heat gain in a human systems while exercing

[rivase]

A quick question that someone may be able to help me with. I am designing a research study exercising humans in a hot environmental chamber. How would I go about figuring out heat gain during exercise.

I will have external work output - in watts (they will exercise on a cycle ergometer).

I will have an estimation of metabolic heat generation - in kcals (measuring indirect metabolism by measuring volume of oxygen utilized in liters per minute - and can convert to kcals or kJ).

The human body (blood and tissues) has a specific heat capacity of 3.49 kj kg c.

Any help would be appreciated on how I would go about predicting the rate of increase in body temperature. I am a physiologist and not an engineer. We are doing a heat acclimation study - 10 days exercising at 40C/ 60% humidity and would be helpful to predict heat gain during 90 minutes of exercise.

I am a physiologist and not an engineer. I figured after reading several similar questions on various materials, this should be doable. Thanks!

 

[racookpe1978]

OK - You are a physiologist, right? So: How much energy is a human using each minute "static" (at rest just standing still)? That is a medically-determined number. As an engineer, we don't know it!

Now, take that same human and "heat stress" him (or her) at 40 deg C and 60% humidity - but NOT exercising, still at static rest. This will become your baseload (before exercising the person), but again, as engineers, we don't know it. A single human's static hourly heat output is a very, very small number even in HVAC studies of single family buildings.

Now that you know the baseload point (a single human not moving at 40 deg C and 60% humidity), add to it the amount of maximum energy a human uses while heavily exercising - but again, that is a medical value based on body weight and type of exercise,right? It is not a engineering term.

 

[rivase]

Resting energy expenditure is about.2-.5Lmin/O2, and there are 5kcal per liter of O2, which is about 1-2.5kcal per min. We can measure this value indirectly for each person.

 

[willard3]

You could consult the tables in ASHRAE volumes.

 

[LiteYear]

You probably already know this, but another important factor is the efficiency of the human. It depends on fitness level and activity, but is somewhere around 22% for a reasonably fit cyclist. Then again, given you have metabolic heat generation, you probably already have two of the three variables in: heat out = work done / eff. Will be useful to check the numbers anyway.

Unfortunately, I suspect that from there things get very complicated very quickly. In general, a healthy human body does its utmost to maintain homeostasis and not increase in temperature. Having naturally quite a high core temperature means that normal processes like panting and sweating are generally pretty effective at thermoregulation. The actual core variation is quite minor, given that a normal range is 36.5–37.5°C and anything above that is considered hyperthermia. I suspect that the variation in core temperature due to exercise could easily be swamped by changes due to mood, circadian rhythm, circamensal rhythm, clothing, atmospheric effects and general fitness level.

What might change noticeably, is skin temperature. Apart from physically panting and sweating, the body effectively regulates temperature by controlling the temperature gradient from core to skin. If the skin is close to ambient, then there is little heat loss. If the skin is close to core (assuming "normal" ambient) then there is significant heat loss. Normal skin temperature (depending on location of course) might be somewhere around 32°C. I suppose that tracking that as it rises towards the core temperature might be an effective measure, but I'm speaking well outside my area of expertise. Maybe then you could calculate body heat gain using the integral of core to skin with specific heat capacity as the scaling constant.

 

[MintJulep]

You are looking for the increase in body temperature, right?

"Heat gain" has a specific meaning to engineers, which is why you are not getting relevant answers.

I'd think trying to model body temperature rise would be exceedingly non-trivial.

The body has a complex active temperature management system. The heat transfer from skin to environment is highly dependent on air temperature, air velocity, humidity and clothing.

There seems to be no shortage of papers and research on the topic however, so perhaps one of the many existing models could work for you.

But since you have a bunch of subjects in a lab, why not just stick a probe up their asses and measure directly?

 

[chicopee]

There use publication on heat stress values for humans at rest and active by the ACGHI. There is a ACGHI pamphlet (pocket size) on TLV's and BEI's that incorporates heat stress limit values.

 

[rivase]

Willard3 - Where can I look for these ASHRAE volumes?

Liteyear - Yes, we expect gross efficiency to be about 17% in exercising humans. You are correct. Thermoregulation of humans are effective via skin blood flow and skin temperature will actually cool to help with evaporative cooling. I like the idea of including both as one is held constant, we can assume heat loss is minimal and heat gain should rise. Thanks.

MintJulep - We are doing this exactly. I should have stated that I am a thermal physiologist. I study heat stress on the human physiology. We are measuring mean skin temperature and core body temperature (via rectal prob or intestinal temperature sensing pill).

Because we are minimizing heat loss by having a high humidity (60%).. I figured we can measure metabolic heat production - by measure metabolism indirectly and get an estimate of increase of body temperature. We do this by measuring the volume oxygen consumption (in L/min). We know that the kcal is roughly 5kcal per liter of O2. We can get an estimate of heat production. Most studies use body surface area, but I think it should be body mass?

Chiopee - I do not understand the acronyms TLV and BEI?

The design of the study is at 90minute bout of exercise and to increase body temperature from rest (~37C) to 38.5C over the duration of 30minutes. Then adjust work output (lower) to keep at 38C for the next hour. Rather than guessing each subjects heat generation to start at, and either over shooting or undershooting the first 30minutes, I was wanting to estimate it accurately from the start.

Ideally, I'd like to get the body mass of each subject and heat generation (VO2 liters per min) in kcal or watts and predict the increase in body temperature (heat gain) over the first 30 minutes of exercise. The changes in skin temperature should also be in the equation.

From an engineering perspective does this make sense?

 

[rivase]

I meant to increase from 37 to 38.5 in 30minutes then adjust work output to keep at 38.5C..

 

[IRstuff]

The supposed gold standard for core body temperature is rectal. There are temperature sensors that are capsulized and can be swallowed.

http://www.businessobserverfl.com/s...erature-sensor-and-the-cortemp-data-recorder/

Is this for school? It seems to be a fairly well-known issue that skin and external temperature measurements are not particularly reliable in measuring core temperature

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[chicopee]

Search the acronym ACGHI on line and you should see their publications as well as the initials TLV's and BEI's employed in their publications.

 

[LiteYear]

From an engineering perspective does this make sense?

Makes sense to me. Sorry on behalf of the forum if some of the responses are a bit elitist. It's an interesting problem.

I still think you'll have difficulty modelling because it's a highly dynamic system - as soon as you try to adjust something externally the system will change the parameters and you'll chase your tail. The existing experimental design could indeed have something going for it - go out hard to get the body temperature up, then as it stabilises, adjust the work load to achieve the desired temperature. The heat gain is probably highly correlated with heat loss, rather than specific heat - that is, heat gain is a function of how much heat is lost via the skin surface area, and how much mass there is to heat is a minor contribution.

In the engineering world, this kind of system might be modelled as a causal loop. Heat is generated to satisfy the work load, but the heat generated also causes the heat loss system to kick into gear. As time progress the loops continue to operate, but their strengths change. Modelling such systems is notoriously difficult, depending on how reliable your data is. I suggest, and only based on a hunch, that with too much loss of accuracy the problem can be simplified to ignore mass. Roughly speaking, the "go out head and then back off" method of control is how many automatic control algorithms work for systems that are not fully characterised. Depending on your tolerance for overshoot and settling time, the parameters can be adjusted, but the method of control remains the same.

 

[rivase]

Chicopee - Will do. Thanks.

IRstuff - This is not for a school project. It is a research study. We are interested in body temperature and it's influence on glucose metabolism in diabetic populations. So you can see, we are particularly interested in the rate of increase of body temperature because we do not want to over heat our diabetic participants. We have the capabilities of measuring rectal and have the telemetry pills you suggested. We are not trying to predict body temperature from skin and external temps. We are trying to control for ambient temp and humidity and work load to predict heat gain.

Liteyear - Thanks for the feedback. We are starting pilot work to pinpoint the correct work-output.

 

[IRstuff]

"The changes in skin temperature should also be in the equation. "

This is a nontrivial question, since it's dependent on sweating, etc; hence the question about school.

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[MintJulep]

In theory it sounds reasonable.

In practice:

Your estimate of heat production based on oxygen use is likely to be variable based on the individual.

Your implied assumption that body temperature rise will be uniform, and can thus be based on the specific heat of blood and tisssue is likely incorrect.

And the heat loss from the body surface by sweating and condition will also be dependent on the individual.

I think the uncertainty of the result will be too wide to make it a useful prediction.

 

[1gibson]

First you need to take subjects who are "in shape" (90 min cardio a day for at least a month) in COLD weather, then pop them in the heat chamber doing the same or very similar exercise.

If you don't, then you won't so much measure heat acclimation, as you will be measuring the effect of subjects getting into better shape after 10 days of cardio... That affect will be more pronounced than the heat acclimation.

Details are great, but it doesn't make sense to comment on the details until the basic assumptions are valid.

Maybe you've already considered that, but it also doesn't make sense, on an engineering forum, to not list all of your assumptions.

 

[rivase]

MintJulep - I think we will need to prescribe on watts/kg body weight.

We have some data that suggest this may work. In 35C with low humidity (30%), two groups - large (91.5kg) and small (67.6kg), both working at slightly different metabolic cost - 2.14 l/min vs 1.8 l/min, but when converted into watts/kg both are at 11.16 vs 11.07. And rate of increase in body heat is ~.02/min in both. The thing is changing environmental temperature and humidity will alter everything and I was hoping from an engineer's perspective, to get a better idea at how to tackle the problem.

1gibson - great point. We are doing a control under cool conditions/ 10day at similar work rates to distinguish the effect of exercise vs body temperature. Exercise intensity is very low -- about 30-40% of their peak maximal exercise capacity. We assume there will be no training effect, but will test that with the control protocol.

 

[1gibson]

You might need a control for your control . If I were doing it, I'd go maybe 10 days cold training unmonitored (or monitor for troubleshooting of your data acquisition methods, then toss the data.) Then 10 days cold training monitored. Then 10 days hot training monitored.

90 minutes a day of even light cardio can be a big change, depending on your subjects. If you use athletes then maybe you can skip the 10 day unmonitored, but that's still a wild card you might not want to deal with. Assuming no training affect for the typical person whose only exercise is walking to/from their car, and then switches to 90 min cardio a day, is probably not valid.