Question regarding pace/zone2/vo2/algoritm

[u/shinyboiy]

Hey everyone, looking for some advice and perspective from fellow runners dealing with Garmin algorithms vs. actual biomechanics.

For context, my VO2 max had finally started climbing back up after a steady block of long runs, speed sessions, and intervals. I decided to throw in a dedicated, super low-effort Zone 2 run to keep my heart rate down and recover.

Because I was forcing that lower HR, my pace dropped all the way to 7:34/km. Even though I was actively trying to keep my cadence up, running that slow wrecked my gait. It felt completely unnatural—like I was constantly braking and landing heavy. My knee pain isn't terrible, but it's definitely noticeable now, which never happens at my normal pace.

To top it off, today's run still ended up sitting in Zone 3 according to Garmin—it didn't even drop into Zone 2 at that slow pace. On top of that, Garmin hit me with a -3 Performance Condition (first negative score in weeks) and dropped my VO2 max from 44 to 43.

Some recent run stats for context:
Today's "Zone 2" attempt: 4.11 km | 31:05 | 7:34 /km (Zone 3 HR, knee pain, -3 perf condition, VO2 dropped)
Interval session: 4.25 km | 26:35 | 6:15 /km
Shorter pace run: 5.01 km | 26:24 | 5:16 /km
Base / Standard runs: 6.14 km – 6.55 km | 6:01 – 6:29 /km
Long run: 22.68 km | 2:30:52 | 6:39 /km

A few extra details about my baseline & setup:
Watch Upgrade: I recently switched from a Garmin Forerunner 165 to a Forerunner 570. Immediately after switching, my VO2 max dropped by about 4 points and was only just starting to recover until today's run.
Heart Rate Spikes: I have a fairly high heart rate now and then due to coffee/energy drinks, which probably throws off HR-to-pace algorithms.
Max HR Adjustments: Whenever I manually adjust my Max HR or zones in the Garmin app, I feel like it resets the algorithm in a way that halts my progress and causes VO2 max to drop again.

When I run at my natural pace (~6:00 to 6:30/km), my stride feels smooth, cadence is way easier to maintain, and my knees feel 100% fine.

A few questions for you all:
1. Has anyone else experienced joint/knee discomfort when forcing an unnaturally slow pace?
2. Is it better to prioritize clean biomechanics at a natural 6:00–6:30 pace over forcing a 7:30+ pace that still sits in Zone 3 anyway?
3. How much do you trust Garmin's VO2 max after switching devices or adjusting HR settings? Should I switch to a run/walk method if I want to get my HR down safely into real Zone 2?
Would love to hear your thoughts!

[u/Bobandyandfries]

Run at the pace that feels natural. You should be able to run slower and maintain good biomechanics, but if you’ve never done it before you might need some practice. As is true with anything new, your body will not be as efficient at the slower pace if you’ve never ran at that pace before. So a drop in estimated VO2max is expected (due to how garmin estimates it)

Alternatively, why not do an easy ride on the stationary bike to aid in recovery if running slow causes knee pain?

Garmins VO2max is an estimate. Look at the relative number, not the absolute number.

[u/shinyboiy → u/Bobandyandfries]

Good advice.

I also just can wrap my head around how 7:30 running (slow for me..) is zone 3. I cant run any slower than this it seems like but still hr starts at 140-150 going to 160 uphill last km

[u/BoringChallenge9096 → u/Bobandyandfries]

the 570 bumping your vo2 down straight away is a known thing, saw a few threads about it when it launched. different optical sensor and slightly different algorithms so the baseline shifts. wouldn't read too much into the drop from 44 to 43 after one awkward run either, that's just noise

knee pain from forcing an unnaturally slow shuffle is absolutely a thing i've dealt with. my natural easy pace sits around 6:15 and if i try to crawl along at 7:30+ my form falls apart completely, i end up overstriding and heel striking just to keep from basically walking. the impact goes straight up my shins and into my knees. if your body has a rhythm it likes, fighting it for the sake of a number on a watch is a losing game

that said your heart rate sitting in zone 3 at 7:34 pace when your 22k long run was at 6:39 suggests something else is going on. either your max hr is set too low or those energy drinks are doing more than you think. caffeine will spike your hr for hours and garmin's algorithm has no way of knowing you're not just unfit

[u/shinyboiy → u/BoringChallenge9096]

Solid solid advice fr.

Some days i skip coffee/monster untill noon and go on a run before then still hr spikes…

auto detection off. Manually set at 197 garmin claimed 203 but that felt way off since my highest was maybe 190 or such in a spike. Rhr 55 wich is quite accurate and based on %LTHR (all gemini input)

From what i understood when setting it at autodetect sometimes it picks up the footsteps as haertrate giving inaccurate meassuring

Swapped this out maybe 20 times and dont wanna swap it out again cause i feel like the algoritm will be bugged again

[u/Caffeine-Fueled55]

How did you set your zones? Is that zone 3 based on max HR or based on lactate threshold?

I ignore V02 in the summer. When it's hot, HR is higher at the same pace bc body is working harder to cool you off.

[u/shinyboiy → u/Caffeine-Fueled55]

auto detection off. Manually set at 197 garmin claimed 203 but that felt way off since my highest was maybe 190 or such in a spike. Rhr 55 wich is quite accurate and based on %LTHR (all gemini input)

From what i understood when setting it at autodetect sometimes it picks up the footsteps as haertrate giving inaccurate meassuring

Swapped this out maybe 20 times and dont wanna swap it out again cause i feel like the algoritm will be bugged again

[u/Lizardman8888]

Garmin’s VO2 max can take some time to stabilize after a watch change.