Grade-adjusted pace (GAP) is the flat-ground pace that would have cost you the same effort as the pace you actually ran on a slope. It exists because raw pace is a flatlander’s number: honest on a track, a liar in the hills.
Why raw pace lies on hills
Every metre of climb makes the metre under it more expensive. A 6:00/km up a steady 6% grade can be harder work than 4:40/km on the flat, but the clock only sees the 6:00. Judged on raw pace, a hilly run reads as a bad day, and the runner who held effort perfectly up the climb reads as the one who faded.
Splits make it worse. Put the climb in the second half and the watch shows you slowing late, which looks like fatigue and may be nothing of the kind. Any judgment built on raw pace inherits the route’s profile. GAP takes the profile back out.
How the adjustment works
The cost of running on a slope is not a guess; it was measured. Minetti and colleagues put runners on steep treadmills in 2002 and mapped the energy cost of every gradient. Uphill, cost climbs steeply: at the extremes they measured, a metre costs more than five times its flat price. Downhill refunds some of the bill, but only some, and only to a point: past roughly a 20% descent the cost rises again, because your legs stop saving energy and start braking.
GAP runs your recorded speed and grade through that curve, moment by moment, and reports the flat pace with the same price tag. debrief reads the grade from a satellite terrain model of your route, so a spiky elevation trace doesn’t invent hills that were never there.
Why your apps disagree about it
Strava reports one GAP for a run, your watch another, debrief a third. Nobody is lying. Every tool smooths differently, samples differently, and fits its own version of the curve. debrief applies the published cost curve and claims no agreement with anyone else’s number.
That is less of a problem than it looks, because the number’s job is not to match another app. Its job is to be consistent with your own history, so this week’s hilly tempo can stand next to last month’s flat one and mean something.
Where the adjustment ends
GAP corrects grade. It does not correct surface or wind: mud, sand and a headwind all slow you at an unchanged grade, and GAP reads that as you slowing. On a treadmill there is no grade signal worth trusting, so no adjustment is applied indoors. And no curve knows your downhill skill: technical descents reward practice, and two runners with the same engine pay different prices on the way down.
What debrief does with it
Every stress score starts from grade-adjusted pace, so a hilly hour counts as the effort it was, not the splits the hills allowed; that number feeds your training load. Long-run drift is read in GAP too, so the climb in your back half doesn’t get read as fading. And in the note itself, GAP is named once and then speaks as plain pace: what the run cost, with the hills already priced in. That reading is one part of what an AI running coach should do with your file.