CoreTemp Heat Accumulation Test

I have been experimenting for the last few weeks with the CORE BodyTemp sensor. This solid little device estimates core temperature from measuring thermal energy transfer at the skin (which I’ve learned is not the same as skin temperature) and using heart rate from a connected strap.

I heard about this device from the excellent Endurance Innovation Podcast, and immediately bought one for myself to play around with. I’ve used active and passive heat training before, and I think there is a lot of value to be gained through including heat exposure in a periodized training program. But I haven’t had any way to quantify outcomes until now.

Just FYI, this isn’t an immediately applicable article on how to implement heat training. That will come later.

This is more of a concept synthesis article trying to develop my own understanding of the constructs of thermal flux, thermoregulation, etc. as they are applied (operationalized) in the context of this novel heat accumulation protocol. With familiar physiological and performance data to help provide context for what we are seeing, and how to eventually interpret these data into actionable information. There are more questions than answers here, for other smarter people to help answer!


Heat Accumulation Protocol

The CORE development team recently posted a ‘heat ramp test’, which I’m calling a ‘Heat Accumulation protocol’ because it involves more than just a ramp test. The protocol is designed to compare changes in physiological and performance responses over time, ostensibly after a heat training/acclimation intervention. It is also used to prescribe a heat training ‘zone’.



CORE BodyTemp heat accumulation test, demo image from CORE

The protocol begins with a gradual warm-up, or ‘thermal accumulation phase‘. They recommend starting at 50% of FTP/CP/Threshold power and gradually increasing workload over ~20 minutes to a maximum of ~80% Threshold Power. The body should gradually warm to 38 °C.

Once we reach this first checkpoint at 38 °C, we clamp heart rate by allowing power to decline to counteract cardiac drift (increasing HR relative to power). I’m going to call this the ‘thermal-metabolic phase‘, as it’s a period where mechanical and metabolic work are modulated (reduced) in order to maintain a steady physiological HR response, while CoreTemp continues to rise.

Once our power output has declined by 20% we have reached checkpoint #2 and report CoreTemp 1 (confusingly named), which will be above 38 °C. The workload is stopped and we rest passively on the bike, allowing HR to decline. I’m calling this the ‘thermal dissipation phase‘ as no additional thermal load is being generated by the working muscles, so heat dissipation will slow and then reverse the rate of heat accumulation. CoreTemp will continue to gradually rise until reaching a peak (CoreTemp 2) and decline thereafter.

More information on the test protocol itself and advice on heat training prescription can be found on the CORE website.


Experimenting with Hot and Cool Conditions

I performed a few experiments of the heat accumulation protocol over the holidays. I modified the thermal accumulation phase slightly, using a 5-1 (5-min work, 1-min rest) intermittent step test protocol which I typically use for muscle oxygenation assessments using Moxy. This shouldn’t affect the thermal accumulation phase too much, since CoreTemp rises so gradually. And I will be repeating the same protocol, so reliability should be preserved.



I’ve kitted up like this in my living room before, so I only felt mildly silly to do it again for science!

In addition to the CoreTemp sensor I wore VO2 Master Pro to measure VO2 & ventilation (VE). I was very interested to see VE, as I know ventilation is affected by thermoregulation.

For the first trial I turned up the heat in my living room to 23 °C and layered up with full winter kit. This is still far from typical ‘hot’ conditions in a dedicated heat chamber – it might barely be considered room temperature south of Canada – but it represents a moderate increase from my typical living room conditions. I typically train in ~18-19 °C ambient temperature without a fan, so I figured I would have to add a greater environmental challenge to really see a good heat accumulation response.

Heat Accumulation Trial #1 – HOT!



Trial #1 – Heat Accumulation Test – HOT!
CoreTemp, VMPro, & cycling data integrated in WKO5. Legend on top

Vertical green lines indicates checkpoints #1 and #2. The first checkpoint after the thermal accumulation phase, at 38 °C. Then the thermal-metabolic phase begins, where HR is clamped and power is decreased to compensate for cardiac drift until -20% decline, and checkpoint #2.

At checkpoint #2, CoreTemp is reported and work is stopped. During the thermal dissipation phase, CoreTemp reaches a delayed peak and subsequently begins to dissipate.

The thermal-accumulation phase started with 130 W + 30 W/step, until I reached checkpoint #1 at 38 °C. I then continued working continuously through the thermal-metabolic phase until checkpoint #2, when power had declined by ~20% at the clamped HR (~150 bpm).

I started noticing the heat and the perception of greater effort and discomfort around step 4. I felt like I was breathing harder and could feel my pulse strongly in my ears. At this point CoreTemp had barely increased half a degree, but I was sweating and feeling the heat.

VO2 was elevated compared to where it should have been from around step 4. On the charts here, the scale of power and VO2 are matched such that at expected physiological gross efficiency, the power and VO2 lines should be approximately at the same height on the Y-axis. Clearly this becomes less and less true through the test, showing a significant excess VO2 relative to what would be expected from power alone.

From this heat accumulation protocol I now had some performance and physiological outcome measures. But I didn’t really have any context for the numbers. So I decided to repeat another trial at the same power output, but this time with full cooling conditions: 19 °C environmental temperature, no extra clothing layers, and a big fan blowing straight on me. I wanted to see how much the outcome measures would be affected by the environmental conditions, with no change in fitness or heat acclimation.

Heat Accumulation Trial #2 – Cool



Trial #2 – Heat Accumulation Test – cool conditions
CoreTemp, VMPro, & cycling data integrated in WKO5. Legend on top

Same protocol and display as above. Heat accumulation was far more gradual, data suggested much lower physiological strain, and there was essentially no additional thermal dissipation phase, as thermal load began to dissipate and CoreTemp began to drop virtually as soon as activity was stopped.

For this second cool trial, I repeated the workout at virtually identical power output and let CoreTemp, HR, VO2, and VE be my outcome measures.

The first few steps were essentially identical. My baseline CoreTemp was consistent at 37.0 °C, and I started to accumulate heat above that baseline with about the same timing during the first two steps. Beyond that however, the rate of thermal accumulation did not rise as fast and things began to diverge.

By the start of the thermal-metabolic phase I was only at 37.8 °C and HR ~15 bpm lower than the hot trial. VO2 and VE were lower as well. All as expected, given less physiological strain from the lower thermal load. During the thermal-metabolic phase I had far less cardiac drift to compensate for, resulting in HR declining by ~12% as power was decreased by 20%.

I was surprised however that VO2 and VE, although starting from lower values, declined by the same magnitude in both trials. ΔHR/ΔPower, a relative measure of how much each variable changed, was ~0.38 bpm/W in the cool trial, compared to zero (by definition) in the hot trial. ΔVO2/ΔPower and ΔVE/ΔPower each had very similar slopes in both the hot and the cool trials (~17 ml/min/W and ~0.34 L/min/W, respectively).

Meanwhile, CoreTemp had increased another +0.4 °C, compared to +0.7 °C in the hot trial. And just about as soon as I stopped pedaling after checkpoint #2, CoreTemp began to dissipate from a peak of 38.2 °C. So there was virtually no thermal dissipation phase. With the cooler environmental conditions I was easily able to dissipate the heat generated from the metabolic workload.


Interpreting the Heat Accumulation Test

To summarize the data points we get from the heat accumulation test, we have:

  • Duration of Thermal Accumulation Phase
  • Power ramp rate (linear slope, W/min) to 38 °C
  • Power at 38 °C (checkpoint #1)
  • HR at 38 °C
  • VO2 & VE at 38 °C

  • Duration of Thermal-Metabolic Phase
  • CoreTemp at checkpoint #2
  • Power ramp rate (W/min) to checkpoint #2 (20% decline)
  • VO2 & VE ramp rates (ΔVO2/ΔPower & ΔVE/ΔPower)

  • Duration of Thermal Dissipation Phase
  • Peak CoreTemp

In every case, we are looking at continuous time series data that are affected by the real variable of interest: thermal load, or more accurately: thermal flux.

Thermal load is what the CoreTemp sensor is estimating. Thermal flux is the balance of the rate of heat generation within the body, primarily from the working muscles in our context. And the rate of heat dissipation to the environment via both passive and active thermoregulation.



Using the classic bucket analogy: Heat generation is predominantly from active muscles producing mechanical work and losing energy as heat from metabolic inefficiencies.

Heat dissipation is through passive (eg. environmental conditions) and active (eg. sweating) thermoregulation.

And the size of the bucket is our functional thermal load capacity, or how much heat accumulation we can tolerate and still meet the activity demands, before task-intolerance.

Thermal Accumulation Phase

Duration of the thermal accumulation phase will be dependent on the ramp rate of power, eg. 30 W/min, or in my case above 30 W / 6-min = 5 W/min. This is important in determining our initial rate of thermal flux, as increasing workload will generate more heat and begin filling our thermal load bucket. Then as we begin to accumulate more thermal load, our active thermoregulatory mechanisms kick in to begin to dissipate more of that heat.

Thermoregulation is primarily dependent on the immediate environmental conditions: a cooler environment allows greater passive heat dissipation, before active thermoregulatory mechanisms (eg. sweating, vasodilation to the skin, and increased ventilation) need to ramp up.

So both the workload ramp rate, and the environmental conditions should be controlled as much as possible between trials. If they are consistent, then the change in duration of the thermal accumulation phase to reach 38 °C will be predominantly related to active thermoregulation, which itself is related to fitness and heat acclimation.

I would expect HR to be particularly sensitive to changes in fitness and heat acclimation, as both of these improvements would elicit increased blood plasma volume. Enhanced venous return and stroke volume will result in lower HR relative to power, and greater heat dissipation away from the working muscles. Meaning it may take longer to reach 38 °C, and we would be able to reach a higher power at checkpoint #1.

VO2 and VE are interesting. In my experiment comparing hot and cool conditions, we could see considerable differences in VO2 and VE at checkpoint #1 (by ~15% and ~7% between trials, respectively). I would expect to see higher VO2 relative to power when active thermoregulation has to work harder; ie. greater thermal flux to preserve the same thermal load.

If thermoregulation were enhanced following a period of heat training, I’m not sure whether we would expect to see higher or lower VO2 relative to power at checkpoint #1: Would enhanced active thermoregulatory capacity impose a higher energy cost and therefore higher VO2? Or would we be more efficient at active & passive thermoregulation, resulting in lower energy cost and less ‘excess’ VO2 relative to power?

Thermal-Metabolic Phase

This is the major component of the test protocol: By clamping HR as a proxy for metabolic strain, we get an indication of our ability to preserve metabolic steady-state under a near-maximal rate of thermal flux.

At this point thermal flux is probably approaching maximum. The mechanical workload is high generating a lot of waste heat, and thermoregulation is working hard to avoid accumulating thermal load too quickly. In order to avoid over-filling the bucket and reaching task-intolerance we have to allow mechanical work to decrease, which will generate less heat and slow the rate of thermal load accumulation.

The ramp rate at which power declines to maintain steady HR, and the duration of the thermal-metabolic phase before a 20% decline in power, give us an indication of our maximum sustainable thermal flux: how much heat generation and heat dissipation can we sustain at this relative metabolic intensity.

At the end of this phase CoreTemp at checkpoint #2 indicates the size or functional capacity of our thermal load bucket. Functional meaning, the thermal load we can theoretically accumulate before reaching task-intolerance.

I’m sure CORE has tested different interations of this test protocol and settled on the targets of 38 °C at checkpoint #1, and 20% power decline at checkpoint #2, as sustainable functional benchmarks. I’m also sure these targets could be further optimized and individualized as we become more comfortable with predicting our thermal and metabolic responses.

If we have gas exchange analysis, we can also measure a ramp rate for both VO2 & VE. Since VO2 is driven primarily by the metabolic demand of the workload, the ramp rate can be normalized to power as ΔVO2/ΔPower and ΔVE/ΔPower.

Interestingly, I noticed in my experiments that the absolute magnitude of change in both VO2 & VE were similar across two very different environmental conditions. Therefore it seems like the response in O2 uptake and ventilation were more directly related to the decline in power than to the thermal flux or load?

Once again, I’m not sure whether we would expect to see the ramp rate of VO2 & VE change in response to fitness and heat acclimation, or whether these adaptations would be accounted for in the magnitude of VO2 & VE at 38 °C checkpoint #1?

Thermal-Dissipation Phase

Finally, the thermal dissipation phase gives us a peak thermal load (peak CoreTemp). I’m honestly not sure how this value is meaningful for heat training prescription. But it may simple be a ceiling to our functional/tolerable core temperature limit.

The duration it takes to reach peak CoreTemp and begin to decline reflects when our rate of thermal flux begins to drain the bucket, ie. when heat dissipation exceeds heat generation. With a greater rate of thermal flux during the thermal-metabolic phase, we might expect this inflection point to occur sooner after active muscle heat generation is stopped, leaving the elevated rate of heat dissipation.

I don’t know enough about thermoregulatory mechanisms and metabolic energy costs to say how much residual heat generation will remain after stopping external work. I can assume this number won’t go to zero, but I would assume given fitness & acclimation changes, that heat dissipation would be relatively greater than residual heat generation.

We saw an example of this in my hot and cool conditions trials, where in the hot trial the thermal dissipation phase was 8 minutes while I sat passively on the bike feeling very uncomfortable… While in the cool trial my thermal dissipation phase was literally non-existent, as my rate of thermal flux (driven by a much greater rate of passive environmental heat dissipation) was sufficient to immediately begin decreasing thermal load as soon as I turned off the active muscle heat generation.

This phase may actually reveal more of what thermal flux is doing behind the scenes: By removing the largest source of heat generation (working muscle activity) we will be able to see how quickly the underlying heat dissipation rate is able to drop the thermal load. Giving us an indication of our thermoregulatory capacity at the end of activity.

Alternative Protocol Designs?

I wonder if it would be useful to continue the resting thermal dissipation phase until CoreTemp has decreased back to checkpoint #2 at the end of the thermal-metabolic phase? That might reveal more about how heat dissipation is proportional to thermal load.

Or the thermal-metabolic phase activity could be continued until either power output or CoreTemp reached a balance at the steady HR? Which could give us a ‘thermal flux balance point’, where heat generation equals heat dissipation. Assuming exercise intolerance doesn’t occur first…

I’m not as up to date as I should be on thermoregulation literature, so I’ll be interested to learn more about these processes and keep experimenting. As always, I find it far more informative to feel the effects of training directly, along with observing the data, in order to generate questions and hypothesis, which then leads me to read and learn more about the mechanisms and processes involved.


Skin Temperature Recording

I should have, but didn’t look at skin temperature during the two heat accumulation trials, above. CORE reports skin temp in their mobile app, but for some reason they don’t record it to a Garmin Connect IQ data field. I would be very interested in seeing this data field added, since it’s already being measured. Skin temp would give us the thermal gradient from core to skin, which might be another way to evaluate thermal flux along with total thermal load.

Consider this a feature request!

Subsequently I’ve also heard that perceptual sense of thermal discomfort comes more from skin temperature than from core temperature per se. I’m not sure how this relationship works exactly, but I’ve been paying more attention to skin temperature lately.

In fact, I noticed an interesting phenomena recently during high intensity interval training (HIIT): Skin temperature appeared to consistently rise during the rest intervals and actually fell during the work intervals, with an amplitude of up to 1.0 °C between work and rest. Unfortunately I can’t show this because skin temperature wasn’t recorded!



4x 6-min Standing HIIT workout

Note CoreTemp also shows a marginal decline during the final two work intervals, and increase during the rest intervals.

This was unexpected. I wonder if this pattern is related to dissipating more heat via elevated ventilation during work intervals? But I would also be generating more heat, which would produce a greater thermal flux, but not necessarily a change in net thermal load.

Maybe it was because I was standing during the work intervals and moving around more in the fan airflow? (I have to stand during high intensity work, or else my iliac artery gets cranky).

Maybe it’s related to enhanced blood flow during work intervals from muscle pump venous return? Greater blood flow could redistribute more heat to the peripheries to dissipate into the environment.


Future Directions

Lots of remaining questions to ask and experiments to run!

Of course, I measured muscle oxygenation with Moxy for all of these trials, but I left out those data for now. There was enough to talk about just considering the novelty of the heat accumulation protocol.

I am very interested in how this device works from an engineering and physiological perspective: how exactly does thermal energy transfer differ from ‘temperature’ in a biological medium? How are skin and environmental temperature modeled with heart rate to estimate core temp? What actually drives perception of thermal discomfort and task-intolerance? (skin temp, core temp, muscle temp??) And of course to what extent are all the metrics I observed here trainable and sensitive to heat acclimation and fitness changes?

I would be interested in seeing (or collecting) validation data on the Core BodyTemp device. I’m sure that has been/is being done. And the more I experiment, the more the measurements appear to have at least reasonable face validity. Now I just need to be able to record skin temperature to evaluate thermal flux with even greater precision!

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