The number one purpose for studying martial arts is to not have a rigid fate. I wish more schools explained this up front. This idea is closely linked to the area known as the mingmen. When the lower back is stiff and deficient we literally have a rigid fate.
How is it possible that a person gets stuck on an idea in their twenties and despite heaps of evidence which accumulates during their lifetime which contradicts that idea, they still cling to it. Traditionally these rigid ideas or notions or ideologies have been conceived as hungry ghosts or wandering demons that are invisible to us but slowly eat away at our kidneys whenever we "check out." By "checking out" I mean staying up too late, forgetting to eat, taking drugs, or unleashing political rants. The day after we become food for little hungry demons, our lower back gets stiff and starts to hurt.
I don't think there is a perfect correlation between physical rigidity and a person's inability to freely make choices based entirely on what is real. There is some correlation, but I've met some amazing people with pretty screwed up bodies. Still, sit-ups are dumb. Six-pack abdominal muscles are O.K. against a boxer with gloves on and that's it. Like the "core-strength" fad, sit-ups break the unity of a person's body, they restrict the freedom of the torso and they tighten the breath. Why choose a rigid fate?