Lactate threshold is the exercise intensity at which lactate begins to accumulate in the blood faster than your body can clear it. Below it, you can sustain effort for hours. Above it, fatigue is a countdown.
The physiology in one breath
During hard exercise, muscle cells produce lactate through glycolysis. At low intensities your body recycles it efficiently — as fuel for the heart, brain, and slow-twitch fibers. Push past threshold and production outruns clearance. Blood lactate climbs, pH drops, and the burning sensation follows.
The lactate curve
During a graded test, blood lactate stays flat or rises slowly, then bends sharply upward. That inflection is your threshold. Drag the slider to see how training shifts the whole curve rightward — same lactate at a higher intensity.
Why it predicts performance better than VO2max
Two athletes can have identical VO2max values and wildly different race results. The one who can hold a higher percentage of that max without lactate stacking wins. Threshold is the better proxy for what you can actually sustain in a race.
How it's measured
The standard method: graded exercise test with blood draws at each stage, plotted as a lactate curve. The inflection point — where the curve bends upward — is your threshold. Definitions vary (first rise, fixed 2 or 4 mmol, D-max, individual anaerobic threshold), which is why comparing studies requires checking which criterion was used.
Coming next
How threshold shifts with age in trained masters athletes — and why it rises as a percentage of VO2max even as absolute capacity falls. Field protocols are up for running and cycling. Device reviews are on the reviews page.