Terrain and grade-adjusted pace

Running uphill is slower at the same effort, so Runsei is terrain-aware. When it judges a workout or estimates your fitness pace, it grade-adjusts using the run's elevation — the flat-ground pace that would cost the same energy (based on the Minetti 2002 running-cost model). Uphill reads faster than the raw pace; downhill slower.

This is why a hilly rep held at threshold heart rate still counts as on-target even though the pace looks slow, and why one hilly workout won't drag your estimated threshold pace.