Physical Education - The muscles say, 'If you deliver it, we will use it'
The muscles say, 'If you deliver it, we will use it'
Several experiments of different types support the concept that it is
oxygen delivery, not oxygen utilisation, that limits VO2 max. By performing
an exercise with one leg and directly measuring muscle oxygen consumption
of a small mass of muscle (using arterial catheterisation), it has been
shown that the muscle's capacity to use oxygen exceeds the heart's capacity
for delivery. Thus, although the average male has about 30 to 35kg of
muscle, only a portion of this muscle can be well perfused with blood at
any one time. The heart cannot deliver a high volume to all skeletal muscle
and still maintain adequate blood pressure . As further evidence for a
delivery limitation, long-term endurance training can result in a 300%
increase in muscle oxidative capacity, but only about a 15 to 25% increase
in VO2 max. VO2 max. can be altered artificially by changing the oxygen
concentration in the air. VO2 max. also increases in previously untrained
subjects before a change in skeletal muscle aerobic capacity occurs. All of
these observations demonstrate that VO2 max. can be dissociated from
skeletal muscle characteristics.
Stroke volume , in contrast, is linearly related to VO2 max. Training
results in an increase in stroke volume and therefore an increase in
maximal cardiac output. Greater capacity for oxygen delivery is the result.
More muscle can be supplied with oxygen simultaneously while still
maintaining necessary blood pressure  levels.
Now, having convinced you that heart performance dictates VO2 max., it is
important to also explain the contributing, or accepting, role of muscle
oxidative capacity. Measured directly, oxygen consumption = cardiac output
x arterial-venous oxygen difference (a-VO2 diff). As the oxygen-rich blood
passes through the capillary  network of a working skeletal muscle,
oxygen diffuses out of the capillaries and to the mitochondria (following
the concentration gradient). The higher the oxygen consumption rate by the
mitochondria, the greater the oxygen extraction, and the higher the a-VO2
difference at any given blood flow rate. Delivery is the limiting factor,
because even the best trained muscle cannot use oxygen that isn't
delivered. But if the blood is delivered to muscles that are poorly trained
for endurance; VO2 max. will be lower, despite a high delivery capacity.
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