a controlled “lab” environment (I think at this point I’m allowed to call my living room a ‘lab’?) compared to out on the road in real-world conditions.

During both efforts I maintained essentially the same seated position on the bike. I tried to control for nutrition, hydration, time of day, fatigue, work/life stress, etc. leading into both trials. Both trials of course used the exact same measurement devices including power meter, HR monitor, Moxy SmO2 sensors, and VO2 Master Pro mask.

I could not control for temperature or cooling inside vs outside. Inside I had a large fan on full blast to try and replicate riding outside with open airflow. This will come up later. I could not perfectly control cadence outside, however I simply tried to maintain my natural comfortable cadence while maintaining 260 W during both trials. There were a few points out on the road where I had to turn corners or wait for cars, so the outside interval was slightly interrupted at times. But I’m satisfied that the power line was about as flat and consistent as I could have hoped for in real-world open-road conditions.

Before we look at the charts below, let’s use this opportunity to discuss the ventilation metrics I was collecting, and their expected response during a steady-state constant workload interval:

Ventilation Metrics

  • Respiratory Frequency (Rf): breathing rate, in breaths per minute (BPM).
  • Tidal Volume (Vt): volume of air exchanged per breath, ie. passed through the lungs during a single inhale and exhale cycle, in Litres (L).
  • Ventilation (VE): volume of air exchanged in Litres per minute (L/min) and the product of Rf * Vt.
  • Fraction of Expired Oxygen (FEO2): percentage of O2 in exhaled air (as %), where inhaled air is ~21% O2. The difference between inhaled and exhaled air must then have been taken up by the body for metabolic oxidation.
  • Volume of Oxygen Consumption (VO2): volume of O2 consumption in millilitres per minute (mL/min), ie. how much O2 is being taken up and utilized by the whole body to meet metabolic demands.

Ventilation (VE) responds very rapidly to an increase in workload. At lower intensities VE increases linearly with increasing Tidal Volume (Vt), while Respiratory Frequency (Rf) remains near resting levels. FEO2 is typically higher at rest and very low intensities, as quiet breathing is more than sufficient to deliver sufficient O2 to meet resting VO2 demands.

As workload and VO2 continue to increase, Rf and VE will both begin to rise linearly, while Vt rises to a maximum plateau. FEO2 typically decreases to a minimum plateau during this phase, as O2 demand (VO2) is increasing and gas exchange is able to efficiently deliver Oxygen.

At higher intensities Vt will begin to decrease, and further increases in VE are driven by a rapidly increasing Rf. This elevated respiration rate, or tachypnea results in less efficient gas exchange at the lungs, resulting in FEO2 increasing. ie. more ‘waste’ O2 being exhaled, even as VO2 is rising.

Vickery, 2008; Ward, 2014

Inside vs Outside Submaximal Steady-State Trials

30min @ 260 W – Trial 1: Inside


All charts show combined Performance (power, L/R Power, HR etc.), VO2 & ventilation, and SmO2 & tHb data from bottom to top, respectively.


30min @ 260 W – Trial 2: Outside

30min @ 260 W outside on the Road


Both Trials Overlaid


in gif format!



  • Top Chart – Muscle Oxygenation
  • Muscle Oxygen Saturation (SmO2) for Left (affected) Quad, measured at two locations (Vastus Lateralis and Rectus Femoris).
  • Total Hemoglobin (tHb, surrogate for blood volume) for Left Quad given below SmO2


  • Middle Chart – Ventilation Metrics
  • Respiratory Frequency (Rf) in red
  • Tidal Volume (Vt) in green, with an optimal range indicated between the dotted lines
  • Minute Ventilation (VE) in blue
  • Fraction of Expired Oxygen (FEO2) in yellow, with an optimal maximum indicated at the dotted line


  • Bottom Chart – Performance & VO2 data
  • Power in yellow
  • R power in blue
  • L power in pink
  • Heart Rate in red, highlighted above 90% HRmax
  • VO2 in Light Blue area

Performance Data

Let’s start at the bottom of the charts with the more familiar stuff: Power, VO2, & HR. The two trials were very consistent at 260 W, aside from a few spots on the road where I had to turn some corners and dodge a car or two. More interesting is R/L power balance reported by my 4iiii dual-sided power meter. While I had established that 260 W should be under systemic anaerobic threshold, I saw a small but significant deviation of L/R power balance during both trials. This was an indication that there was a dis-equilibrium between my L (affected) and R (un-affected) legs and how they were contributing toward the overall systemic metabolic picture.

Surprisingly the L/R difference was worse out on the road. This aligned with my perception of symptoms, where the power felt like a moderate effort at most, maybe 4-5/10 RPE. However symptoms worsened through each interval to a very uncomfortable 7-8/10 pain-scale. We’ll get back to this when we look at muscle oxygenation below.

Comparing VO2 for both trials showed very good consistency between the inside & outside intervals (avg 3720 and 3840 mL/min, respectively) at the same 260 W workload. VO2 in both trials maintained a steady-state, without any significant drift after the first few minutes. This steady-state oxygen uptake suggested that I was under systemic anaerobic threshold during both trials.

Heart rate told a different story on the trainer, with a significant drift from ~155 to 173 bpm, just touching 90% HRmax. Whereas on the road HR showed very little drift, remaining at ~160 bpm. This was likely due to differences in thermoregulation on the trainer compared to on the road. I’m sure we’re all familiar with the differences in how hot it gets and how much more we sweat sitting inside on a trainer compared to riding outside .

Ventilation Metrics

If we move up to look at ventilation metrics in the middle of each chart, we can see a few differences. Breathing rate (Rf) was slightly higher outside on the road, while breathing depth (Vt) showed the reverse trend, slightly higher on the trainer. Although Rf begins lower inside vs outside, it ends up drifting higher through the first inside trial, while Vt remains mostly constant throughout both trials.

If Rf & Vt were inversely different between trials, but at the same relative scale, we would expect Ventilation (VE) to remain mostly constant. And we do see similar VE during the first ~10min of each trial. However during the first inside trial where Rf begins to drift higher, VE follows by gradually rising to a higher magnitude during the final ~10min of the inside trial, compared to the outside trial.

This is likely again related to thermoregulation. Increasing ventilation is one of the body’s methods of controlling core temperature (Zila & Calkovska, 2011; No & Kwak, 2016)

Note on thermoregulation: the VO2 Master Pro reports temperature of expired air, although I haven’t included this metric in the charts above. I probably should have. Temperature range inside on the trainer was 27-28°C, while outside on the road was 24-26°C. Doesn’t sound like a massive difference, but probably reflects an even greater core temperature gradient, and the extra energy required to lower core temp on the trainer.

I wasn’t trying to control my breathing either way, but I think naturally my breathing was smoother and deeper (higher Vt, lower Rf) on the trainer. Out on the road I had to pay attention to a lot more moving parts around me, and this heightened physiological stress probably contributed to a less relaxed breathing pattern, ie. slightly faster (higher Rf) and less deep (lower Vt).

VO2 (volume of O2 uptake) was consistent between trials, but ventilation (volume of air exchanged) was higher during the inside trial. So the trade-off was that Fraction of Expired O2 (FEO2) was therefore slightly higher on the trainer. This is a reflection of gas exchange efficiency: VO2 must be maintained in order to meet the required energy demands of the workload, but due to the other factors (eg. thermoregulation, ‘road stress’) effect on ventilation, the rate of gas exchange required to meet that VO2 was different. The excess O2 inhaled with a higher VE was then just re-exhaled as ‘waste’, measurable as a higher FEO2.

Muscle Oxygenation

At the top of the charts and unmentioned so far, we have Muscle Oxygen Saturation (SmO2) at two locations on the Left Quad: the Vastus Lateralis (VL) and the Rectus Femoris (RF). I only had two Moxy sensors for this test and I wanted to observe changes specifically in the affected leg. Although in retrospect it would have been more interesting to look at Right vs Left.(current experiments ongoing!)

What we saw was a very close match between the trainer and road trials. L RF appeared slightly lower on the trainer, potentially showing greater metabolic involvement (less SmO2 ‘coming out’ means the working muscle is consuming more O2 to produce work). I actually expected greater RF recruitment outside on the road, where I assumed more varied muscle mass would be recruited to stabilize on a moving bike.

This difference between trials was not statistically or clinically significant, so it probably reflects a positional difference or movement artefact, rather than a meaningful physiological difference. But if it does reflect a true change, it could imply higher recruitment of RF on the trainer, possibly related to more consistent quad muscle recruitment through the pedal stroke. This leads to a bunch of interesting questions on using muscle oxygenation to look at biomechanics and muscle recruitment through the pedal stroke… an experiment for another time!

Finally there might be a hint of a suggestion of L VL gradually de-saturating SmO2 through the 30min interval on the road, more than on the trainer. If this reflects a real change it would align with slightly worse symptoms and worse L/R power balance on the road. If power was the same (actually, L power was lower on the road due to lower L/R balance) the O2 demand was likely the same (or lower). But if SmO2 was lower, it might mean the O2 supply-demand mismatch was greater, and therefore O2 supply was more limited on the road. However the trend is too slight to make any firm conclusions from this one experiment.


This was a great confirmation of the consistency of measured data collected in a more controlled environment on the trainer, and in a real-world training environment out on the road. We should be able to directly compare workouts and data collected in both conditions and begin to investigate how well our traditional lab-based experiments and tests translate to real-world open-road situations.

I thought this simple experiment was a good opportunity to discuss some expected trends in metabolic signals. Again, as a kind of introduction to the charts and metrics I will look at further in the future. When I show a confusing chart from a race, with colourful data lines all over the place, we’ll be able to make better sense of it!

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