Cadence vs HR at a constant Power - is there an explainable relationship?

I’m completing an adaption week, so had a low power ‘Andrews’ session. In the last 15 minutes instead of sticking rigorously to the plan, I decided to test this relationship, but would like to know more what it tells me.
I set my Power at 75% and held this rigorously. Over 5 x 3 minute intervals I stepped up the cadence twice then stepped it down twice and let TR monitor my HR. The results:
Cadence - 87 … … 96… … …105… …96… … 87
HR… … 102->112… ->117… . ->123… ->120… ->115

I wasn’t surprised to see the last two HR being a few beats higher than their initial intervals. But don’t understand why the HR rises/falls purely as a result of leg cadence, while power is constant. It seems to imply their is a sweet spot for the cadence/HR/power hence FTP relationship?

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It’s actually very easy to understand and quite logical: at a higher cadence, you need to produce less force to achieve the same power output. That shifts the load from your leg muscles to your cardiovascular system. Similarly, at a lower cadence, your legs must produce more force and the load shifts from your cardiovascular system to your leg muscles.

Put another way, at equal power and higher cadence, your heart rate tends to increase compared with the same power output at a lower cadence.

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I would also suggest it’s at least partly the weight of your legs. Your leg mass is a fixed, fairly substantial quantity, and the faster you spin the more effort you expend moving them around. This added effort doesn’t show up in the power meter, but you have to generate it all the same.

There’s probably some interplay with self-selected cadence optimizing this leg-spin power loss relative to the actual output power effort (and other factors…).

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This. +1

Use this on climbs to spread the workload across cardio vs muscular energy systems.

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Yes, and you can make a conscious decision to tax either system, depending on which system is more fatigued. I found out that when I do certain VO2max intervals and I am quite fatigued, I fare better reducing my cadence from 100–110 rpm to 90–95ish rpm. It seems my cardiovascular system is maxed out, but my leg muscles aren’t (yet).

I don’t understand the explanation.

I understand, at least from my assumptions, which could well be totally incorrect, that Power is force x speed. Neurology triggers both the speed and force applied in the muscle. That Energy is expended to cause both motions. That the cardio vascular system is directly related to the energy supply. Muscle mass is directly related to muscle strength.

If your argument is complete. The inference is that high strength low cadence will always require less energy from the cardiovascular system, then that implies that muscular athletes will always out perform less muscular, which we know isn’t correct, eg, explosive vs endurance sports.

It seems this isn’t a clear cut straight line relationship.

HR changes with cadence in part due to higher VO2 with higher cadence (despite less motor unit recruitment), which is due to more SERCA activity from increased contraction frequency, and also just to plain old pendulum energy. Also with higher cadence is increased muscle pump, ergo venous return. If you’re not at vo2max, the HR will increase for this. If you are at vo2max, HR will usually drop a few bpm.

The relationship between HR and power is not at all linear. It’s almost impossible to assign a HR to a power output, as for various reasons many people will show cardiac drift, in addition to the relationship with cadence.

We don’t address it specifically for subthreshold, but we discuss all this in a podcast series. https://www.empiricalcycling.com/podcast-episodes watts doc # 18-24. The first half of #23 addresses cadence, and #18/19 goes into the relationship between power, vo2, and HR.

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Surely I will be glossing over fine details. High force efforts require more of your fast twitch muscle fiber to participate. Such muscle fibers exhaust quickly and recover slowly (ie they are the “matches” in your matchbox). Low force efforts predominantly use the slow twitch fibers which last a long time and when they do exhaust recover quickly.

Fast twitch muscle usually use anaerobic respiration and slow twitch aerobic respiration. The result being low force but high power efforts demand more of your cardiovascular system than high force high power efforts.

As usual there is no “free lunch”. Wanton use of high force efforts will exhaust your fast twitch fibers (ie burn your matches) making them unavailable for efforts for the rest of the ride. This is why sprinters work so hard avoiding work until the last 100 meters or so.

It’s a perfect display of the fact that you are always using multiple energy systems, it’s never as simple as one energy system.

Higher cadence is more aerobic(using more oxygen) due to the more frequent but lighter muscle contractions.

Lower cadence increases the anaerobic load(reducing your muscles need for oxygen) due to the less frequent but more forceful muscle contractions.

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Am listening to some of the linked podcasts. They are somewhat verbose! Some gems within for those willing to spend several hours at it. Will get back to you as they certainly raise some questions. Not least how do TR training plans fulfill and fit with EC findings?

Intervals.icu (shameless plug) has a chart you might want to look at exploring this topic (on bottom of the activity power page):

It looks a bit messy with cadence added.