The University of Western Australia explains that human bones are hollow because this structure makes them light enough to easily move but sturdy enough to avoid easy breakage or collapse. In addition, the hollow areas of bones store fat.
The university observes that if existing bones were solid, muscles would need to be much larger to move them, necessitating a different shape to the human body. If bones were simply thinner and solid, they would be less flexible and more breakable, the university notes.. According to the Baylor College of Medicine, the calcium and collagen in the bones play significant roles in the strength and sturdiness of bones.