Oxygen Plateau: Definition, Identification, and Its Role in VO2 Max

What is the meaning of oxygen plateau?

The oxygen plateau, often referred to as a "true" VO2 max, signifies the point during incremental exercise where oxygen consumption no longer increases despite an increase in exercise intensity, indicating that the individual's maximal aerobic capacity has been reached.

Defining the Oxygen Plateau

The concept of an oxygen plateau is central to understanding maximal aerobic capacity, commonly known as VO2 max (maximal oxygen uptake). During a progressive, incremental exercise test (e.g., on a treadmill or cycle ergometer), an individual's oxygen consumption (VO2) typically increases linearly with increases in workload. The oxygen plateau represents the point at which this linear relationship breaks down; despite further increases in exercise intensity, the rate of oxygen consumption plateaus or even slightly decreases. This indicates that the body's cardiorespiratory and muscular systems have reached their absolute limit in their ability to transport and utilize oxygen for energy production.

The Context: VO2 Max Testing

To understand the oxygen plateau, it's essential to grasp its context within VO2 max testing. VO2 max is widely recognized as the gold standard measure of cardiorespiratory fitness. It quantifies the maximum amount of oxygen an individual can utilize during intense, maximal exercise.

During a typical VO2 max test:

• The subject performs exercise at progressively increasing intensities.
• Oxygen consumption and carbon dioxide production are continuously measured using gas analysis equipment.
• Various physiological parameters, such as heart rate, blood pressure, and ratings of perceived exertion (RPE), are also monitored.

The primary goal is to determine the highest rate at which the body can consume oxygen. The identification of an oxygen plateau is considered the most definitive criterion for having achieved a "true" VO2 max.

Identifying a "True" Oxygen Plateau

While the concept is straightforward, observing a clear oxygen plateau can be challenging, and it doesn't occur in all individuals. For an oxygen plateau to be definitively identified, the following criterion is typically used:

• A lack of increase in VO2: Oxygen consumption increases by less than 2.1 mL/kg/min (or less than 150 mL/min for absolute VO2) despite an increase in workload during the final stages of the test.

Because a distinct plateau is not always evident, especially in untrained individuals or children, exercise physiologists often rely on secondary criteria to confirm that a maximal effort has been achieved and that the measured VO2 is indeed a maximal value (often termed "peak VO2" if a plateau isn't observed). These secondary criteria include:

• Respiratory Exchange Ratio (RER) greater than 1.10-1.15: Indicating high reliance on anaerobic metabolism.
• Heart rate within 10-12 beats per minute of age-predicted maximum heart rate.
• Blood lactate concentration exceeding 8-10 mmol/L.
• Rating of Perceived Exertion (RPE) of 17 or higher on the 6-20 Borg scale.

Physiological Mechanisms Underlying the Plateau

The oxygen plateau occurs because one or more components of the oxygen transport and utilization system become limiting. These physiological bottlenecks can occur at various stages:

• Oxygen Delivery Limitations:
  • Cardiac Output: The heart's maximal ability to pump blood (and thus oxygen) to the working muscles. This is determined by maximal heart rate and stroke volume.
  • Pulmonary Diffusion: The efficiency of oxygen transfer from the lungs into the bloodstream.
  • Blood Oxygen Carrying Capacity: The amount of hemoglobin available in the blood to bind with oxygen.
• Oxygen Utilization Limitations:
  • Muscle Capillarization: The density of capillaries within the muscles, which facilitates oxygen delivery to muscle cells.
  • Mitochondrial Density and Enzyme Activity: The quantity and activity of mitochondria (the "powerhouses" of the cell) and the enzymes involved in aerobic metabolism within the muscle cells.

When the most limiting of these factors is reached, the body can no longer increase its oxygen uptake, regardless of how much harder the individual tries or how much the external workload increases.

Significance in Exercise Science and Performance

The oxygen plateau holds significant importance in exercise science and sports performance:

• Gold Standard of Aerobic Fitness: When observed, it provides unequivocal evidence of an individual's maximal aerobic power, making VO2 max a highly reliable indicator of cardiorespiratory health and endurance performance potential.
• Understanding Physiological Limits: It helps researchers and practitioners understand the precise physiological limits of the human body during sustained, high-intensity exercise.
• Training Prescription and Monitoring: While VO2 max itself is genetically influenced and difficult to change dramatically, observing a plateau (or the lack thereof) and changes in peak VO2 can inform training strategies and help monitor the effectiveness of endurance training programs. For example, a higher VO2 max typically correlates with better endurance performance.
• Clinical Assessment: In clinical settings, VO2 max testing (and the presence of a plateau) can be used to assess functional capacity in individuals with cardiovascular or pulmonary diseases.

Challenges and Variability in Observation

Despite its theoretical importance, the clear observation of an oxygen plateau is not universal. Studies have shown that a distinct plateau may only be seen in a percentage of individuals tested, with figures varying between 50% and 80% depending on the population and testing protocol. This has led to ongoing debate in exercise physiology regarding the necessity of observing a plateau to define "true" VO2 max, with many researchers now accepting "peak VO2" (the highest oxygen uptake achieved during a maximal test, even without a plateau) as a valid and practical measure of aerobic capacity.

Factors that can influence the observation of a plateau include:

• Training Status: Highly trained endurance athletes are often more likely to exhibit a clear plateau.
• Test Protocol: The specific ramp rate and increment size of the exercise test can affect whether a plateau is observed.
• Individual Variability: Genetic factors and individual physiological responses play a role.

Conclusion

The oxygen plateau is a fascinating and critical concept in exercise physiology, representing the absolute ceiling of an individual's aerobic capacity. It signifies the point where the body can no longer increase its oxygen consumption, despite escalating exercise intensity, due to limitations in oxygen delivery or utilization. While not always definitively observed in every individual, its presence confirms the achievement of a "true" VO2 max, providing invaluable insight into an individual's cardiorespiratory fitness and physiological limits during maximal exertion.