What Is the Anaerobic Threshold?
The anaerobic threshold, often called the lactate threshold or first ventilatory threshold, is the exercise intensity at which a gradual, non‑linear increase in ventilation and carbon‑dioxide production begins relative to oxygen uptake. It reflects a shift toward greater reliance on glycolysis with increased lactate and hydrogen ion production, which are buffered by bicarbonate to generate additional carbon dioxide, prompting a disproportionate rise in breathing. This is not an on/off switch between aerobic and anaerobic metabolism—the body uses all energy systems all the time—but rather a continuum where the balance tilts as intensity rises.
In laboratory settings, cardiopulmonary exercise testing identifies this point from breath‑by‑breath gas exchange. Methods include the V‑slope technique, where the slope of carbon‑dioxide output versus oxygen uptake shows a breakpoint, and analysis of ventilatory equivalents and end‑tidal gases: at the first ventilatory threshold, the ventilatory equivalent for oxygen increases while the ventilatory equivalent for carbon dioxide remains stable, end‑tidal oxygen rises, and end‑tidal carbon dioxide is maintained. A second threshold, the respiratory compensation point, occurs at higher intensities when the ventilatory equivalent for carbon dioxide also rises and end‑tidal carbon dioxide falls as hyperventilation compensates for accumulating acid. In the field, simple cues like the talk test approximate these thresholds: speaking in full sentences is possible below the first threshold and becomes progressively harder as intensity exceeds it.
Exercise performed above the anaerobic threshold is characterized by faster lactate and proton accumulation, higher sympathetic activation, rising heart rate and perceived exertion, and a shorter sustainable duration. For healthy, trained individuals, bouts above this level are a normal part of structured training when progressed sensibly and balanced with recovery. For people with cardiovascular, pulmonary, or metabolic disease—or those in early rehabilitation—prescribing intensity relative to ventilatory thresholds helps keep work sustainable and symptoms controlled while still driving adaptation.
Practically, thresholds are used to individualize training zones. Work near and just below the first threshold improves aerobic capacity and endurance economy, while targeted intervals and tempo efforts can raise the threshold over time. In clinical and rehabilitation contexts, setting workloads below or between thresholds can reduce ischemia, excessive dyspnea, and premature fatigue, and repeat testing can track improvement objectively. Because thresholds reflect integrated cardiovascular, pulmonary, and muscular responses, they offer insights that resting measures cannot provide.
Safe application relies on gradual progression, adequate warm‑up and cool‑down, and attention to red‑flag symptoms such as chest pain, marked breathlessness out of proportion to effort, dizziness, or palpitations. Individuals with chronic conditions, recent illness, or new medications should seek guidance from a clinician or qualified exercise professional to translate threshold findings into day‑to‑day training targets.
