Corpus: Tendon

Tendons are connective tissue fibers that attach muscles to bones or, in the case of an intermediate tendon, connect two muscle bellies.

Depending on their function, tendons are also referred to as tendons of origin or attachment tendons. Tendons are present in every muscle, even when it appears that the muscle attaches directly to the bone. Additionally, some muscles have tendons, known as intermediate tendons, that insert into the muscle fibers without directly contacting the bone.

Broad, flat tendon sheats, such as those found in the external oblique muscle of the abdomen, are referred to as aponeuroses.

Tendon fibers either extend into the periosteum (the membrane covering the bone) or anchor directly within the bone. Tendons that cross multiple joints are surrounded by tendon sheaths, which help minimize friction with the surrounding tissues.

2.1. Classification

Tendons are classified into traction tendons and gliding tendons. Traction tendons run in the same direction as the muscle’s action and are subjected primarily to tension. In contrast, gliding tendons have a different alignment from the muscle’s action, as they are routed around a bone (e.g., the tendon of the long head of the biceps brachii muscle). At the point where pressure is applied, these tendons are usually protected by a layer of fibrocartilaginous tissue.

The histological structure of tendons reveals a connective tissue that is rich in fibers but sparse in cells, with isolated fibrocytes, also known as wing cells due to their distinctive appearance. The fibers are primarily composed of collagen, with smaller amounts of elastin. The dominant fiber protein is type I collagen (about 98 %), while other collagens (types II, III, IV, V, IX, and X) constitute a minor portion. The extracellular matrix between the fibers is rich in proteoglycans, such as decorin and aggrecan.

Externally, tendons are surrounded by a thin sheath called the peritendineum, or 'tendon skin.'

Tendons are classified as bradytrophic tissue, meaning they have a low metabolic rate and contain few nerves and blood vessels. Consequently, tendons have a limited ability to regenerate.

Tendons transmit the force generated by muscle contraction to the bones, enabling movement. They are composed of strong collagen fibers, which provide excellent tensile strength.

In addition to force transmission, tendons also exhibit a spring-like effect due to their elasticity. This allows them to temporarily store kinetic energy and release it, making many movements more energy-efficient. For example, when the foot makes contact with the ground, the Achilles tendon is passively stretched by dorsiflexion. It then releases this stored energy during the subsequent plantar flexion, reducing the workload on the calf muscles.

• Achilles tendon

Due to the high mechanical demands placed on them, tendons are often prone to degenerative changes, collectively referred to as tendinopathy.

When a tendon anchored to bone tears, it often leads to an avulsion fracture, where a fragment of bone is pulled away with the tendon.