The relationships between heart rate, work rate and duration over time

Firstly, thanks for reading this post.

I’d like to tap into the experience of TR users to try to shed some light on a few vague ideas of mine which revolve around the relationships between heart rate, work rate and duration.

Firstly, do you find that your AeTHR and AnTHR (LT2HR etc) remain stable over time even when outputs at these intensities change? I’m also interested in the relationships between these heart rates so if you could post your heart rates as well that would be really interesting.

Secondly, where does your heart rate at sweetspot (let’s say 88% FTP for the purposes of the question) sit in relation to your AeTHR? If you could mention your age and training status that would be cool.

Lastly, during long sub-threshold sessions my HR tends to rise linearly before eventually a gentle exponential rise sets in as decoupling takes hold. My thought would be that this serves as a marker of having exceeded current capacity to the extent where meaningful adaptation will be triggered. As an example, a recent 90 minute sweetspot (88% FTP) interval saw my heart rate stabilise at 140BPM between 20 mins and 80 mins before heading to 150 BPM in the final minutes of the interval.

Background: a reading of Fast After 50 last year sent me down a rabbit hole of further reading from which I have yet to emerge. I’m 51, long time cycle tourist but not a racer, just started the build phase in training for my first 24hr race this summer (at the Nürburgring), self coached. FTP currently set at 260 but this might be a shade low. AeTHR 140 ish; AnTHR 160 ish, lab-tested last November.

Any thoughts on any of the above greatly appreciated.

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Heart rate zones are stable over time. You might see a little movement on HR-aerobic-lower and HR-aerobic-upper, but practically speaking they stay the same.

Power threshold and zones are reflective of your metabolic fitness, and cardio fitness. Power zones will move in response to training.

FWIW I’m eleven years older and my field tested LTHR has been at 160bpm +/- 2bpm since I started training 8 years ago.

85% tempo is usually in the 144-148bpm range.

HR is a little higher when fresh (after a short break), and returns to my normal range while training 4-5 days a week.

At 90% its usually in the 149-153bpm for example here is a steady 60-min at 90% and only looking at the last 40 minutes:

and time view of the 30-sec power and HR:

Variability index of power was 1.00 so very steady outside effort (its flat here). HR dips when 30-sec power drops, and HR rises when power increases. Over those 40 minutes and fitting regression lines to power and HR, there is an increase in 5W for a 3bpm increase in HR - no decoupling. Again power and HR are tracking each other.

Now I compare that to a 90-minute HC climb (1000 meters) where I remove the first 20 minutes… this is final 70 minutes at 84% (variability of 1.01)

and the highest amount of time is spent between 145-148bpm.

Some of those HR variations are due to the wide range of cadence (and some from altitude change):


because higher cadence will raise HR, and lower cadence will reduce HR, here is the time view with 30-sec cadence:

you can see some level of cadence influencing HR in that view, not as strong as power but can still be seen.

could be hydration or fueling or heat or metabolic fitness. In general train by power, and if you were feeling muscle fatigue (RPE) then your HR was confirming you pushed yourself just beyond your current fitness at upper tempo.

Thanks for sharing, WindWarrior, fascinating stuff.

I was fairly sure I had read somewhere that HR-threshold relationships are quite stable over time; nice to hear your experience confirms this. I’ve been keeping an eye on power against HR at various intensities over the time since my lab test. What I seem to be seeing is a right-shift in the power curve, more pronounced at the sub-threshold levels; less so above LT. This is what I would hope to see at the end of Base and heading into Build. It will be interesting to see if more supra-threshold and threshold work shifts the upper part of the curve as I head towards Peak and the race itself.

‘HR is a little higher when fresh (after a short break), and returns to my normal range while training 4-5 days a week.’ I read a paper recently which reported a 5% +/-2 decrease after prolonged training, returning to previous levels only after sustained rest. I had wondered whether some of my apparent increase in fitness might be attributable to this decline in maxHR (on the assumption that threshold HR values would decline in proportion) but, as you’ve confirmed, the HR values at AeT and AnT don’t change.

As an aside, getting VI down to 1.0 outside is impressive!

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If you test in a lab, both HR and lactate will right-shift as you build fitness.

Max heart rate will decline with age, and the heart loses elasticity. And if your FTP as % vo2max is say 75%, and you raise it to 90%, then your LTHR (upper HR threshold) will increase - over time I’ve seen mine fluctuate between 158-162bpm and I simply split the difference at 160bpm. Back in 2016 I have one or two datapoints from 30-min at threshold and initially my LTHR was 156bpm, after some training its been between 158-162bpm.

All that said you should be able to treat those heart rate thresholds as relatively fixed unless you go from exercising in spring like German temperatures of say 16C, to summer temperatures of Spain at 38C. And then of course your HR will increase due to the additional cooling demand.

err, HR curve shifts down, lactate to the right. Same power, lower HR.

Another power-to-HR phenomena - I have bad allergies and find it difficult to breath at times. When that happens I start mouth breathing at tempo and sweet spot. If I focus on breathing in thru my nose, and exhaling thru mouth, my HR will drop 2-3bpm. During long 60+ minute intervals, sometimes I will notice/feel HR climbing, simply focusing on breathing and changing body position will usually stop it from continuing to climb (assume constant power).

(as I understand it, nose breathing allows your nasal cavities to produce nitric oxide, which is sent to lungs, and that increases blood flow and boosts oxygen levels in the body - somebody correct me if wrong)