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tendons are the tough extensions of muscles that attach them to bones, usually focusing the pull of the muscle on a relatively small area. They also allow muscles to act at a distance. For example, the fleshy bulk of the forearm muscles is well out of the way of the hand and finger movements that they control, whilst their tendons reach as far as the bone of the fingertip. Tendons come in many shapes and sizes. There are the long, narrow, flattened tendons (sinews) at the wrist and ankle, and those visible on the back of the hand. The broad and thick ‘Achilles’ tendon links the calf muscles to the foot. The patellar tendon links the quadriceps to the tibia via attachments to the knee cap. The rounded hamstring tendons are readily felt behind the knee. Whilst some tendons are attached to the end of a muscle, others are attached along the side or down the centre. Tendinous sheets or strips attach the edges of flatter trunk muscles to bony ridges, such as the ridge on the shoulder blade or the upper rim of the pelvic bone. Other sheets of tendon link muscles to each other or blend with the outer capsules of joints.

Tendons consist almost entirely of parallel collagen fibres, with elongated cells scattered among them. At the junction with muscle there are strengthened connections to infoldings of the muscle fibre membrane; at the junction with bone there are strong links to the fibrous covering (periosteum) adherent to it. To cause movement in exact proportion to the shortening of the muscle, a tendon would need to be inextensible; there is, however, a small degree of elasticity. Tearing of tendons is rare, because their breaking strength is high, but the Achilles tendon for example is sometimes the victim of sports injury. The solid structure, with few blood vessels, is then a disadvantage, making healing tediously slow.

At any site where a tendon lies in a tunnel or groove, it is surrounded by a smooth, double-layered, fluid-containing (synovial) sheath, facilitating mobility with minimal friction. Inflammation in such a sheath, for example at the wrist, is the painful condition of tenosynovitis.

Tendons are furnished with sensory receptors (Golgi tendon organs) that detect the tension within them and therefore the extent to which the attached muscle is exerting force upon them. Excessive nerve impulses generated by them cause reflex inhibition of the muscle contraction via the spinal cord. This provides defence against overstretching and potential tearing.

Sheila Jennett

See musculo-skeletal system.See also skeletal muscle.