# Durability field test — fatigued-state power on a fixed hill

A repeatable field test for **fatigue resistance**: how much of your fresh power survives after
substantial work. Fatigued-state power discriminates real-world cycling performance better than
fresh power (see `references/durability.md` and `references/citations.md`).

> ⚠️ **Under-researched dose.** The *concept* is well supported; the *optimal* kJ pre-load and
> effort length are not standardized. Treat the numbers below as a sensible, repeatable default —
> hold them **constant** across tests so the comparison is valid, and track your own response.

## The protocol

**Fresh baseline (do once, on a rested day):**
- Warm up, then a maximal **1-minute effort** on your chosen fixed hill.
- Record: average power, the segment, gearing, and conditions. This is your *fresh* anchor.

**Fatigued test (the actual durability measure):**
1. Ride until you have accumulated **~1,500 kJ of work** (typically ~2.5 h; read the kJ total from
   the MCP `get_activity_*` data / head unit — do **not** estimate it by feel). Keep this ride
   mostly endurance so the fatigue is from *duration*, not a second hard effort.
2. Immediately do the **same 1-minute maximal effort on the same hill**, same gearing.
3. Record average power.

**The number that matters:**
```
Durability % = fatigued 1-min power ÷ fresh 1-min power × 100
```
Higher = more durable. Track the *trend* across a season, not a single value.

## Keep it a valid comparison (hold these constant)

| Variable | Why it must be fixed |
|---|---|
| The hill / segment & gearing | Grade and cadence change the power–speed relationship |
| The **1,500 kJ** pre-load | The whole point is matched accumulated work |
| Nutrition during the pre-load | Glycogen status drives fatigued power; fuel it consistently |
| Time of day, rough temperature | Heat and circadian effects move power (`references/data-confounds.md`) |

## Reading it

- **Prescribe and judge from power, not HR.** HR after 1,500 kJ is confounded by cardiac drift,
  heat, and fueling — use it only as a secondary decoupling signal (`references/data-confounds.md`).
- A rising durability % across a block, with fresh power flat or up, is the signal you want.
- A durability % that only holds when overall training load is high is *expected* — durability
  depends on sustained load, unlike fresh power (see Spragg et al. in `references/citations.md`).

## Logging it back to Intervals.icu

Record the fresh and fatigued efforts as tagged activities/intervals, or note the durability % in
the activity message / wellness comment, so the MCP server can retrieve the series later. This
skill does not store data — Intervals.icu does.