MAF 180 — Maffetone maximum aerobic heart rate
Phil Maffetone's formula for your maximum aerobic HR. Training below this number for 3-4 months builds pure aerobic base — you'll run faster at the same HR.
The MAF method is unfashionable but works. The test: every month, run 5 km at your MAF max HR. If pace improves over time, your aerobic engine is growing.