In the untrained, skeletal muscle capacity can be limiting
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.