How is low/high aerobic load determined? Heart rate or running power?

There is only a very indirect way to determine what is considered low/high aerobic or anaerobic.

Low/High Aerobic, Anaerobic effect = F (duration, repetition and level of effort during intervals of a workout)

Low aerobic, high aerobic and anaerobic are Training Focus zones that you can see in dotted lines in the training focus screen. The placement (more or less to the right) and the length are determined by your acute training load area automatically. Essentially, Garmin/Firstbeat scientist have determined that a balanced training (ie an optimized training) should have contributions reaching these areas.

These things are not directly related to your HR or power zones, although they use the same words as labels.

You contribute to these Training Focus zones by executing workouts, depending on how you solicited your aerobic and your anaerobic metabolism. The measure of this solicitation is represented as Training Effect scores when you save your workout.

Further, depending on the duration, repetition and level of effort of intervals during the workout, using a proprietary Firstbeat algorithm, the workout Training Effect data is split in the three Focus Zones. You can see a glimpse of the rational in the example Firstbeat gives of how the Training Labels (i.e. the summary of the different combinations of Training Effect) work here:

https://www.firstbeatanalytics.com/en/features/workout-labels/

The level of effort is determined by the heart rate data during the intervals divided by HR at VO2Max. Your performance condition is determined by your performance (pace for running or power for cycling) for that heart rate data, compared with the corresponding performance associated with the similar heart rate data in your VO2Max model. It is possible that in the future the performance level for running will be power-based, but it is not the case yet. Running power is an independent silo of data for the time being.

Since the level of effort is in relation to your VO2Max model data, there is no direct way for you to set "zones" correspondingly.

How do we target the Training Focus zones, then? Three different approaches:

a) Faith. The easiest way is to follow the daily workout suggestions. The watch will continuously try to have to do workouts that give you points and "feed the bars".

b) Benchmark. You can run/bike steady pace/power 45mn-1h intervals at different levels and benchmark your contribution to the zones, and possibly adjust your zones

c) Holistic. Use Lactate Threshold zones, after executing the lactate threshold tests on the watch. At least, this will give you zones that use the same individual physiological data as the one used for your VO2Max model. Then, as you keep a balanced training program, everything (the VO2Max underlying model and the training zones) will be in harmony :-)

Note that the quality of your VO2Max model is paramount. Make sure your HRMax value is correct and you perform maximal effort (like short sprints) regularly (this is the hardest part, honestly).

Thank you for the detailed information! So you can't be sure how your activity will be rated until you actually save it.. well staying in HR Zone 2 for most of the activity still seems to be a pretty reliable way to get low aerobic load higher.

Hi,

I am hoping you can help me understand how these two activities are evaluated. Both performed on Tacx Neo Bike and tracked with Edge 830

High aerobe: (tempo 3,2 training load)

Low aerobe: (base 3,3 training load)

This is a harder one to explain, because the activities are very similar on average (214W and 205W Normalized Power), and the training effect is very similar (3.2 vs 3.3)

From the numbers you share, only the power distribution seems to correlate with the tempo (more time in zone 3, 214W NP) vs aerobic (more time in zone 2, 205W NP) assessment. The HR zones and training load would indicate that the aerobic one (TL: 95) was slightly more intense than the tempo one (TL 90), though...

A couple of things to know that will impact the TE:

- The EPOC calculation will be influenced by a change in HR Max value, if the watch is left on auto-detection. HR Max is better when tested in the field and auto-detection if turned off (because it doesn't change, except because of age).

- The EPOC calculation will be influenced by the HR measurements errors, if any, from the wrist HR or the chest strap (dry strap, moving strap, etc.)

- For 2 consecutive strictly equal workouts, the TE calculation can be lower for the second workout than the first workout , because TE takes training history into account on top of EPOC

I am not sure this explains fully what is going on there.

Also, on a side note, it looks like your HR and power zones are not quite in sync or your workout was not well designed as a tempo workout, or you have designed your zones in a non-traditional way: your heart was never in the zone 3. This could happen if the workout was made of short tempo bursts, and the HR was lagging. If the tempo intervals were 10mn or more, your heart should get to HR zone 3.

Thank you for your phenomenal response!

I just did a FTP test in Zwift while logging with the Garmin Edge simultanously. The result was 290 which is a 7,5% increase... as you suggested ;)

Thank you so much!