Why track function rather than appearance
Functional measurements answer the question people actually care about: can I still do the things I want to do? They are also harder to fool. A timed walk or a set of sit-to-stands is indifferent to how the week felt.
They cost nothing to repeat, which matters, because a measurement you take once tells you almost nothing.
Testing so the numbers mean something
Repeatability is the whole game. Use the same shoes, the same surface, the same approximate time of day, and the same instructions each time. Note anything unusual — poor sleep, a heavy session the day before, heat.
Retest every six to eight weeks rather than every week. Adaptation is slower than curiosity.
Reading your own trend
Look at direction across at least three entries before concluding anything. A single low result is usually the day, not the trend.
Where a number has been flat for several months, that is useful information too: it tells you which system is not currently being trained, which is often exactly the one to prioritise next.