Flexibility is a topic that is interesting. I think it's important to understand different peoples perception of what they think 'flexibility' means.
First, lets address movement
The human body, it seems, paritially relies on movement to activate, and manually operate certain functions. Circulation in the extremeties, hand and feet which are far from the heart, are an example. A "pooling" effect of the blood occurs if movement doesn't occur, like in bed sores - typical for patients who are treated long term in hospitals. The blood can stagnate in different areas especially the pelvic region, causing all kinds of problems. Movement then becomes essential in helping out the cirulatory system and heart by easing their work. Another example is the Lymphatic system. The functions of this system include the absorbtion of excess fluid and its return to the blood stream, absorption of fat (in the villi of the small intestine) and the immune system function. The network of channels that return the excess fluid to the blood stream rely on body movement. The movement propels the fluid back to the blood stream, the lymphatic system, as far as i know, has no pump dedicated to it (like the heart is to the circulatory system). So it seems the body is engineered to utilize movement to function properly.
"You are as old as your spine is flexible"
Along with movement comes 'range of movement'. This is were people start to differ on opinions. They differ on how much range of movement one should have.Template:Underonstruction