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Feb 2016
11:43am, 26 Feb 2016
5,347 posts
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paul the builder
Intirguing question - isn't the key that power on a bike an 'output measure'? The power meter knows nothing of how hard you're having to work (in RPE terms) to produce 130W. So I guess my answer would be that it *is* about efficiency still. You might need to burn say 200, or 250 kcal in order to produce the 65Wh (56 kcal, or 234KJ).
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Feb 2016
12:02pm, 26 Feb 2016
17,580 posts
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GlennR
Yes Paul, I see the point, but over time HR will decrease for a given session even if the mechanical efficiency was consistent throughout. The Garmin (at least one that uses the HR algorithm to calculate calories) will therefore indicate a lower calorie burn, and that's what I can't get my head around.
What I am asserting - and am happy to be challenged on - is that mechanical efficiency and heart fitness are different things, although they are very likely to change together over time.
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Feb 2016
12:40pm, 26 Feb 2016
5,348 posts
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paul the builder
I think I agree with your final assertion. You could think of 'heart fitness' as an indicator of, or a part of, 'chemical efficiency'. Your mechanical efficiency is only one part of the pathway between converting stored chemical energy into turning pedals.
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Feb 2016
2:09pm, 26 Feb 2016
1,729 posts
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Canute
The Garmin does not measure energy consumption directly. As implied by Paul, it almost certainly uses an algorithm based on the fact that heart rate rises as energy consumption increases. At aerobic paces, energy consumption is proportional to oxygen utilization, which in turn is proportion to heart rate, stroke volume, the difference in oxygen content between arterial and venous blood. The algorithm in effect assumes that your values for stroke volume and arterial-venous difference in oxygen content are typical values. If the algorithm is really clever it might take account of factors such as your fitness level and also the fact that arterio-venous oxygen difference increases with increasing power output. But at best, the algorithm assumes a relationship between HR and energy consumption based on typical values.
There are several other issues. One other important issue is the fact that the Garmin estimates metabolic cost the Watt bike measures mechanical work done. The efficiency of conversion of metabolic energy to mechanical energy varies between individuals and also within an individual with power output, and with changing level of type 2 fibre recruitment as you become exhausted.
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Feb 2016
2:27pm, 26 Feb 2016
17,585 posts
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GlennR
Just to give some background, Icelandic Trigirl started a thread to ask if she should trust the calories burned figure on her treadmill or her Garmin HR calculated data for the same session more.
At first I confidently answered 'Garmin', but thinking about it realised I wasn't so sure.
I used my own situation for further evidence of the issue. Using the Wattbike in the gym it gives me a number for calories, which is not all that far out from the figure that Garmin Connect provides if I enter the session manually. However, my new watch links directly to the Wattbike as well as measuring heart rate, and that is giving a much lower (less than 50%) calorie burn for the same session.
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Feb 2016
2:36pm, 26 Feb 2016
5,349 posts
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paul the builder
Canute wrote that Garmin estimates energy cost (i.e. calories burned) and the Wattbike (presumably) estimates work done (power x duration). That's what I was getting at too with my 'output' measure comment first.
I think I've read (probably Ross Tucker) that typical cycling efficiency is in the region of 25% (for when he's off calculating levels of suspicion/belief based on professional cycling climbing feats).
Was your 130W for 30min a real-world example? Because that converts to 56 calories only. I could easily believe that it 'cost' you 200 calories of energy expenditure, which is what a light-moderate 30min exercise might be.
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Feb 2016
2:41pm, 26 Feb 2016
17,586 posts
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GlennR
Yes, it was real Paul, and that sort of thing gives me a nominal speed of around 20 mph which, while not compatible with being out on the road, is not a bad pace for an easy session on a Wattbike - average HR was 117 bpm if I remember correctly.
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Feb 2016
2:52pm, 26 Feb 2016
5,350 posts
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paul the builder
And if you could bring yourself to go for an easy run for half an hour, with no interval session involved  - and you pootled along with that sort of HR, you might cover (say) 2.5 miles? At the crude rule-of-thumb of 100 cal/mile for a typical(*) 70kg male, this all *roughly* ties up.
* Not that you're typical of anything, obviously.:-)
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Feb 2016
2:58pm, 26 Feb 2016
17,587 posts
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GlennR
I'm currently rather more than 70kg, and will be for a while yet, which is why the Garmin gives me more calorie burn than that when running.
None the less I agree with your point. At least I'm reassured that I didn't give Icelandic Trigirl particularly bad advice.
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Jun 2016
7:01pm, 11 Jun 2016
9,004 posts
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FenlandRunner
Blast from the past today, while at Peterborough Athletic Track today, I saw MJB, brought back happy memories
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