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Limit of VO2 max and muscle mass

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What would be a physiologic rationale to support or refute the contention that either (1) central circulatory factors or (2) peripheral factors residing within the active muscle mass limit VO2 max?

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The Limitation to Vo2 Max Is Central!

Introduction
Maximal oxygen uptake (VO2max) is defined as the highest rate of oxygen, which can be taken up and utilized by the body during severe exercise (Basset and Howley 2000).
It is frequently used as an indicator of an individual's cardiorespiratory fitness as oxygen consumption is linearly related to energy expenditure (Basset and Howley 2000). VO2max is commonly used in scientific literature to evaluate changes in maximal ability to work aerobically and in exercise prescription (Basset and Howley 2000).
To determine VO2max, incremental exercise testing is used. It is taken as the point at which oxygen uptake peaks with additional power failing to produce VO2 gains (Lindstedt et al 1988). Secondary criteria to verify VO2 max include:
1. expiratory exchange ratio > 1.15
2. blood lactate level > 8-9 mM (Duncan 1997)
It is generally expressed in ml.kg —1.min —1
There has been longstanding interest in identifying physiological factors that control VO2 max and especially debate over whether the limitation is centrally or peripherally regulated.

Background Knowledge
In order to examine VO2max limitation an understanding of those factors affecting O2 transport from the atmosphere to mitochondria of the muscle must be examined.
The O2 delivery system can be viewed as including the pulmonary and cardiovascular systems. Central physiological factors, which may affect VO2 max limitation include:
1. pulmonary diffusing capacity,
2. maximal cardiac output,
3. O2 carrying capacity of the blood.
Peripheral skeletal muscle limitations include:
1. peripheral diffusion gradients,
2. mitochondrial enzyme levels,
3. capillary density.
Centrally Limited VO2max
Traditional View:
Hill who postulated that developed the concept of maximal oxygen uptake:
1. Oxygen uptake had an upper limit;
2. VO2 max was limited by the rate at which the cardiorespiratory system could transport O2 to the muscles
(Bassett and Howley 2000)
The majority of the following research supports the concept that VO2max is limited by the rate of oxygen delivery (central factors) not the ability of the muscles to take up O2 (peripheral factors).
Pulmonary System
The ability of the pulmonary system to maintain arterial oxygen level (% SaO2) in healthy subjects remains high even during maximal work. Richardson et al (2000) examined whether the quadriceps are O2 supply dependant at maximal exercise. The VO2max of the muscle group was measured during altered inspired O2 levels (hypoxia 12%; normoxia 21% and hyperoxia 100%). It was found VO2 max increased with increased O2 delivery. It was concluded therefore that in normal conditions of isolated knee extension, VO2 max of trained subjects was limited not by peripheral factors but centrally by O2 supply.
A similar study examining the effects of breathing hyperoxic gas on the VO2max of trained athletes again demonstrated an increase in VO2max (Powers 1989).
The rationale for the increase in VO2 max while breathing hypoxic gas is thought to be due to an expended arterial-venous O2 difference (a-v O2 difference). Small increases in hemoglobin (Hb) saturation and in O2 dissolved in plasma produce an increased O2 supply. Such an increase in arterial PO2 can produce a 10% improvement in VO2max (McArdle1986).
Daniels and Oldridge ...

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