I noticed that my body responds in a drastically different manner (heart rate / breathing rate/RPE/fatigue) when I perform a 5x5 FTP power workout with different cadences. When I perform the intervals at 55 rpm, my heart rate is low, breathing is controlled, fatigue reasonable, and RPE fairly low. When I perform the same 5x5 interval set at the same power level but 110 rpm, my heart rate never hits steady state, my breathing rate keeps increasing, my RPE keeps increasing…
Does anyone know what is happening to my body physiologically? What type of stimulus is each exercise imparting? Clearly they are not the same exercise. Are there benefits of one vs the other? Am I the only one who has experienced this effect?
Great observation. You are perfectly normal.
The difference is in the type of muscle fibers you use and the energy system feeding those.
Low cadence will give you a workout focused on strength development, while the high cadence variant will target your aerobic fibers.
Since the fast twitchers are less dependent on oxygen supply, the heart will not provide it.
Thank you for your comment. Are you saying that lower cadence requires larger force and that larger force results in recruitment of both type 1 and type 2 muscle fibers, whereas at higher cadence because of the lower force, the same person would only recruit Type 1 fibers?
yes.
In more detail: you might recruit type 2 even at high cadence, depending on how strong your type 1’s are.
There are definitely fiber recruitment differences between 55 and 110 rpm (and I’ll get back to that), but I would suggest the larger effect that you’re observing is the increase in blood flow and enhanced muscle O2 delivery (QmO2) during low cadence, related to longer relaxation time between contractions.
Very simply, cycling economy (CE = W/L/min VO2) is higher at lower cadence. CP is higher at the same VO2, Wpeak is higher at the same VO2peak, VT2/RCP is higher at the same VO2.
So at the same power output at lower cadence, you’re observing the higher metabolic efficiency.
See below
Not ‘only’ type I. In biology, nothing goes to zero. Everything is happening all the time, just at different rates. I have no idea who said that, but it’s a good one.
I honestly don’t fully understand fiber recruitment patterns related to cadence and torque, but Type II’s will tend to be recruited more at both higher torque (muscle force output) and higher cadence (contraction velocity).
Maybe someone will have a force-velocity curve that’s relevant here?
Now those are the questions! What are the trade-offs of working at higher or lower economy? What are the central effects of optimising for systemic O2 uptake or external workload? What are the peripheral effects of torque vs contraction velocity? 
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