Labored breathing is very familiar to runners.
They call it the ventilatory threshold. It’s that intensity of exercise above which your breathing becomes labored and you feel you just can’t draw in as much air as your body wants. Every runner is experientially familiar with the ventilatory threshold. When you run easy you breathe easy. As your speed increases, your breathing deepens, but gradually. However, as you continue to increase your speed, suddenly it seems as if a switch is flipped and your breathing races off ahead of your legs.
While the experience of the ventilatory threshold is familiar to every runner, the concept of the ventilatory threshold is less familiar to the average runner than is that of the lactate threshold. During exercise of gradually increasing intensity, the amount of lactate, an intermediate product of glycogen (carbohydrate) metabolism, increases in the blood as the muscles burn glycogen faster and faster. Just like the breathing rate, the blood lactate concentration increases gradually for a while and then, at a certain intensity, suddenly increases much more rapidly.
In fact, in most laboratory exercise tests, the ventilatory and lactate thresholds fall close to the same exercise intensity. Observing this coincidence led exercise physiologists to speculate that increasing blood lactate concentrations somehow trigger increased ventilation. But a study by Robert McMurray at the University of North Carolina proves it does not.
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The design of the study was very clever. Knowing that the muscles’ ability to produce lactate is limited by the amount of glycogen they store, McMurray had a group of eight experienced triathletes perform incremental exercise tests in two conditions: once with normal muscle glycogen stores and again with glycogen stores depleted by low carbohydrate intake before the test. McMurray found that the relationship between blood lactate concentration and ventilation differed between the two trials, a clear indication that breathing rate and depth are not directly controlled by blood lactate.
So what does cause the ventilatory threshold? According to McMurray, the evidence suggests that it is the activation of fast-twitch muscle fibers. As you may know, there are three basic types of fibers in muscles: slow-twitch fibers with poor speed but excellent endurance that are used during low-intensity exercise (actually, they are used at all intensities, but they are used to the exclusion of the other two types at low intensities); fast-twitch fibers with excellent speed but poor endurance that are used only during high-intensity exercise; and hybrid fibers with a mixture and slow and fast characteristics that kick in at moderate intensities. Different brain cells are used to activate each fiber type. When the exercise intensity increases to the point where brain cells connected to fast-twitch muscle fibers must become active, that’s when breathing rate and depth increase geometrically (as opposed to linearly).
What is the practical upshot of this finding for you? It means that you shouldn’t bother to submit to blood lactate testing to determine your lactate threshold. Your blood lactate levels during exercise are essentially meaningless. Instead, have your ventilatory threshold determined through a VO2 exercise test. Or just pay closer attention to your pace and/or heart rate the next time you experience that loss-of-breath-control feeling. That’s your ventilatory threshold right there. Most of your training should be done below it; a modest amount right at it; and a small but consistent amount above it.
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