HR drift

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I've read some brief notes about and experienced an effect called HR drift, and would like to know if any others have experienced this and what causes it ?
i.e. If you ride at a fixed power over time, HR seems to slowly increase.

e.g. After a 10 minute warmup, if I start riding at a moderate power output (for me) of around 250W @ 90RPM, my HR would be around 145. After say 30 minutes at this same power, my HR will have drifted to around 155.
Is this perhaps due to the legs tiring+increase of lactate, and therefore more oxygen/blood is required to maintain the same effort ?

Yes fatique and the production of lactate is a cause for HR drift. Other obvious causes are core temperature rise (as your body temp increases you have to pump more blood to cool yourself) and dehydration. I experience this also. Kevin

Originally posted by bradgregory
I've read some brief notes about and experienced an effect called HR drift, and would like to know if any others have experienced this and what causes it ?
i.e. If you ride at a fixed power over time, HR seems to slowly increase.

e.g. After a 10 minute warmup, if I start riding at a moderate power output (for me) of around 250W @ 90RPM, my HR would be around 145. After say 30 minutes at this same power, my HR will have drifted to around 155.
Is this perhaps due to the legs tiring+increase of lactate, and therefore more oxygen/blood is required to maintain the same effort ?