Aerobic efficiency is pace per heartbeat. If you run 5:00/km at 150 bpm, your efficiency is higher than running 5:00/km at 160 bpm. Track it over weeks — a rising trend means your aerobic fitness is improving.
GAP converts your actual pace to what it would be on flat ground. Running 6:00/km uphill might equal 4:30/km GAP. This lets you fairly compare hilly and flat runs, and understand your true effort on varied terrain.
During steady runs, your heart rate naturally rises over time even at constant effort. This is cardiac drift:
Low drift on long easy runs is a sign of strong aerobic base.
TRIMP combines duration and heart rate intensity into a training load score. Unlike power-based training load, TRIMP works for any activity with heart rate data. Higher values mean greater training stress.