While physically connected and working together to create movement, muscles and tendons are distinct biological tissues with fundamentally different structures and functions. They form a single functional partnership responsible for moving the body, but their cellular makeup and primary roles are entirely separate. Understanding this difference is necessary for understanding how the body generates force and executes movement.
Muscle Structure and Primary Role
Muscle tissue is composed of specialized cells known as muscle fibers, which are the only cells capable of shortening. These fibers contain thousands of tiny contractile units called sarcomeres. The primary function of muscle is to generate force and produce movement through active contraction. This occurs when the protein filaments, myosin and actin, within the sarcomeres slide past each other, causing the fiber to shorten.
Skeletal muscle, the type associated with tendons, is under voluntary control and attached to the bones of the skeleton. The arrangement of these filaments gives skeletal muscle its characteristic striated appearance. Muscle also plays a role in maintaining posture, stabilizing joints, and generating body heat.
Tendon Structure and Primary Role
In contrast to muscle, a tendon is a dense, fibrous connective tissue that acts as a physical bridge between muscle and bone. Its structure is composed of tightly packed, parallel bundles of collagen fibers, which provides the tissue with incredible tensile strength. This strength allows the tendon to resist large pulling forces without tearing.
The specialized cells within the tendon, called tenocytes, are responsible for maintaining this collagen-rich matrix. The primary role of the tendon is to transmit the force generated by the muscle’s contraction to the skeletal system. Unlike muscle, tendons are passive and cannot contract or actively shorten.
The Musculotendinous Unit
The muscle and tendon are functionally inseparable, forming the musculotendinous unit (MTU). The junction where the muscle fibers meet the tendon is known as the myotendinous junction, which is specialized for the efficient transmission of force. This unit works in a coordinated sequence to produce skeletal movement.
When a muscle fiber contracts, it shortens and generates tension. This tension is immediately transferred through the myotendinous junction to the attached tendon. The tendon then pulls on the anchored bone, causing the limb to move across a joint axis.
Distinguishing Common Injuries
The distinct composition of these two tissues means they are susceptible to different types of injury. A muscle injury is commonly referred to as a strain or a “pull,” involving the tearing of the contractile muscle fibers themselves. These injuries typically result from a sudden, excessive force or overstretching, causing immediate, sharp pain within the belly of the muscle.
Tendon injuries, often grouped under the term tendinopathy, reflect damage to the dense connective tissue. These conditions, such as tendinitis or tendinosis, are typically degenerative and result from repetitive, overuse-related stress. Pain from a tendinopathy is usually felt near the joint, precisely where the tendon connects the muscle to the bone.