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Development Team Member
Posts: 55
With the warmer and humid weather I have exprienced significant
cardiac drift both during intervals as well as asy sessions. 

For example, during recent interval session (in 30C weather), the difference
in HR between the first and the sixth (in 6x10') interval was almost 20 beats.
All other measures of effort (perceived effort, power, pace, etc.) stayed the same
and there was no change in SmO2/ThB from interval to interval.

Has anybody noticed any changes in SmO2/ThB purely due to
heat and/or humidity?

I am trying to quantify the cardiac drift and distinguish between the drift and
HR increase due to effort.


Development Team Member
Posts: 159
Hi runner

Yes I posted something a while ago. I find that about when I start sweating, about 30 minutes into the ride, my non priority muscle SmO2 drops. I may be at a steady SmO2 level it then drops to a lower SmO2 level with no change in workload. I put this down to blood being redirected to the skin for cooling purposes.

Fortiori Design LLC
Posts: 65

A few comments about HR drift and changes in SmO2 in different muscles overtime. 

This goes directly into one problem with performance testing that usually end within an hour and do short steps at different intensities. The fact is that time itself causes fatigue and time causes physiological changes. This is seen simply during steady state exercise in the mentioned HR drift. Essentially what we see with HR drift is that with time, if you want to maintain intensity you need to provide more physiological effort to maintain balance. You see this in SmO2, as mentioned as well. More relative O2 is extracted. Heat is a major concern, which causes blood redistribution to the skin in order to control heat. This is a major loss performance as this blood and oxygen is no longer effectively being used in the muscles. This will happen in non-priority muscles first, and then at some point the question of cardiac power in relation to blood distribution comes into play. So Craig, I would say that your conclusion is likely a major component.


Development Team Member
Posts: 1
Hi Craig.  Years ago when I was competing in ironman triathlons I monitored my HR throughout an entire race.  Half way into the marathon the drift was ridiculous and I thought my watch was malfunctioning.  My effort did not match the high HR.  I have been cognizant of this ever since.

Few points to remember with heat.  Not only does it vasodilate vessels, it shifts the hemoglobin dissociation curve to the right making hemoglobin slippery and therefore O2 falls off easily.  The question is does it make o2 more available? Perhaps this a is compensation for the blood that is redirected to the skin for cooling so we can keep up with the energy demands? 

I strongly agree with Andri and would like to add another tidbit or two.  Training in the heat increases sweating so therefore dehydration can also play a huge role as this changes blood volume.   I do not believe there is one single reason for cardiac drift (unless there is a medical condition).  It can result as an accumulation of fatigue from several systems.  The autonomic nervous and neural drive being one of the major components.  Not many people discuss the fatigue of the autonomic nervous system or how this effects neural drive.  If you monitor your Heart rate Variability you will make the connection as it is quite sensitive to sympathetic and parasympathetic fatigue.  If that directly effects HRV because of altered neural drive then we could infer the same phenomenon occurring at the muscular level.  After all the lumen size of vessels are directly controlled by the nervous system.

What I have observed (just did this the other day in the heat) is a higher than expected HR for the given exercise intensity and a very minor change in Smo2 and tHb.  I still had to work really hard to desaturate.  So it appears Smo2 is less subject to change compared to HR in this case.  It makes sense.  The heart is working way harder to help with cooling AND deliver O2 to muscles to keep up with energy demands.         
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