Prepend stationary GPS records to the start of a FIT.

Two main reasons. First, forgot-to-press-start: you rode the first kilometre from your house before realising the Garmin was not recording, and you want the activity to cover the real beginning. Second, anchor an indoor warmup to a real coordinate so it slots correctly on a map view before the outdoor portion of a brick session or commute begins. A third niche reason: padding the start of a course file so the head unit shows the route a little before you reach the actual start line.

No - speed and distance on those records are zero, so any moving-time calculation (Strava, Garmin Connect, our own analytics) treats them as a stationary warmup. Total elapsed time goes up by the prepended seconds; moving time stays the same. This is what you want - the prepend exists to fix the timeline, not to inflate your numbers.

Not for this tool. FIT only. Start extension needs the lap and session messages and the file_id.time_created field that GPX and TCX do not carry. If you have a GPX or TCX recording you want to backdate, convert it to FIT first with our [FIT Converter](/tools/fit-converter), then extend.

Yes - the output is a fully valid FIT re-encoded via the official Garmin SDK with proper CRC and headers. Strava will treat it as a normal activity. If you are re-uploading because the original was missing the start, delete the original from Strava first or the duplicate-detection logic may reject the new upload.

It does not need to be perfectly precise - a real lat/lon close to where you actually were is fine. The point is to give the file a believable starting position rather than (0, 0) or some weird default. Drag the map pin or paste in coordinates from Google Maps. A few metres of error makes no difference to anything downstream.

That is not what this tool does - all prepended records have zero speed and the same coordinate. If you need a moving warmup, you really want to merge two FIT files together, which our [FIT Combiner](/tools/fit-combiner) handles. Record the warmup separately and combine, rather than synthesising a fake moving warmup.

No - the prepended records all share the elevation of the first original record (extrapolated from the activity's actual start altitude). They form a flat plateau at the start of the elevation profile and add zero metres of gain. Cumulative ascent for the activity stays unchanged.

Technically yes, but the longer the prepend, the weirder the activity gets - hours of stationary records at the start looks suspicious to platforms and to anyone reading the file. Keep it under 10 minutes for sane results. For longer backdating, you usually want a different tool: use [FIT Time Adjuster](/tools/fit-time-adjuster) to shift the whole activity, or combine two recordings.