LTHR from a 30-minute time trial

The average heart rate of the last 20 minutes of a 30-minute time trial defines LTHR — lactate threshold heart rate.

Protocol

After warming up, perform a 30-minute time trial at the hardest pace you can sustain (running, cycling, or another cyclic sport), recording heart rate throughout the whole test. Use the average HR of the last 20 minutes — the first 10 minutes are discarded because they still reflect HR drift as it climbs toward a stable level.

If the series has less than 20 minutes available, LTHR is approximated from whatever minutes you have, with a warning flagging the limitation.

Formula

LTHR = average(HR of the last 20 min)

About the source

A field protocol popularized by Joe Friel in Total Heart Rate Training (2006) — a widely accepted sports authority, but a field protocol, not itself a derivation of a peer-reviewed study, hence classified as moderate strength.