A tendon is not a muscle. They are two entirely different types of tissue with different structures, different cells, and different jobs. A tendon is a tough cord of connective tissue that attaches a muscle to a bone, acting as a bridge that transmits the pulling force a muscle generates into actual movement at a joint.
What Makes Tendons and Muscles Different
Muscles are made of contractile cells that can shorten and lengthen on command from your nervous system. That ability to contract is what generates force. Tendons have no contractile ability at all. They’re built from densely packed collagen fibers arranged in parallel rows, designed to resist being pulled apart rather than to move on their own. About 65% to 80% of a tendon’s solid material is type I collagen, the strongest variety your body produces.
The cells inside each tissue are also completely different. Muscle contains muscle fibers (myocytes), which are long cells packed with proteins that slide past each other to create contraction. Tendons contain tenocytes, a type of fibroblast that makes up roughly 95% of tendon cells. Tenocytes produce and maintain the collagen matrix that gives tendons their remarkable toughness. A tendon is about 70% water by weight, with the remaining 30% almost entirely collagen and a small amount of elastin (about 2%).
How They Work Together
Muscles and tendons function as a single unit, even though they’re distinct tissues. When a muscle contracts, the force travels through a specialized connection point called the myotendinous junction, where muscle cells interlock with tendon fibers through a series of membrane folds. These folds increase the surface area of the connection, spreading the load so the junction doesn’t tear under stress. A chain of structural proteins at this junction passes the force from the contracting muscle into the collagen fibers of the tendon, which then pulls on the bone.
Tendons also let muscles operate efficiently by maintaining the right distance between a muscle and the joint it controls. Without tendons, muscles would need to be much longer to reach their attachment points on bone. Instead, a compact muscle in your forearm can control fingers through long, thin tendons that run all the way to your fingertips. The shape of a tendon even matches its muscle: flat muscles produce flat, sheet-like tendons, while round muscles produce cord-shaped ones.
Why Tendons Are So Strong but Heal Slowly
Tendons can withstand enormous pulling forces. Measured tensile strength in human tendons can exceed 2,000 megapascals, which is comparable to some industrial materials. That strength comes from the parallel arrangement of collagen fibers, which are optimized to resist force in one direction.
The tradeoff for that toughness is very poor blood supply. Early medical literature described tendons as “virtually dead during life” because they contain so few blood vessels and so few cells compared to other tissues. Muscles, skin, and bone are all richly supplied with blood, which is why they heal relatively quickly after injury. Tendons lack that advantage. When a tendon is damaged, the body’s pool of local stem cells often fails to fully restore normal tissue quality. New blood vessels do grow into an injured tendon, but unlike in skin or bone, that new vessel growth is actually associated with degeneration rather than healthy repair. This is why tendon injuries like Achilles tendonitis or rotator cuff tears can linger for months.
Tendons vs. Ligaments
Tendons are often confused with ligaments, which look and feel similar. The distinction is straightforward: tendons connect muscle to bone, while ligaments connect bone to bone. Ligaments hold joints together and keep them stable. Both are made primarily of collagen, but they serve completely different mechanical roles. A torn ligament (like an ACL tear in the knee) is a joint stability problem. A torn tendon is a movement problem, because the muscle can no longer transmit its force to the skeleton.
How to Tell a Muscle Injury From a Tendon Injury
Because muscles and tendons sit so close together and work as a unit, it’s common to wonder which one is actually hurt. The key difference is location. A muscle strain produces pain in the fleshy belly of the muscle itself, often in the middle of the limb. Tendon pain, by contrast, is felt near where the muscle attaches to the bone, typically close to a joint. Tendonitis pain tends to worsen with repetitive motion and may feel like a sharp or burning sensation right at the attachment point, while a muscle strain often produces a broader, deeper ache with sudden onset during exertion.
Recovery timelines also differ significantly. A mild muscle strain might heal in a few weeks thanks to muscle tissue’s rich blood supply. A tendon injury of similar severity can take two to three times as long, and chronic tendon problems can persist for months because of that limited blood flow and poor regenerative capacity.