Measurable

Stretches can be divided into two types. Those where performance is measurable. Those where it is not.

When performance isn't measurable, I simply push the position until I feel my muscles become taught, and then hold for half a minute. Over time, the muscles need to be pushed further in order to become taught and I become more flexible.

When performance is measurable, I push the position until I am at least at the same level as I was last time, and then hold for half a minute. Frequently this is painful. It doesn't take many stretching sessions before my muscles hurt; twinges of pain as I push the stretches. This is followed by periods of recovery where I can't do any stretching whatsoever. Periods long enough that many of the gains I made have been lost.