Several experiments of different types support the concept that, in trained individuals, it is oxygen delivery, not oxygen utilization that limits VO2 max. By performing 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 capacity of skeletal muscle to use oxygen exceeds the heart's capacity for delivery. Thus although the average male has about 30 to 35 kg of muscle, only a portion of this muscle can be well perfused with blood at any one time. The heart can't deliver a high blood flow to all skeletal muscle, and still maintain adequate blood pressure. This limitation is analogous to the water pressure in your house. If you turn all the faucets on while trying to take a shower, the shower pressure will be inadequate because there is not enough driving pressure. Without getting in to deep on the hemodynamics, it seems that blood pressure is a centrally controlled variable; the body will not "open the valves" to more muscle than can be perfused without compromising central pressure, and blood flow to the brain. The bigger the pumping capacity of the heart, the more muscle can be perfused while maintaining all-important 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.
In the untrained, skeletal muscle capacity can be limiting
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-v O2 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-v O2 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. When we perform VO2 max tests on untrained persons, we often see that they stop at a at a time point in the test when their VO2 max seems to still be on the way up. The problem is that they just do not have the aerobic capacity in their working muscles and become fatigued locally prior to fully exploiting their cardiovascular capacity. In contrast, when we test athletes, they will usually show a nice flattening out of VO2 despite increasing intensity towards the end of the test. Heart rate peaks out, VO2 maxes out, and even though some of the best trained can hold out at VO2 max for several minutes, max is max and they eventually hit a wall due to the accumulation of protons and other changes at the muscular level that inhibit muscular force production and bring on exhaustion.
