Have you ever looked at a skeleton and seen the amount of space that exists within the human body without the muscles, tendons, cartilage, etc.? The amount of space between the bones is beyond expansive than what I typically feel within the movement of my body.
Bones are denser than muscles, yet often times it feels as if the muscles are denser then bones internally. A tight back, an achy neck, a stiff shoulder, and on and on. Yet the muscles are actually malleable.