Tendons are robust, cord-like structures composed primarily of collagen, a strong protein found throughout the body. They function as a mechanical bridge, connecting muscles to bones and enabling movement across joints. This connection allows the force generated by muscle contraction to be transmitted to the skeletal system, facilitating a wide range of actions. Tendons possess high tensile strength, allowing them to withstand significant forces while being relatively inelastic, which minimizes energy loss during movement. These specialized tissues are crucial for the intricate movements of the fingers and hand, allowing for both powerful grips and delicate manipulations.
The Tendon System of the Fingers
The finger and hand rely on two primary groups of tendons to facilitate movement: flexor tendons and extensor tendons. These tendons originate from muscles located in the forearm and extend into the hand and fingers, connecting to specific bones to produce motion.
Flexor tendons are situated on the palm side of the hand and fingers, responsible for bending or curling them. The main flexor tendons are the flexor digitorum superficialis (FDS) and the flexor digitorum profundus (FDP), which originate from muscles in the forearm. The FDS tendons attach to the middle bone of each finger, enabling bending at the middle finger joint, while the FDP tendons run deeper, attaching to the very tip bones of the fingers, allowing for bending at the fingertip joint. For the thumb, the flexor pollicis longus (FPL) tendon connects to its end bone. These flexor tendons glide within protective sheaths, which keep them close to the bones and provide lubrication for smooth movement.
Extensor tendons are located on the back of the hand and fingers, enabling their straightening. The extensor digitorum communis (EDC) tendons are the primary extensors for the four fingers, originating from a common muscle in the forearm. They travel down the forearm and across the wrist, held in place by a tough band of tissue called the retinaculum, which allows them to slide. As they reach the fingers, these tendons fan out and form a complex structure over the knuckles and finger joints, known as the extensor hood or expansion, including central slips to the middle bone and lateral bands to the fingertip bone, coordinating the straightening of all finger joints. Specific extensor tendons, like the extensor indicis proprius (EIP) for the index finger and extensor digiti minimi (EDM) for the pinky finger, also contribute to these movements.
How Finger Tendons Enable Movement
Finger tendons function by transmitting the mechanical force generated by muscle contractions. When the muscles in the forearm contract, they pull on their respective tendons, which then act like ropes to move the bones of the fingers. This pulling action causes the joints to bend or straighten, depending on which set of tendons is activated.
The flexor tendons are responsible for the various bending movements of the fingers. When the flexor muscles in the forearm contract, they pull on the FDS and FDP tendons, causing the fingers to curl inward, as seen when making a fist or grasping an object. The FDS specifically bends the middle joint, while the FDP bends the joint closest to the fingertip. This allows for both gross grasping and fine motor control.
Conversely, the extensor tendons facilitate the straightening of the fingers. When the extensor muscles in the forearm contract, they pull on the EDC and other extensor tendons, lifting the fingers away from the palm. This action extends the fingers at the knuckle joints and the joints within the fingers themselves. The coordinated interplay between these opposing sets of flexor and extensor tendons allows for highly precise and powerful movements, from typing and playing musical instruments to gripping heavy objects.