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Usually heat. As your core temperature rises, more blood goes to the skin to cool you and less returns to the heart with each beat, so your heart beats faster to keep output the same. Expect a higher heart rate indoors at any given power, and expect it to climb through a session.
Hold a steady power for an hour and your heart rate will usually climb even though the effort has not changed. That is cardiac drift, and heat is the main reason for it.
As you warm up, blood is sent towards the skin to shed heat. That means slightly less blood coming back to fill the heart between beats, so each beat moves a little less. To keep delivering the same oxygen, your heart beats more often.
Outdoors, airflow keeps a lid on this. Indoors with no fan it can be dramatic: the same 200 watts that sat at 140 beats in the first ten minutes can be at 160 by the end of the hour.
If you train to heart rate zones indoors, drift will quietly ruin your sessions. Thirty minutes in, your heart rate hits the top of the zone, you back off the power to stay in it, and you end up doing a much easier session than you planned without ever deciding to.
Indoors, power is the more reliable instruction and heart rate is useful context. Set the session by watts and let the heart rate do whatever it does.
If you do not have power, pay attention to your breathing and how hard it feels rather than chasing a number that is going to drift regardless.
Day to day variation of a few beats means very little. These are the patterns worth noticing:
The first three usually mean fatigue or illness and are a reason to take some easy days. The last one is a reason to stop and talk to a doctor. None of this is medical advice, and a sports doctor or cardiologist is the right person for anything that persists or worries you.
Five to fifteen beats at the same power is common, and more in a hot room with no airflow. What matters more is whether it is consistent for you.
If you train by heart rate, it is reasonable to accept slightly higher numbers indoors rather than reducing the work. Training by power indoors sidesteps the problem entirely.
Chest straps need moisture to read properly and can misbehave when dry at the start. Dry skin under an optical sensor, or a loose strap, cause the same thing. Wet the contacts before you start.
That is the problem we are actually trying to solve. Turbo Trainer Games turns your watts into the controls of a browser game — your power flies the aircraft, climbs the mountain, pulls the sledge. Nothing to install, and it works with a smart trainer you already own.
It is in closed testing at the moment, so there is nothing to play yet. But you can put your name down.
Ask for early access