Field note
Pacing is how you spread your effort across a run, with hills counted as the extra work they actually cost.
The cleanest field read is simple. Split the route in half by distance. Measure how fast each half was after converting climbs to the flat work they were worth. Then compare the two. The comparison names the shape. It does not grade the run.
Distance, not time, is the fair cut. A time split on a hilly route can hide a slowdown that was only a climb. It can also invent a fade that never happened. Equal-distance halves keep the same ground in each bucket.
Grade-adjusted pace converts every climb to the flat pace it was worth. Minetti and colleagues (2002) measured the energy cost of walking and running on extreme uphill and downhill slopes. That energy-cost model is why an uphill second half is not counted as a slowdown that never happened. The climb is translated into the flat work it asked of you. Then the two halves can be compared on the same scale.
If the second half was meaningfully quicker, that is a negative split. You finished stronger. If the two halves sat within a hair of each other, the splits were even. If the second half was gentler, you eased off later. The margin that separates those three is a small one just above GPS noise. It is a field calibration, not a scientific line.
| Shape | Plain reading |
|---|---|
| Negative split | Second half quicker after grade adjustment |
| Even splits | The two halves sat within a hair of each other |
| Eased later | Second half gentler; you backed off or faded |
This is a running read. The energy-cost numbers come from running. Rides, indoor sessions, and short non-GPS efforts have no clean half to compare.
Abbiss and Laursen (2008) reviewed how athletes spread effort across a race. The named shapes are the ones coaches already talk about: even, negative, and the fade that starts too hard.
Hanley (2015) mapped pacing profiles at a world half-marathon championship, including how pack running changes what a lone split would have looked like. Championship data is not a prescription for your Tuesday loop. It is evidence that the shape of a run is a real thing people study, not a watch gimmick.
Runners have long found that holding pace, or finishing a touch faster than they started, tends to go with a well-judged effort. Starting too hard and drifting is the classic way a run comes apart. Naming the shape just says which of those yours was.
A gentler second half can still be exactly right. Easy days, long runs, and routes that climb at the end all invite a quieter finish. Read it as the story of the run. Not a mark out of ten.
This sits next to What is durability? Durability asks whether pace and heart-rate cost held up in the second half. Pacing asks only how the speed was spread, hills accounted for. The two can disagree. Even splits can still cost more heart later. A negative split can still fade on heart cost.
A structured session is a different kind of pacing. The question there is whether each rep held, not whether kilometre two beat kilometre one. That lives in What are interval workouts?
Raw GPS pace treats a climb as a slowdown. Your legs know it was extra work. Grade adjustment is the attempt to speak the legs' language.
Minetti's group measured cost on slopes far steeper than most road loops. The useful takeaway for a watch owner is modest. Uphill costs more than the same speed on the flat. Downhill costs less, but not in a simple mirror of the climb. Downhills are the least certain part of the model.
Trust the read most on runs without long steep descents. A rolling loop is a fairer test than a mountain drop. GPS pace also wobbles on switchbacks and under trees. The grade correction cannot fix a noisy track.
Walking pace is a different question. Everyday gait speed is a fitness summary, not a race-split story. That lives in What does walking pace say about fitness?
Grade adjustment softens the terrain effect. It never erases it. Heat, wind, and simple fatigue all move a second half. One run is never a verdict.
Short sessions and indoor work do not give a clean half. Stay quiet when there is no honest comparison. The energy-cost model is measured for running, so the same split does not travel to a ride.
Pacing is the shape of how you spent the run, hills counted. It is a description, not a score. That is the line worth remembering.
I write these because I build Aera, an iPhone app that reads Apple Health against your own baseline and puts the research and the error bars underneath every number it shows you, including this one. You do not need it to use anything above.