Efficiency Factor

3 minute read

In my last post, I showed the results of FTP test measurement.  Here I’m trying to measure my Aerobic Efficiency.  To calculate this, I did a 10 minute warm-up (not shown), followed by a steady Z2 heart rate workout for 35 minutes.  For me, I should be around 145 to be in the middle of HR zone 2 on the bike – which I can figure out from the values measured in my FTP test.  It was harder than I expected to keep my heart rate constant, and I ended up with it a little low for the duration of the test – around 141 according the the data.  As before, I’m not used to pacing on the Wattbike, and I lost my concentration near the end – unintentionally speeding up a bit before correcting.  I’m also starting to get down on the drops as my arm flexibility improves, which does vary my output somewhat.  Next time I’ll know roughly the setting I should be shooting for, and be a little more disciplined!

Although 30 minutes should work for this test, I suspect an hour or so might give a more even set of data.  To make this test accurate, you simply need to keep your heart as steady as possible in whatever zone you’ve picked.  I wanted an ‘easy’ workout, so I chose Z2 for the test.

This time the dip in the graph is intentional, since I got up to open a window!  I also smoothed the graph to give a better view of what’s going on.  I guess it’s an issue that my workouts are typically done in a much warmer environment that I would be in outside.  But as long as this is consistent, it shouldn’t be a problem.

Firstly, as you can see from Training Peaks, you get great analysis data of your ride – much better than Strava.  The ‘EF‘ (Efficiency Factor) number is 1.18.  This is the ratio of Watts I generated to HR.  The idea here is that as I get more aerobically fit, the number should go up – since the watts I generate for a given HR during an aerobic workout should slowly rise – as long as I keep the HR at the same value each time.  The nice thing about this workout is that I can throw it in on any day when I don’t feel up to a harder interval session.  It will be interesting to see the number rise.  Additionally, if the number starts to level out, that means I need to do something to push myself to improve.

The second interesting thing here is commonly known as decoupling. Training Peaks display this as Pwr:Hr, and the number is ~8% in this case.  What this represents is the change in my heart rate vs wattage for the duration of the test.  It’s quite visible in the graph – you can see the pink wattage line diverging from the red line.  According to Joe Friel, this is a little high and represents ‘poor’ aerobic efficiency.  No surprise there, since I’ve been sat on my backside for most of this year.  But I don’t think it’s too bad, and I suspect it is a little skewed due to this being my first go at the test.  I like a number I know I can improve; since that’s the point of measuring this stuff…..

And for completeness, here are the other interesting values that Training Peaks gives you, among others:

IF – Intensity Factor – the % of my FTP during this workout.  Here, 75% of my FTP; around the intensity I would typically hope to do a Half Ironman bike at.

VI – Variability Index – the variability of my performance during the test (about 1% in this case – as it should be)

Power Balance – The ratio of left to right leg power.  Very close in my case, though I have noticed that during intense workouts my ‘right’ side tends to work a bit harder than my left.

W/Kg – The wattage I generated per Kg of body weight (not expected to be high, since this was a Z2 workout)

NP – Normalized Power – a better measure of Power based on the variable intensity of the ride (note that it is almost the same as average wattage in this case – as you’d expect for a steady ride).  If I’d climbed a few mountains you’d expect NP to be much greater than Average Power – giving a much better representation of the effort I put in.  I’m not sure why Strava choose a different version of this (theirs is ‘Average Weighted Power’, and seems consistently lower than NP).

EF Test

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