The 52 that never was
Since mid-July, Tuesday on my training plan had been “VO2 Max Intervals”: four-minute efforts at 90 percent of max heart rate, prescribed by Coach, my AI training agent. VO2 max is my watch’s estimate of aerobic capacity, and it had been a goal in the program for months. The July rebuild gave Tuesday its reason. The estimate had slipped from 52 to 50, the story went, and Tuesday existed to win it back.
I did the work, and the work paid off. Stairmasters used to lay me out; now I max their speed and hold my own for 30 or 40 minutes, and the challenge has pivoted from keeping up to finding equipment that lets me fully exert myself without first exhausting the machine. I’ve always gravitated toward strength training over cardio, so a prescription for more of it landed the way your doctor telling you that you need less salt or more sleep lands. I did it anyway. The number barely moved.
I know the body is slow to adapt, so a flat reading the morning after an exhausting workout never surprised me. A flat reading after months of targeted work was something else. My first instinct, honestly, was ego: I hoped my body was wrong, or the watch was, rather than the prescription or the numbers on the agent side. I went for an all-out 5K to test the theory that my VO2 max was higher and just being measured badly. I am not a runner and never have been. All I walked away with was a couple of new calluses and a reminder that I’m closer to 40 than I care to admit.
So instead of asking for another workout tweak, I asked for an audit: pull everything the watch has ever recorded and show me what has ever moved this number. The full history came back in two bulk calls, 286 daily readings and 933 activities.
The 52 never existed. My all-time maximum is 51.2, and the estimate has been essentially flat for a year: on 190 of the 258 days recorded since last September, it didn’t change at all. Its whole early rise happened in the watch’s first 22 days, and the activity driving it was daily walks. The audit’s read is a new device converging from a seeded starting value. That ramp had been read as a fitness gain ever since, and a remembered peak just above it became the baseline every later document cited.
The prescribed work couldn’t have moved it either. Six weeks of VO2 Tuesdays ran on a Stairmaster, and the estimate never reads a Stairmaster session. Cycling told a related story. My AI tooling had told me third-party imports never feed the estimate and that I should disconnect my Peloton, but every cycling reading in my history lands on a Peloton import day and stops at the disconnect. The audit’s read is that the estimate needs power and heart rate together and the imports carried both. That part is inference from my own records, not anything the watch’s maker documents.
Part of why this fitness story belongs on an AI blog is how recognizable the failure is. My read of it: with limited context, a model over-indexes on whatever sparse data you give it, and my VO2 max was exactly that kind of data, one number with an authoritative feel. It grabbed the number early, fixated on it, and over-rotated without understanding what it measured, where it came from, or how to move it. The claim then survived two later reviews of the program, because it had been copied into four of Coach’s own documents and they kept agreeing with each other. The series that settled it was a bulk download away the whole time, and nobody, me included, thought to pull it before rebuilding a training program around it.
VO2 max is no longer a goal in my program: no target, no checkpoints, nothing bent toward feeding it, and the daily briefs don’t bring it up. The watch still tracks it alongside dozens of other metrics, and I still look at the trend. I’m just unwilling to reshape a regimen that’s working for me to fit a wearable’s prescription. Athletes who need this number get it measured in a lab next to a pair of doctors. I don’t have the doctors or a reason to need them, so what I get is directional, and the daily readings get some grace.
Tuesday is still intervals, still unpleasant, and on the plan it’s called Conditioning Intervals now.