Structure and biochemistry only take you so far. Function is the part that determines whether you can carry your own suitcase at eighty, and it is measurable to a precision most people find surprising.
Why function predicts so well
Cardiorespiratory fitness and grip strength are among the strongest predictors of all-cause mortality in the literature — stronger than most blood markers, and both highly responsive to training at any age we typically see. They are also honest: you cannot supplement your way to a better VO2 max.
How we test
All functional testing runs on our own calibrated equipment, using the same protocol every time, so that your re-test is genuinely comparable to your baseline.