Collagen fibers, while flexible, have high tensile strength, resist stretching, and provide the distinctive resilience and strength of ligaments and tendons. These fibers keep connective tissues together even while the body is moving.

Collagen fibers are structural elements that transfer stresses, store energy, and dissipate energy in vertebrate tissues. Collagen fibers have a hierarchical structure that includes collagen molecules, microfibrils, fibrils, fibers, and fascicles, and they limit the deformation of tendon and other load-bearing tissues.

Collagenous fibers are found in all types of connective tissue and are made up of type I, II, or III collagen. The ratio of collagen fibers to ground substance divides collagenous connective tissue into two types: The most common type of collagenous connective tissue is loose (areolar connective tissue). It appears in small, elongated bundles separated by regions that contain ground substance.
Collagen fibers are abundant in dense connective tissue, which has little ground substance. It is called regular if the tightly packed bundles of fibers are aligned in one direction; it is called irregular if they are orientated in many directions.

Tendons are an example of regular dense connective tissue, while the dermis is an example of irregular dense connective tissue.