Is the anaerobic threshold truly anaerobic?

Area of Science:

• Exercise Physiology
• Metabolic Regulation
• Cardiorespiratory Function

Background:

• Understanding the relationship between oxygen supply and muscle energy production during exercise is crucial.
• The existence and characteristics of an anaerobic threshold (AT) have been debated.
• Cellular redox state and lactate accumulation are key indicators of metabolic stress during physical activity.

Purpose of the Study:

• To determine if an exercise metabolic rate exists below which oxygen supply is adequate and above which it becomes inadequate (anaerobic threshold).
• To investigate the behavior of the arterial lactate/pyruvate ratio in relation to oxygen uptake (VO2) during exercise.
• To assess the impact of reduced oxygen delivery on VO2 and cardiorespiratory responses at different exercise intensities.

Main Methods:

• Measured arterial lactate and pyruvate ratios in subjects during graded exercise.
• Analyzed VO2 kinetics and lactate accumulation before and after inducing varying levels of carboxyhemoglobin (COHb).
• Compared threshold and continuous models to explain lactate/pyruvate ratio changes.

Main Results:

• The arterial lactate/pyruvate ratio remained stable at lower work rates but increased sharply above a specific VO2 threshold (AT).
• Elevated COHb levels (reducing oxygen delivery) decreased the AT and slowed VO2 kinetics.
• Exercise-induced lactate increases were sensitive to oxygen flow, particularly above the AT.

Conclusions:

• Exercise at moderate intensities can be sustained without altering cellular redox state, but this changes rapidly above the AT.
• The anaerobic threshold represents a critical point where muscle oxygen supply becomes insufficient.
• Lactate accumulation during exercise is directly influenced by oxygen availability, becoming significant only above the identified AT.