As the workload is progressively increased during exercise, an equilibrium point is reached betw een the production and clearance of lactic acid. This is termed the lactate threshold (or anaerobic threshold). At higher workloads, lactic acid accumulates.
Practically speaking, this point can be measured noninvasively using gas exchange. As lactic acid is buffered. COn is released and ventilation must be increased. The lactate threshold is identified as the point where the ratio of minute ventilation to 02 consumption increases while the ratio of minute ventilation to CO, production is constant.
This transition, referred to as the ventilatory threshold, has also been labeled the anaerobic threshold. Yet the inference that the body suddently shifts to anaerobic metabolism as the workload increases or that there is inadequate oxygen delivery at the tissue level is hotly debated. In fact, radioisotope studies reveal that lactate is a by-product and a fuel at lower work rates, and can serve as a substrate shuttle between less active muscle groups and active groups, allowing for some degree of local glycogen sparing during prolonged exercise.
A better explanation for the progressive net appearance of lactate in the blood beyond some exercise threshold relates to increased reliance on muscle fibers with less oxidative capacity and decreased lactate clearance. As work rate increases, there is increased recruitment of motor units that have less oxidative capacity and greater glycolytic capacity (fast-twitch muscle fibers). In essence, these fibers are biased toward glycolysis, and thus their recruitment results in increased _etic acid release. Concurrently, as the work rate increases, there is proportionate decrease in splanchnic”namely, hepatic”blood flow and thus less lactate clearance. Other factors likely contribute but are beyond the scope of this discussion.
What is the lactate threshold? Photo Gallery
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