Ventilation and Anaerobic Threshold

The anaerobic threshold is defined as that intensity of work load or oxygen consumption in which anaerobic metabolism is accelerated, (i.e., where changes in both volume and lactic acid accumulation in the blood begin to increase dramatically. Over the years, the concept has been subject to critical review and refinement. The issues center on whether the change in the rate of ventilation is reflective of the rapid increase in blood lactic acid accumulation or represent independent events. 

The concept has been defined, as indicated, by both lactate and ventilation changes during progressively increasing exercise intensity. The point at which ventilation departs from a linear increase with work load and CO2 production has been identified as the ventilatory threshold (Tvent) and the point where lactate accumulation accelerates in the blood has been called the lactate threshold (Tlact). 

An increase in anaerobic metabolism (glycolysis) results in accumulation of lactic acid in the blood and muscles. Lactic acid is the prime suspect a cause of muscular fatigue. This, plus the fact that the anaerobic threshold is different for trained and untrained individuals, has generated interest in its applications to athlete conditions. 

One way to determine the anaerobic threshold is to measure blood lactic acid periodically during continuous and progressively increasing work loads, such as those performed on a stationary cycle or a treadmill. However, this technique requires drawing multiple blood samples, which may be uncomfortable  for the subject, and which requires a certain amount of time for the chemical analysis of the blood samples. ]

If the ventilation threshold and the lactate threshold are coincidental , and related, then it would be attractive to have a relatively simple, noninvasive technique for determining the anaerobic threshold, such as being able to observe changes in minute ventilation and gas exchange variables, such as carbon dioxide production and the ventilatory equivalent for oxygen (Vebpts/VO2). These variables increase linearly (i.e., in a straight line fashion) with increasing work loads until the anaerobic threshold is reached. At this point, their rate of increase is greatly accelerated. The acceleration in ventilation is probably due to the additional increased production of CO2 through the buffering of lactic acid through the bicarbonate system.  

The hydrogen ion (H+) derived from lactic acid is responsible for the production of both carbonic acid and CO2. Thus, for exercise exceeding the anaerobic threshold ventilation appears to be driven by two sources of CO2. The first is metabolic CO2 from aerobic metabolism, and the second is the “excess” CO2 resulting from buffering lactic acid. Thus we see that minute ventilation appears to be a reliable and relatively easily obtained indicator of the onset of increased anaerobic metabolism during exercise.

Other evidence suggest that the anaerobic threshold is really a ventilatory threshold (Tvent) and that there is a separate lactate threshold, Tlact). Although there is evidence for distinguishing Tvent and Tlact, from the practical point of view for the athlete, the following observations are pertinent. First, a good predictor of performance in long distance running is the fractional utilization of VO2 max at the ventilatory theshold. Second, the ventilatory threshold can be increased with conditioning. Third, adaptationin the ventilatory threshold may be specific to the muscle group being fatigued, as in triathlon training.

In summary, the anaerobic threshold may or may not consist of a related change in ventilation and lactic acid accumulation. If the events are not related, they do occur very closely in time, from simultaneous occurrence to 1 or 2 minutes apart. While scientists wrestle with the associations and concepts, experienced athletes can feel when they are running at a pace, in a particular event, that will allow them to reach the finish line in the shortest possible time or “crash” before finishing the race. 

Daryl Conant, M.Ed