Power, Outside Temp and calories (kJ)



tomUK

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Oct 20, 2003
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I am somewhat interested in the relationship between the above. I'm told that all the body cares about is the actually amount of work done and hence this is how fitness is improve - demand more and the body adapts.

However, I am not quite sure I understand. Yesterday I rode for a couple of hours at 170W, the temp outside was 38 degrees C. Percieved exertion felt rather high. It seemed like my body, amongst working hard to turn the pedals was also taxed by the high heat.

Maintaining this wattage when it is cooler out - 20C - feels much easier.

And so my question: Is my body not burning more calories during the hot ride in light that it feels much harder? I'm trying to understand the dynamics of it. (needless to say, in the hot weather my heart rate is higher).

Note: prior to both rides I am sufficently rested.
 
tomUK said:
I am somewhat interested in the relationship between the above. I'm told that all the body cares about is the actually amount of work done and hence this is how fitness is improve - demand more and the body adapts.

However, I am not quite sure I understand. Yesterday I rode for a couple of hours at 170W, the temp outside was 38 degrees C. Percieved exertion felt rather high. It seemed like my body, amongst working hard to turn the pedals was also taxed by the high heat.

Maintaining this wattage when it is cooler out - 20C - feels much easier.

And so my question: Is my body not burning more calories during the hot ride in light that it feels much harder? I'm trying to understand the dynamics of it. (needless to say, in the hot weather my heart rate is higher).

Note: prior to both rides I am sufficently rested.
It's just your body has far greater difficulty dissipating the heat generated when it's hot out, and if body core temp rises, performance degrades somewhat and you'll ultimately be forced to slow down.

For every watt going through the cranks, there's about another 3 watts of heat being generated within your body. If you are averaging 170 watts, your body is pumping out ~ 500 watts of heat energy.

this is one reason why indoor training needs bloody big powerful fans for effective cooling.

Remember Flandis pouring water over his head/neck all thru that famous TdF stage on his solo break? It was simply a way to stay cooler and be able to sustain a higher power output for longer.
 
tomUK said:
...And so my question: Is my body not burning more calories during the hot ride in light that it feels much harder? I'm trying to understand the dynamics of it. (needless to say, in the hot weather my heart rate is higher)....
Alex nailed the answer of course, but I wouldn't be surprised if there are slight changes in an athlete's gross metabolic efficiency when exercising in particularly hot or cold climates. But since gross metabolic efficiency is limited to a fairly narrow range across humans in general I doubt the change due to climate is very large, perhaps a percentage point or two.

But Alex's point about waste heat is huge and often overlooked. Deliver a sustained 330 watts to the pedals and roughly a kilowatt is dissipated. That's a lot of heat to get rid of and a hard thing to do on hot and humid days.

-Dave
 
Is it therefore fair to say that FTP would be lower in higher heat, as it would be at altitude?

When heart rate is increase does the body not burn more calories then? for example if I were to crank the heat up in my room to 35 and not move an inch then my heart rate may increase somewhat; however, would I be burning any more calories?

Alex Simmons said:
It's just your body has far greater difficulty dissipating the heat generated when it's hot out, and if body core temp rises, performance degrades somewhat and you'll ultimately be forced to slow down.

For every watt going through the cranks, there's about another 3 watts of heat being generated within your body. If you are averaging 170 watts, your body is pumping out ~ 500 watts of heat energy.

this is one reason why indoor training needs bloody big powerful fans for effective cooling.

Remember Flandis pouring water over his head/neck all thru that famous TdF stage on his solo break? It was simply a way to stay cooler and be able to sustain a higher power output for longer.
 
tomUK said:
Is it therefore fair to say that FTP would be lower in higher heat, as it would be at altitude?
It is not unreasonable to see a performance decrease in unusually hot conditions.

Whether you choose to alter your FTP depends more on whether that is the normal state of affairs, or represents are large proportion of your training/riding.
 
Alex Simmons said:
It is not unreasonable to see a performance decrease in unusually hot conditions.

Whether you choose to alter your FTP depends more on whether that is the normal state of affairs, or represents are large proportion of your training/riding.
I'd go as far to say that it's not unreasonable to see a performance decrease in any unusual condition, whether that be heat, cold or altitude.
 

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