Why are tendons hard?

Answer 1

Yes, hard but supple. There is a fine line there.

When you say hard, I assume you mean tougher than muscle and skin. All tissues are held together primarily by proteins secreted by cells that surround the cells called extracellular matrix. Cells alone are generally not very sturdy. The extracellular matrix is a combination of proteins that vary depending on what part of the body and what cells are secreting the proteins. The main structural proteins are collagens and elastin which are long helical (corkscrew shaped) proteins. Tendons and ligaments have more collagen and elastin than most tissues and this is what accounts for most of their durability. The types of collagen (and other proteins) and how they are hooked together (crosslinked) also has a large effect on the properties of the tissue. For example, the durability of cartilage is also due to extracellular matrix proteins but its character is very different from tendons and ligaments because the types and associations of the proteins is different.