Lactate threshold

The lactate threshold is the exercise intensity at which blood lactate begins to rise sharply, because production outpaces clearance. Above it, effort becomes unsustainable relatively quickly.

It matters because it is a strong predictor of endurance performance — often more so than VO₂max among trained athletes — and marks the boundary between comfortably-hard and unsustainable pacing.

In practice, threshold pace or heart rate anchors interval and tempo training, and improving it lets you hold a faster pace for longer.