.......losses in appendicular muscle mass in these subjects (∼2.7 kg) do appear to contribute importantly to the reduced whole bodyV˙o2 max values of older endurance-trained men and women
WRT to the question I raised whether loss of VO2 max or FTP could be related to the loss of muscle mass, I could only find one study that looked at older trained athletes. It looks like half of the aerobic power loss might be attributed to loss of muscle.
https://journals.physiology.org/doi/...1997.82.5.1411