The ventilatory thresholds are physiological markers used in exercise science to define shifts in metabolism and exercise intensity. They are identified during a ramp or graded exercise test, where workload is progressively increased until exhaustion. The Second Ventilatory Threshold (VT2) represents a turning point in the body’s ability to maintain a steady metabolic state during high-intensity exercise. Understanding VT2 is important for assessing performance capacity and designing effective training programs.
Defining the Ventilatory Threshold 2 (VT2)
VT2 is defined by changes in respiratory mechanics and gas exchange, marking the point where breathing accelerates disproportionately to the body’s oxygen consumption (\(\text{V}_{\text{O}_2}\)). As exercise intensity increases, the volume of air breathed per minute (\(\text{V}_{\text{E}}\)) normally increases linearly with oxygen consumed and carbon dioxide produced (\(\text{V}_{\text{CO}_2}\)). At VT2, minute ventilation begins to increase exponentially, while \(\text{V}_{\text{O}_2}\) continues to rise linearly.
This increase in breathing rate and depth is the body’s attempt to blow off excess \(\text{CO}_2\). This point is often referred to as the Respiratory Compensation Point (RCP) because the respiratory system compensates for an accumulating metabolic disturbance. Beyond this threshold, the exercise intensity becomes unsustainable, and the duration of effort is severely limited by fatigue.
The Connection to Lactate and Metabolism
The underlying cause of the hyperventilation observed at VT2 is the body’s struggle to manage the rapid accumulation of hydrogen ions, which leads to metabolic acidosis. As muscle activity increases, energy production through anaerobic pathways accelerates, leading to a surge in lactate production. The hydrogen ions, which cause the burning sensation and fatigue, accumulate alongside the lactate.
To prevent a drop in blood pH, the body activates the bicarbonate buffering system. This system uses bicarbonate (\(\text{HCO}_3^-\)) to neutralize the accumulating hydrogen ions. This neutralization produces water and, significantly, carbon dioxide (\(\text{CO}_2\)).
At the intensity corresponding to VT2, the production of these metabolic acids exceeds the body’s capacity to clear them efficiently. The extra \(\text{CO}_2\) produced by the buffering reaction triggers a strong signal to the respiratory center, forcing the disproportionate increase in ventilation. This event is closely associated with the Onset of Blood Lactate Accumulation (OBLA), where blood lactate levels rise sharply and progressively, typically around \(4.0 \text{ mmol/L}\).
Identifying VT2 Through Testing
Exercise physiologists identify VT2 in a laboratory setting using a metabolic cart to analyze the gases exchanged during a graded exercise test. The V-slope method is a common technique that visually analyzes the relationship between \(\text{V}_{\text{CO}_2}\) and \(\text{V}_{\text{O}_2}\) data. VT2 is the point where the ratio of \(\text{V}_{\text{E}}\) to \(\text{V}_{\text{O}_2}\) rises, or where the slope of the \(\text{V}_{\text{E}}/\text{V}_{\text{CO}_2}\) curve begins to increase.
Another indicator is the Respiratory Exchange Ratio (RER), which is the ratio of \(\text{V}_{\text{CO}_2}\) produced to \(\text{V}_{\text{O}_2}\) consumed. At rest, RER is typically around \(0.80\), but as intensity rises, VT2 is often marked by the RER crossing the \(1.0\) threshold. This value indicates that the volume of \(\text{CO}_2\) being exhaled is greater than the oxygen being consumed.
Utilizing VT2 for Training Intensity
For endurance athletes and fitness enthusiasts, VT2 is an important marker because it defines the maximum intensity that can be sustained for a limited period. It represents the upper boundary of the heavy exercise domain and is a strong predictor of performance in events lasting between \(30\) and \(60\) minutes. The intensity at VT2 is often linked to concepts like Functional Threshold Power (FTP) in cycling or Critical Speed in running.
Knowing the heart rate or power output at which VT2 occurs is necessary for prescribing high-intensity training zones, typically Zone \(4\) or \(5\). Training just below this threshold helps improve the body’s capacity to buffer metabolic acids and clear lactate, raising the intensity at which VT2 is reached. High-intensity interval training (HIIT) sessions are frequently structured around efforts at or slightly above VT2 to maximize these physiological adaptations.