Fractional Utilization

Hello! I have a question about fractional utilization - I understand it as the relationship between the anaerobic threshold and VO2Max. However - presumably there is a fractional utilization relationship between the aerobic threshold and the anaerobic threshold as well (or indeed between any two points you might want to choose)? I’m curious because based on heart rate, power, RPE (and Efficiency Factor in TrainingPeaks - which I believe reflects this too) - I have seen a significant improvement below threshold in recent months after being much more disciplined with my polarised training. I feel that i’ve set some good ground work and that an increase in my threshold is actually possible. Even if it is not, i’m certainly capable of holding for example a sweetspot intensity for more prolonged periods - which is a good thing to be able to draw upon!

I’m really interested in this lower intensity training and fractional utilization - does anyone have any research recommendations that I could read up on it? or maybe a podcast :wink:

Or please correct me, if i’m completely confused about this topic!

In ‘Training for the Uphill Athlete’, the authors use the relationship between LT1 and LT2 as an index of ‘aerobic’ fitness. IIRC, they define a difference of more than 10% (i.e. a fractional utilisation of less that 90%) as ‘aerobic deficiency syndrome’, with the suggestion that you would then need to spend more time developing your power/pace at LT1. I’m not sure how they arrived at this, I cannot remember seeing any references to support it. Regardless, it’s a good read and you might be able to find some of their writing, and reasoning, on this subject on the site referenced above.

This article explains the concept, but there’s no explanation of how they arrived at 10%:

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@Carolynsgaskell, great question. So I’m not aware of any research that speaks to this in terms of aerobic/anaerobic thresholds. The fractional utilization has mostly been referring to % of VO2 max, as you said.

I think as we evolve in this field, we will look at different ways to interpret data and give us clues around our progress. In the Uphill Athlete article that @davidmosborne linked to, this is the positive outcome you would find with nearly any performance test. As you improve from base training, the classic movement of the lactate curve is “down and right,” indicating improved submaximal performance, enhanced mitochondrial function/density, etc. etc.

Pulling out the 10% rule is a novel way to consider when it might be time to add high intensity training to the mix. For ultra distance events (like mountaineering), I have seen success with good blocks of base work with very little intensity. So if you are approaching training from that perspective, this could be an easy method to suggest when you’ve maybe done too much low intensity training and need to work that top end.

Personally, I would not rely on a certain % as the decision point to indicate when to add high-intensity as I’m not sure this percentage has any scientific basis. We’ve talked about this before around threshold, and the anaerobic threshold, particularly, is not going to be the same day to day. So it’s always a range. In that case, you might be at 10% one day, 6% another day, and 18% on a different day depending on recovery, fueling, HRV, stress, etc.

Using this in conjunction with your other performance data and day to day sensations would be my suggestion.

Coach Ryan


Brilliant… thank you - i’m going to check these out. Thank you for taking the time to reply.