Detraining Status - help

I am 5 weeks into the 12 week FTP builder, it is my first proper use of the platform.

I have used various other platforms prior to this with consistent result. I had a cycling Vo2 max of 54 before using Zwift which was consistent across all other platforms.

I did an FTP test and then started using Zwift and now 5 weeks and I have been continually unproductive, my vo2 max has dropped to 46, and this morning I am in a detraining status.

What is going on where Zwift is so fundamentally different to other platforms when it comes to training and results?

I’m close to giving up on zwift and moving back to other platforms.

Thanks for any input or advice

Obviously all the other platforms could be wrong and zwift could just be providing reality.

Riding a Wattbike Atom, with wahoo HRM. Recording sessions on a Garmin Fenix 5.

Garmin’s training status comes from the sensor(s) that are connected to your watch and not from the applications that you use. I would first make sure height/weight/etc. are set correctly.

Garmin’s VO2Max estimate compares HR (as a % of maxHR) to your power output.

As such, factors that affect HR (other than fitness) can show up in the VO2Max estimate. A common issue as you switch to cycling indoors is that the lack of air movement means your core temperature rises, and your HR increases to keep you cool (your heart starts working as a heat pump, as well as just an oxygen pump). A higher HR for any given power output leads to a lower VO2Max estimate, even if your fitness is actually unchanged.

If raising your FTP is your goal, I would recommend just doing the ramp test every few weeks, and let that be your metric for measuring progress, rather than VO2Max. Garmin’s VO2Max estimate is subject to too many external factors (as well as just heat).

I agree with the other poster as well - look at your connected sensors, and how you are measuring power.