A tendon is the structure that attaches a muscle to a bone. Tendons are tough, flexible cords of connective tissue made primarily of collagen, and they exist at the ends of nearly every skeletal muscle in your body. When a muscle contracts, the tendon pulls on the bone it’s anchored to, creating movement at the joint.
How Tendons Are Built
A tendon’s internal structure resembles a rope or fiber-optic cable. Small collagen fibers are arranged into progressively larger bundles, and this layered bundling is what gives tendons their remarkable strength. The cells responsible for producing and maintaining that collagen are called fibroblasts, which continuously lay down new protein fibers to keep the tissue intact.
Where a tendon meets bone, specialized collagen strands called Sharpey fibers anchor the tendon directly into the bone’s surface. This junction is incredibly secure. The tensile strength of tendons ranges from 50 to 150 megapascals, and the Achilles tendon alone handles forces between 750 and 2,360 newtons during ordinary walking. That’s roughly equivalent to supporting 170 to 530 pounds of force with each step.
Not All Muscle-to-Bone Connections Look the Same
Most people picture a tendon as a thick, rope-like cord, and many tendons do look exactly like that. But some muscles attach to bone through a flat, sheet-like version called an aponeurosis. An aponeurosis is made of the same collagen-based connective tissue as a tendon, just spread out into a thin, broad layer instead of a rounded cord.
You have aponeuroses in several places:
- Palm of the hand: The palmar aponeurosis stretches from your wrist crease to the base of your fingers, helping stabilize your grip.
- Sole of the foot: The plantar aponeurosis (often called the plantar fascia) runs from your heel to the front of your foot and supports your arch.
- Top of the skull: The epicranial aponeurosis sits like a thin helmet beneath your scalp, connecting the muscles that move your forehead and the back of your head.
- Inner elbow: The bicipital aponeurosis is a wide sheet of tissue extending from the biceps muscle near the elbow joint.
Tendons vs. Ligaments
Tendons and ligaments are easy to confuse because they’re both made of fibrous connective tissue and they both exist around joints. The key difference is what they connect. Tendons attach muscle to bone, and their job is to transmit force so you can move. Ligaments attach bone to bone, and their job is to hold joints together and keep them stable. You need both working in tandem for any joint to function properly.
Why Tendons Heal Slowly
Tendons have a limited blood supply compared to muscle or skin. They receive blood from small vessels in the surrounding tissue, and in areas like the fingers, blood reaches the tendon through tiny folds of tissue called vincula. Some tendon nutrition doesn’t come from blood flow at all. Instead, tendons absorb nutrients through diffusion from the synovial fluid that surrounds them inside joint sheaths.
This low vascularity is the main reason tendon injuries take longer to heal than muscle strains or skin wounds. A mild tendon injury typically improves within two to three weeks, but more significant damage, especially chronic degeneration where the collagen structure breaks down over time, can take months to fully resolve. Repetitive stress injuries to tendons are common in athletes, manual laborers, and anyone who performs the same motion repeatedly.
Tendons as Sensory Organs
Tendons do more than just pull on bones. Embedded within them are tiny sensory structures called Golgi tendon organs that act as built-in tension monitors. Each one consists of a small capsule of connective tissue threaded with nerve fibers that weave between the collagen bundles inside. When the tendon is stretched or compressed during muscle contraction, these nerve endings fire signals to the spinal cord and brain.
This feedback loop is part of your proprioceptive system, which is how your body senses its own position and movement without you having to look. Active muscle contraction is more effective at triggering these sensors than passive stretching, which means your nervous system gets the strongest feedback during voluntary movement. The information travels to the cerebellum, the part of the brain that coordinates balance and smooth motion, and also loops back to the muscles themselves to fine-tune how hard they contract. This is one reason you can pick up a glass of water without crushing it or letting it slip: your tendons are constantly reporting back on exactly how much force your muscles are generating.