How to fix a heart rate spike in a FIT file
The chart shows 210 for the first ten minutes of an easy run, then drops to something believable. Or a single jump to 230 in the middle of a ride. The average is wrong, the max is wrong, and the training load your platform calculates from it is wrong too. The data around the spike is fine, so the fix is to correct that stretch and leave the rest alone.
Where spikes come from
- A dry chest strap. The electrodes need moisture to read. Until you sweat, a dry strap picks up noise and reports it as a very high rate. This is the classic spike at the start of a session in cold or dry weather.
- Static from clothing. Synthetic jerseys in dry air build up charge, and the strap reads it. Often shows as a long block of nonsense rather than a single jump.
- A loose or worn strap. A belt that has stretched, or a battery on its way out, gives intermittent dropouts and jumps.
- Cadence lock on an optical sensor. A wrist sensor sometimes locks onto your running cadence (around 170 to 180) instead of your pulse. This looks like a plausible but wrong plateau.
Prevention is easy for the first two: wet the strap before you start, and if static is the problem, a little electrode gel or plain water on the contacts fixes it.
Fixing it in the file
- Open the file in FIT File Editor. The heart rate stream is on the timeline under the map.
- Drag across the spike. Be generous: a little of the good data on either side is fine, because the fix only changes values that need changing.
- Pick Heart rate as the field to edit.
- Choose how to fix it:
- Cap sets a ceiling, say 175, and clamps anything above it. Good for a single jump or a block of high noise where the underlying rate was roughly steady.
- Smooth replaces the selection with a smooth curve through the neighbouring values. Good for spikes in the middle of a normal trace.
- Fill interpolates from the value before the selection to the value after. Good for a dry-strap start, where the real rate was climbing gently.
- Clear removes heart rate from the selection entirely. The platform then treats that stretch as no data, which is the honest choice if you truly have no idea what your heart was doing.
- Watch the preview. The average and max in the stats bar update as you choose.
- Press Export.
Uploading the corrected file
Delete the original activity on Strava or Garmin Connect, then upload the edited one. If you upload first you'll get a duplicate activity error.
What about power spikes and dropouts
The same tool works on power, cadence, speed, and elevation. A power meter that reads 1,800 watts for one second after a bad calibration gets capped; a dropout gets filled or smoothed. The Fix FIT file data page covers each field.