What happens to your heart rate during say a 20 minute FTP test interval? I have found a slow steady rise with a sustained effort. I think this probably represents heat stress. What do you find? I am not sure whether a rise with a relatively short 20 minute interval represents true decoupling.
What happens to your heart rate during say a 20 minute FTP test interval? I have found a slow steady rise with a sustained effort. I think this probably represents heat stress. What do you find? I am not sure whether a rise with a relatively short 20 minute interval represents true decoupling.
It’s completely normal to see one’s heart rate increase continually over the course of 20 minutes of intense iso-power effort.
Hugh
I think a rising HR is common on sustained efforts - usually called “HR Drift”…
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I don’t think heat has that much to do with it. Your HR only shifts a relatively small amount to cool the body to push more blood to the skin surface. You body has a lot of surface area, so removing lets say 200w of heat isn’t really that hard even without sweating in cooler temperatures… until its very humid. With evaporation, 1L of water removed 0.265kJ of energy. Over 20 minutes, that’s 220W. But again, you remove a lot of heat from radiation and convection first. Just consider that you easily cool your body without sweating in about a 75F with no air movement… that’s only a 20F temp difference.
Yes, normal to take a long time to rise. You have to remember that heart rate is an output not an input. Meaning that it responds to conditions and is a completely passive. The blood transports nutrients. A well trained athlete has a lot of oxygen and glycogen storage available so their heart rate changes very slowly. The more fit you are, the slower it changes.
Here’s my most resent FTP test last weekend. http://tpks.ws/KoIu
It took 2 minutes to rise from 112 to 154bpm. It didn’t really level out until 15 minutes in, then I made a final push the last 1 minute (I went out a little too easy). It also wasn’t a perfect test as I had some technical difficulties and stopped briefly early in the ride. But it’s off season, and I just wanted a new baseline for training zones.
On the flip side HR, can do some weird things in races. In my IM, it continuously dropped over the course of 5 hours.
I don’t think heat has that much to do with it. Your HR only shifts a relatively small amount to cool the body to push more blood to the skin surface. You body has a lot of surface area, so removing lets say 200w of heat isn’t really that hard even without sweating in cooler temperatures… until its very humid. With evaporation, 1L of water removed 0.265kJ of energy. Over 20 minutes, that’s 220W. But again, you remove a lot of heat from radiation and convection first. Just consider that you easily cool your body without sweating in about a 75F with no air movement… that’s only a 20F temp difference.
Good post, you are losing lots of heat due to convection, but heat lost due to radiation is tiny and way less than evaporation.
Fun story about heat loss due to convection. During the Apollo 13 incident they turned off the heating of the capsule to save energy and they also turned off the the fans that circulated the air the in capsule. The astronauts figured out they could stay warm if they stayed totally still, since they would heat the air next to their body. Since they were in 0 gravity, there was no convection, so the warm air did not rise and stayed close to their skin. Since air has a low thermal conductivity, it acted like an insulator. Once they started moving, then they would quickly get cold. This is also why cooling of spacecraft electronics can also be tricky, since we really take convection for granted.
Yes radiation is relatively small, but it’s significant. Evaporation can be very high if needed… but is limited by conditions, but at some point it will limit your power output. But again, you have a huge surface area and even a small amount of airflow will hav a big impact.
That’a cool story. Fortunately when riding or running you have air movement. But that’s why lots of fans is so important, but even then, if you do the math, trying to simulate even 15mph headwind, takes a HUGE fan. Forget 30mph. So you threefore sweat a lot.
it goes up
this represents the fact that it gets harder to sustain an effort the longer you do the effort.
What happens to your heart rate during say a 20 minute FTP test interval? I have found a slow steady rise with a sustained effort. I think this probably represents heat stress. What do you find? I am not sure whether a rise with a relatively short 20 minute interval represents true decoupling.
If the heat goes up in your room with your trainer, then that can drive the HR up and water loss through sweat will also. It’s normal for HR to creep up during FTP tests. That’s why for 30 min LTHR protocols, they make you take the avg of the last 20 and disregard the first 10 min.
It’s likely decoupling due to the high effort involved. If you were doine easy z2 type stuff, it would take a lot longer provided you’re in decent shape, but if you’re really pushing FTP+, it’ll gradually decouple over the hour to the point that you can’t go on anymore (ideally at the 1-hr mark if doing a 60 min TT.)
For the 20 min TT tests, people can actually ride FTP+ (most tests have FTP=95% 20 min max power) so you will expect some decoupling.
If the heat goes up in your room with your trainer, then that can drive the HR up and water loss through sweat will also. It’s normal for HR to creep up during FTP tests. That’s why for 30 min LTHR protocols, they make you take the avg of the last 20 and disregard the first 10 min.
Or why they have you use you ave HR over a 20’ test to represent your FTP, which is in theory 1 hour. You take 95% of the power output.
Yup.
I’ve heard for the 30 min test, you use avg watts for the whole 30 min, but only the last 20 min for LTHR. I’m probably confusing a couple of protocols though.
Vo2= Q x (a-v02dif) . For any given workload, (Vo2) is mostly determined by cardiac output (Q) and that is a product of HR x stroke volume. Once stroke volume is at max, HR must increase to meet the needs of the working tissue. A-Vo2 dif (o2 unloaded at the tissue) is actually increased a bit when the body temp rises (oxyhemoglobin dissociation curve) but not a major factor in the equation of any given workload, specially for such a “short” period. Are there any other Exercise Physiology nerds in here? haha
I would’ve expected your IF to be higher than 0.89. Shouldn’t it be .95+? There’s a lot I don’t know but that seems odd.
Vo2= Q x (a-v02dif) . For any given workload, (Vo2) is mostly determined by cardiac output (Q) and that is a product of HR x stroke volume. Once stroke volume is at max, HR must increase to meet the needs of the working tissue. A-Vo2 dif (o2 unloaded at the tissue) is actually increased a bit when the body temp rises (oxyhemoglobin dissociation curve) but not a major factor in the equation of any given workload, specially for such a “short” period. Are there any other Exercise Physiology nerds in here? haha
to add to this, when you’re sweating and losing body water due to heat production, which happens quite a bit in a stuffy room with no breeze, you’re essentially lowering your cardiac output because end diastolic volume is reduced by having less total body water, which then causes a reduction in stroke volume (SV = EDV - ESV). But assuming your workload doesn’t change, the muscles need the same amount of oxygen, so the variable that can do this is HR, which creeps up. That’s why it’s important to be euhydrated before exercise, especially when it’s hot and humid or if you sweat like a pig.
Mike,
Just wanted to add/clarify a bit.
. You body has a lot of surface area, so removing lets say 200w of heat isn’t really that hard even without sweating in cooler temperatures… until its very humid.
At rest in comfortable conditions we already still lose about 25% of our heat via evaporation. While exercising, don’t forget that we’re only about 20% efficient in converting our stored chemical energy to mechanical work. The upshot of this is that the power meter may show one is doing mechanical work at a rate of 200 watts but in doing so the waste heat you’re producing is on the order of that given off by an 800 watt heater or ~ 2880kJ/ hour. So picture the heat being given off by 8 rather than two one hundred watt light bulbs as the heat one needs to be getting rid off.
With evaporation, 1L of water removed 0.265kJ of energy.
Your decimal point must have skipped there. The latent heat of vaporization of water is ~2,420kJ/l at 37C. So someone cycling at 200 watts would need to evaporate ~ 2880/2420 or ~ 1.2 l/hour to shed the heat generated by working at this rate if all the heat loss was covered by evaporation. Don’t forget that any sweat that drips off on to your frame, floor, etc is absolutely no use in terms of cooling you. It’s only the water that evaporates that cools you.
Over 20 minutes, that’s 220W. But again, you remove a lot of heat from radiation and convection first. Just consider that you easily cool your body without sweating in about a 75F with no air movement…
Heat loss by radiation doesn’t increase significantly when you exercise since it varies with absolute temperature of the body. So any extra heat produced while exercising must be lost via evaporation of sweat or convection.
Not sure what you mean by
that’s only a 20F temp difference.
YMMV,
Hugh
Yes radiation is relatively small, but it’s significant.
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Don’t forget that at ambient temperatures of 36C and higher one actually gains heat via radiation and convection rather than loses it so if it warms to 36C + the only way you dump heat is via evaporation of sweat or contact with cooler materials like ice.
YMMV,
Hugh