Concept
Lactate threshold heart rate
A practical estimate of the heart rate near the highest steady metabolic effort a runner can sustain before fatigue begins to rise much faster. It is a useful training anchor, but it is not a direct measurement of blood lactate.
What the number represents
Lactate threshold heart rate, often shortened to LTHR, links an internal response to sustained hard aerobic work. It is usually more specific to threshold training than a percentage of maximum heart rate.
How it is estimated
A laboratory test can identify threshold responses directly. Field tests, recent races, and watch estimates can provide practical approximations, but protocol, pacing, fatigue, heat, and sensor quality all affect the result.
Why it is not permanent
Training status, recovery, environment, altitude, illness, and measurement method can move the observed value. A useful estimate should be reviewed as a range and retested under comparable conditions.
How it supports zone setting
Threshold-based zones express training intensity relative to LTHR. They are especially useful for controlled harder running, while easy-day decisions should still be checked with breathing, conversation, and perceived effort.
Use a stable average from a credible test or race, cross-check it with pace and effort, and retest under similar conditions before changing zones.
LTHR is not maximum heart rate, and a single watch estimate is not equivalent to a laboratory lactate measurement.