Tendons are living ropes that turn muscle pull into bone motion
A tendon is dense fibrous connective tissue linking muscle to bone, transmitting contraction while enduring tension. Ligaments join bone to bone; both rely on collagen, yet adults carry about four thousand tendons whose parallel fibres act as force cables—and sometimes as elastic springs.
Tenocytes build an extracellular matrix dominated by aligned type I collagen fascicles wrapped by endotenon, then epitenon, with paratenon fat outside and Sharpey's fibres anchoring into bone. Dry mass is thirty to forty-five percent of total mass. Fibrils fifty to five hundred nanometres wide assemble into fascicles millimetres long, then fibres hundreds of micrometres across, cross-linked by proteoglycans such as decorin.
Nerves are scarce inside the bulk but present in sheaths; Golgi tendon organs sit at the myotendinous junction. Blood vessels run with the endotenon. Genetic tendon length, not training, largely sets how long a muscle belly can grow—bodybuilders often have short tendons—while carpal-tunnel routes show how tendons park bulky motors away from crowded joints.
Crimped collagen and water-loving proteoglycans let some tendons store and return elastic energy during gait, passively stabilizing locomotion. They resist tension along fibres and compression where proteoglycans swell. Far from inert string, tendon tissue senses load through gap-junction networks among tenocytes and adapts its matrix to the forces it must carry.
Not every tendon does the same job. Positional tendons, like those steering the fingers during writing, mainly place a limb, while energy-storing ones such as the Achilles lengthen as the ankle flexes mid-stride and recoil efficiently afterward; runners and jumpers gain from a longer Achilles paired with a shorter calf. Under load the tissue behaves viscoelastically. Its stress-strain curve opens with a soft toe region while crimps straighten, then turns stiff and linear until failure. Imaging and cadaver tests suggest healthy tendons are strongly anisotropic and even auxetic, widening in some planes when stretched by up to 2 percent. Because tendinopathy raises glycosaminoglycan content, sodium MRI is being tested as an early warning of matrix change.
Source: Tendon
