The term “sinew” is considered a synonym for a tendon in modern anatomy. A sinew is a cord of strong, flexible connective tissue, similar in appearance to a rope. These structures are a fundamental part of the musculoskeletal system. They provide the necessary link to translate muscle effort into body motion.
Physical Structure and Composition
Sinews are classified as dense regular connective tissue. This tissue is defined by its densely packed fibers arranged in a highly organized, parallel fashion. This organization is necessary for withstanding the immense, unidirectional tension they routinely experience. The primary component of this tissue is collagen, a resilient protein that makes up roughly 60% to 85% of the sinew’s dry mass.
The collagen fibers are bundled into a hierarchical structure. The smallest units of collagen fibrils combine to form primary fiber bundles, which group together into secondary and tertiary bundles, known as fascicles. Each fascicle is encased in a thin layer of connective tissue called the endotenon, which allows the bundles to glide against one another during movement. The entire sinew is then covered by the epitenon, a fine sheath that is sometimes surrounded by a looser layer called the paratenon.
How Sinews Enable Movement
The main function of a sinew is to transmit the mechanical force generated by a muscle’s contraction to the bone. When a muscle shortens, it pulls on the attached sinew, which subsequently pulls on the skeletal structure. This pulling action causes movement to occur at a joint.
Sinews act as biological levers, converting the force of muscle contraction into motion. They are built to withstand large amounts of tension without tearing. In some areas, particularly the lower limbs, sinews also function as elastic structures that store and release energy during movement. For example, the Achilles sinew stretches when the foot lands and recoils to provide a spring-like push-off, which improves movement efficiency.
Distinguishing Sinews, Tendons, and Ligaments
Sinews are interchangeable with tendons, referring to the strong, fibrous cords that connect muscle to bone. They translate muscle effort into skeletal motion. For example, the biceps sinew connects the biceps muscle to the forearm bone to enable bending the elbow.
A ligament is a separate type of connective tissue with a different primary role. Ligaments connect one bone to another bone, providing stability to joints and limiting excessive movement. While both sinews and ligaments are composed mainly of collagen, ligaments contain more elastic fibers, giving them a greater ability to stretch and maintain joint alignment. Injuries to sinews are called strains, resulting from overstretching or tearing the muscle-to-bone connection, whereas injuries to ligaments are called sprains.