>The decline is not just about the heart. VO₂ max falls by approximately 46% between ages 20 and 70, while maximal cardiac output falls by only 31% over the same period. The gap between those two numbers is the first quantitative signal that something beyond cardiac output is driving the loss.

The difference in % is modest enough that it just seems to implie it's not directly 45 degree linear, but has a not that impressive multiplier. Why would vo2max and cardiac output be perfectly direct? After all vo2max also takes into account the lungs and muscles for example.