"CONSTANTLY DIVIDING BY 3.5"...
Constants in science are referred to as a sort of "gold" since they represent something that is always true and constant. For example, 1+1 = 2 is a constant that allows you to use the two mathematical phrases interchangeably. Anytime you see 2 you can write 1+1 instead, and vice versa.
Now imagine a physiological measurement that works the same way. Scientific gold I tell ya', gold. Though humans that are alive and at rest have an RVO2 of 3.3 - 3.7 mL/Kg/min. the vast majority of humans under these conditions have an RVO2 of 3.5 mL/Kg/min. Thus, an RVO2 of 3.5 mL/Kg/min is used as a constant to represent the oxygen consumption at rest, of all humans that are alive, regardless of age, sex, fitness, health, etc.
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Furthermore, an RVO2 of 3.5 mL/Kg/min thus is define as the same as one Metabolic Equivalent (1 MET). The MET as a unit allows us to enjoy several advantages to its use, such as being able to compare how much more is a person working, physiologically, compared to being in the state of physiological rest.
Allow me to remind you that physiological rest can be defined in several ways where the person is in the anatomical position though on their back, and doing the minimal physiologically and metabolically to be alive. This would correlate naturally and logically to the lowest relative oxygen consumption possible (RVO2 at rest; mL/Kg/min).
Accordingly, a person at 3METs is working, physiologically three times more than the same person's physiological work at rest. A person at 7METs is working, physiologically seven times more than the same person's physiological work at rest. And a person at 10METs is working, physiologically three times more than the same person's physiological work at rest. Naturally, the number of METs a person is working at, is tightly connected to their oxygen consumption.
While all people begin at 1MET at rest, regardless of age, sex, fitness, health, etc., their maximal work capacity in units of METs is not the same, and is determined by the maximal aerobic capacity (maximal RVO2). The average person, is considered untrained and unfit, and presents a maximal RVO2 of 45 mL/Kg/min, which is the same as a maximal physiological aerobic work capacity of 12.85METs.
A person of "low" aerobic fitness will less than an RVO2 of 45 mL/Kg/min (12.85METs), while a person of "poor" aerobic fitness will have an RVO2 of 35 mL/Kg/min (10METs) or less. On the other hand, the most aerobically trained people in the world (ultra aerobic athletes) present an maximal RVO2 of 85 mL/Kg/min (24.28METs), allowing them to be amongst the five top ultra aerobic athletes in the world.
In average, a person will require an increase of about 10 heart beats per minute, in order to accommodate an RVO2 increase in their 3.5 mL/Kg/min (1MET), while the ultra aerobically trained person can achieve the same via about 5-6 heart beats per minute. This means not only a greater maximal aerobic capacity, but greater efficiency as well in achieving a given relative oxygen consumption.
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The metabolic equivalent is not only used in exercise or aerobic fitness assessment, it is also used as a means to indicate the extent of physical challenge one is allowed to endure during rehabilitation or while returning to day to day function or work, after a long period of time without substantial physical challenge. In summary, a person's physiological work capacity ranges from 1MET at rest, to their METsmax during maximal aerobic physiological effort.
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