Rigid bodies are tensed, flexed and pumped muscles.
Rigid minds are strained, wired and stretched nerves.
Rigid muscles can be experienced whilst doing exercises, particularly strenuous ones, at the point when your mucles begin to shake from the stress.
Rigid nerves can be experienced when you feel 'wired' from too little sleep and too much coffee, you're extremely sensitive to all kinds of noises (your ear hairs wince at the sound of a cup being tapped with a fingernail, your peripheral, bodily, nervous system jumps when an enexpected noise is heard, or touch is felt.
Things that are rigid are less flexible than things that are not. Thus if you have a body and mind that is too tense, all the time, you are more likely to snap, because you are brittle, rigid. Flexibilty is required to prevent the body and mind from snapping. Too much flexibility leads to a body and mind with no form whatsoever, muscles like jelly that cannot hold any weight, and a mind like a rotting, disintegrating sandwich that can neither discern, nor discriminate nor decide.