How to Combine Power and Heart Rate Data from Two Devices

The Two-Device Setup

Many serious cyclists ride with two recording devices: a head unit (Garmin Edge, Wahoo ELEMNT) on the bars and a watch (Apple Watch, Garmin Fenix, COROS PACE) on the wrist. Each captures different data:

• Head unit — GPS coordinates, power (from ANT+ power meter), cadence, speed, temperature, barometric elevation.
• Watch — Optical heart rate, calories, VO2 max estimates, sometimes GPS as a backup.

The result? Two activity files, each with half the data you want. Your power file has no heart rate. Your heart rate file has no power. Neither tells the full training story.

Why This Matters for Training

Training platforms like TrainingPeaks, Strava, and Intervals.icu calculate key metrics from the combination of power and heart rate:

• IF (Intensity Factor) and TSS (Training Stress Score) — Require power data.
• TRIMP and hrTSS — Require heart rate data.
• Aerobic decoupling (Pw:Hr) — Requires both power AND heart rate in the same file. This is the metric that tells you if you paced your endurance ride correctly.
• Efficiency Factor (EF) — Power divided by heart rate. Tracks aerobic fitness over time. Needs both in one file.

If your power and heart rate are in separate files, these combined metrics simply don't calculate. You're leaving training insights on the table.

Why Not Just Pair Both Sensors to One Device?

In theory, you could pair your heart rate monitor (chest strap or watch broadcast) to your head unit. In practice, it often doesn't work cleanly:

• Apple Watch can't broadcast HR to Garmin — Apple Watch doesn't support ANT+ or standard Bluetooth HR broadcast to third-party head units.
• Chest strap discomfort — Many riders prefer optical wrist HR over wearing a chest strap for 4+ hours.
• Bluetooth conflicts — Some devices can only maintain one Bluetooth HR connection. If your watch is already recording HR, it may not broadcast simultaneously.
• Backup recording — Many riders want the watch recording as a backup in case the head unit fails.

Merging after the ride is the pragmatic solution.

The Hard Way: Spreadsheets and Manual Alignment

Without a merge tool, combining power and heart rate from two files means exporting both as CSV, opening them in a spreadsheet, and manually aligning rows by timestamp. You'd need to:

1. Convert both files to CSV — Use Garmin's FIT SDK or an online converter for each file.
2. Open both CSVs in a spreadsheet — One has power/cadence columns, the other has heart rate.
3. Align rows by timestamp — If recording intervals differ (1s vs 5s), you'll need to interpolate or duplicate rows to match.
4. Copy the heart rate column into the power file, matching each row to the correct timestamp.
5. Re-encode back to FIT or GPX — Convert the merged spreadsheet back into a valid activity file.

For a 2-hour ride at 1-second intervals, that's 7,200 rows to align. One misaligned row shifts every subsequent data point. And even if you get it right, most converters lose metadata during the round-trip. There's a better way.

The Easy Way: Merge with TrailBlender

1. Export both files — Get the .FIT file from your head unit (Garmin Connect or USB) and the .GPX or .FIT from your watch (Apple Health export, Garmin Connect, COROS app).
2. Open TrailBlender and drag both files in.
3. TrailBlender detects the overlap — Both files cover the same ride, so it aligns them by timestamp.
4. Select data channels:
   • GPS → Head unit (better antenna)
   • Power → Head unit (direct power meter connection)
   • Cadence → Head unit (from cadence sensor or power meter)
   • Heart rate → Watch (optical wrist sensor)
   • Elevation → Head unit (barometric altimeter)
5. Preview the merged data — Verify that power and heart rate graphs both appear and look correct.
6. Export as FIT — Upload to Strava, TrainingPeaks, or Intervals.icu. All combined metrics now calculate correctly.