# Study 000B Discussion

## What this flagship companion study adds to Study 000A

`Study 000A` established that later adaptation occurred through selective flexibility under increasing specialization. `Study 000B` asks what that specialization coincided with in terms of internal burden.

The answer is not "cost disappeared" and not "everything got worse." The answer is mixed.

## Best-supported interpretation

The clearest result is that adaptive efficiency improved.

Across the first and last 30 QC-pass runs:

- speed-per-heart-rate efficiency improved by `13.41%`
- power-per-heart-rate efficiency improved by only `2.82%`
- running share of 28-day activity rose from `15.26%` to `56.40%`

That makes it hard to say the later system was merely working harder in a crude sense. It was producing more speed per unit heart-rate cost.

## Why the study still finds persistent burden

The mixed part of the picture is equally important.

The simple speed-plus-power HR model shows that average unexplained HR burden shifted from `-1.60` bpm to `3.09` bpm, an upward shift of `4.69` bpm. In plain language, late sessions still carried slightly more HR than the simple external-output model would predict.

So the late system looks more efficient, but not frictionless.

## Recovery moved in a more favorable direction

Some background recovery markers moved in a favorable direction alongside specialization:

- next-day resting heart rate changed `-19.16%`
- next-day sleep score changed `6.53%`
- late-window HRV became available, but early-window missingness prevents a symmetric early-late comparison

That means the later system may have been better supported in the background even while harder sessions still carried real acute burden.

## Specialization appears relevant, but not sufficient by itself

Specialization tracked better speed efficiency (`r = 0.3103`) and lower next-day resting HR (`r = -0.8127`), but it did not by itself erase the simple run-level unexplained HR burden.

That is the most honest synthesis of the study:

`specialization coincided with better adaptive efficiency and more favorable background recovery markers, while leaving a persistent session-level unexplained HR burden`

## Why this matters for the broader program

This is valuable because it keeps the research program from collapsing into a false choice.

The system did not simply become more costly.
The system did not simply become more efficient.

It appears to have become more efficient in output terms while still carrying some unexplained burden in session terms.

That mixed pattern is likely one of the most important things this dataset can currently say.