Grade-Adjusted Pace (GAP)

What it is

GAP is an estimate that an app makes, and your watch does not measure it. Strava describes it as pace adjusted for the steepness of the terrain, an estimate of the equivalent pace on flat land. On a climb your GAP is faster than your actual pace and on a descent it tends to be slower, and Strava says it does not account for the technical difficulty or condition of the terrain¹. COROS describes standard GAP as built from population data and says its own Effort Pace is built from each athlete's training history². Every maker builds its own model, so compare a GAP figure only with figures from the same app or watch.

What a slope costs

Uphill, the energy cost of each meter rises roughly in a straight line with the grade. Downhill, it falls to a minimum somewhere between 10 and 20 percent downhill and then rises again on steeper slopes³. In a 2022 study of 8 well-trained trail runners on a treadmill at about 60 percent of their maximal oxygen uptake, a meter at +15 percent cost about 2.2 times a level meter, and a meter at -15 percent cost about 70 percent of one⁴. So a 15 percent climb costs more than twice the flat meter, and a 15 percent descent saves less than a third. The descents give back far less than the climbs take, which is why a rolling route is slower than a flat one at the same effort. Do not expect to hold your flat-road pace over hills, and do not read the slower pace as lost fitness.

Where the number misleads

Steep descents are where it misleads most. Strava says its downhill adjustment peaks at around 10 percent downhill and eases on steeper slopes¹. Pace on a steep descent is limited by braking and leg damage more than by energy: in a review, muscular stress from lengthening contractions, and not heart and lung capacity, appeared to be what limits intense descents³. Run a steep descent at a speed you can control and treat its adjusted pace as a rough guess. The guide on downhill running covers how to run and train them.

Heart rate is a poor check on a descent. At the same speed it is lower running down than up, yet hard descents at 70 percent of maximal oxygen uptake pushed heart rate and breathing rate toward their maximum³.

Rough ground costs more than the number knows. In a 2022 study of 10 experienced male trail runners who climbed for about 10.5 minutes at racing pace, oxygen cost on two trails was 10.5 percent higher, and ventilation 21 percent higher, than on a treadmill set to the same slope, speed and distance⁵. On rough trail, expect to be slower than your GAP implies and go by effort.

A fixed adjustment cannot show fatigue. Running economy drops after a big descent³, so the same adjusted pace costs more late in a run with a lot of descent than early in it. Heat and altitude are separate adjustments too: COROS says its Effort Pace adjusts only for hill grade, not for weather or altitude².

How to use it

Use GAP after the run and effort during it.

• After the run, compare a hilly run with a flat one by GAP, and use it to see how hard a climb was in your training log. Keep the comparison inside one app or watch.
• During the run, go by effort and let pace fall on the climbs. Use the talk test and the guide on heart rate, pace and effort to read it.
• Heart rate lags effort, so ease off before a climb starts and not when the number jumps.
• For a race, set a pace for each stretch from the course profile, slower on the climbs and quicker on the descents so effort stays even, and hold back before climbs in the second half (see the guide on race-day pacing).
• If your race is steeper than your training, match the climbing per mile and the grades in training, and measure runs on steep terrain by time and not distance (see the guide on terrain and conditions).