I have thought about cardiac drift recently. From what I understood there seem to be many different reasons for cardiac drift to occur:
- Heat. The body has to work increasing amounts to cool itself.
- Dehydration. The blood volume decreases the heart has to pump more.
- Increased fatigue. The body cycles though different muscle fibres.
- Mechanical reasons e.g. low inertia trainers (not sure about this one though).
What ever the reason, the body has to gradually produce more input (heart rate) to generate the same output (power) over the course of a workout.
I experimented with longer sweet spot workouts like Whiteside (5x20m of SST) this year and observed quite an increase of my heart rate during the workout. I managed to keep the power on target the whole time. The first interval finishes at 155 bpm. At the end of last interval then my HR crept way up close to 170 bpm. This puts me at 94% of max heart rate at the end of the workout, which is not anywhere close to the sweet spot zone.
According to this article where Coach Trevor Connor states:
Suppose you did a four-hour ride at a steady 160 watts. You might start the ride holding 145 beats per minute, but hours later your heart might be pumping 20 beats faster. This is known as cardiac drift. If you ride by power alone, you may be in the right physiological range initially but not at the end. Sticking to a heart rate range will keep you at the right intensity the entire ride.
Now I am slightly confused. Am I training in the right zone for the duration of Whiteside? Do I need to tone it down a little? Shouldn’t the heart rate curve be kind of flat when doing only sub threshold work? I know the general recommendation on the TR podcast is to ignore heart rate, however I am questioning if I am “doing it right” by just muscling through these longer workouts.
And a second thought: Have you ever witnessed different cardiac drift patterns of different indoor training setups? I strongly feel that pedalling inertia plays a big part here too.
Thanks for sharing your thoughts on this.