Activity trim & distance override — fixing FIT-file edge cases

Why this exists

The watch is wrong sometimes. Common cases:

• Athlete forgot to switch sport — swim FIT keeps recording through T1, sometimes into the bike. The trailing samples corrupt avg HR, avg pace, decoupling, W' depletion analysis — every recap stat.
• Watch kept running past the finish line. Same problem at the end of the run FIT.
• Open-water swim FIT routinely over-counts distance by 30–80% because the watch loses GPS during arm strokes and interpolates wildly. A 1.9 km 70.3 swim records as 2.5–3.5 km. Trimming doesn't fix that uniform inflation; only a manual override does.
• Indoor trainer rides record duration but zero distance.

The trim & adjust panel on /athlete/<pid>/activity/<id>/ is the escape hatch for all of these.

Where to find it

/athlete/<pid>/activity/<id>/ → scroll to the "Trim & Adjust" section. Visible to:

• Coaches with roster access for that athlete
• The linked premium athlete (their own activities)

Free-tier athletes can view their own activity but can't edit.

The visual preview chart

The top of the panel shows a Chart.js line plot of the activity:

• HR in red (always when present)
• Power in blue (bike + run with Stryd)
• Cadence / stroke rate in green (sport-aware: swim shows per-arm SR labelled "SR/arm" — multiply by 2 for full SR)

Red shaded regions mark what would be dropped from recap stats based on the current trim values. The chart updates live as you type — no save needed to preview the cut.

The unshaded middle band is what feeds the recap and PDF.

The two trim inputs

• Cut from start (s) — drops the first N seconds of the FIT. Use when the watch was recording before the leg actually began (warm-up before the gun, GPS lock-in time on a swim FIT, watch fumbling).
• Cut from end (s) — drops the last N seconds. Use when the watch kept running after the finish line, into T1, or through a cooldown.

Both are independent edge cuts — there is no from-to window selection. To keep only the middle 30 minutes of a 60-minute activity, you'd cut 15 minutes from each end.

The form rejects trim windows that leave less than 10 seconds of activity (the recap analyses need at least that much data).

Distance override

Below the two trim inputs sits Distance override (m). Use this when the FIT distance is wrong but trimming wouldn't fix it. The canonical case is an open-water swim where the watch over-counted across the entire activity — trimming the start/end doesn't help because the inflation is uniform.

Set the override to the race spec (e.g. 1900 for a 70.3 swim) and the recap, PDF, comparison page, and readiness model all use that number instead of the raw FIT distance. The original FIT distance stays on the row for reference and is shown as "raw: " alongside.

Leave the field empty to fall back to the FIT distance (scaled by the trim window — a 10% time trim trims ~10% of distance too).

Race-day distance flag

When you open a race recap, the recap card now surfaces a flag when the FIT distance differs ≥15% from the race format spec:

• ±15–30% (amber) — "GPS noise + open-water sighting" or "tight tangent line." Usually fine, just a heads-up.
• >30% (red) — "Likely GPS overcount or watch left running. Trim or override." Always action-needed.

The flag includes an Edit → link that jumps straight to the activity's trim page so you can fix it in one click. After overriding (or trimming), reload the recap — the flag disappears and pace recomputes from the corrected distance.

When trim → distance, or distance → trim?

• Trim fixes the watch-recorded-beyond-the-actual-leg problem. HR / power / pace all get a clean window.
• Distance override fixes the watch-recorded-the-wrong-distance problem. HR / power / pace are unaffected; only the distance and derived pace change.

Open-water swims usually need override (uniform GPS inflation). T1 spillover or post-finish recording usually needs trim. They compose: you can trim AND override on the same activity.

What happens when you save

• The precomputed recap-stat JSON columns are wiped (pacing, W', CdA, transition HR proxy).
• The Django streams cache for this activity is evicted.
• The recap and PDF will recompute on the next view — usually within ~1 second per leg the first time, then cached again.
• The athlete's race-comparison page and readiness report inherit the new values automatically.

Deleting an activity

The bottom of the trim panel has a "Delete this activity…" disclosure. Click it to reveal a typed-confirm form:

1. Type DELETE in the confirmation box
2. Click the red Delete button
3. Browser asks one more time (JS confirm)
4. Activity gone, redirected to the calendar

The server enforces the typed-DELETE match too — accidental double-clicks or stale form submissions can't trigger a delete.

For multisport files, deleting the parent cascades to all leg children automatically (no orphaned rows).

Caveats

• The chart streams come from the per-second FIT extract via /api/athlete/<pid>/activity/<id>/streams/?max_points=600 — about 600 points across the full activity. Resolution is enough to see broad shape changes (HR dropping, power spiking) but not individual sub-second spikes. That's fine for trim decisions.
• Trim is non-destructive — the original FIT file stays intact on disk. You can reset by zeroing both trim inputs. Same for distance override.
• The trim window is applied at read time by every recap service; there's no separate "trimmed FIT" file generated. Re-download the original FIT and you'll get the raw watch data back.

See also

• Post-race recap reference
• Race-readiness report
• Race comparison