What is a key feature of immovable joints?

A key feature of immovable joints, also known as synarthroses, is that they are interlocked. This interlocking structure provides stability and strength, which is essential in areas where movement is not needed or could be detrimental. For instance, the sutures in the skull are immovable joints that hold the bony plates of the skull together, protecting the brain while providing a rigid structure.

In contrast, the other options describe characteristics that do not apply to immovable joints. For instance, allowing free movement is a feature of movable joints (like synovial joints) rather than immovable ones. Flexibility also pertains more to joints that allow movement, as immovable joints maintain a fixed position. Finally, the connection of muscles to bones describes tendons' role rather than the nature of joint mobility or stability. Thus, the interlocking nature of immovable joints is what defines their function and structure.