The two structures that make up the muscular system are muscles and tendons. Muscles generate force by contracting, and tendons connect those muscles to bones so the force actually produces movement. Together, they form a functional unit: muscles do the pulling, and tendons deliver that pull to the skeleton.
How Muscles and Tendons Work Together
A skeletal muscle is the engine of movement. It contracts by shortening its fibers, but it can’t move a bone on its own because it isn’t directly fused to the skeleton along its full length. Instead, at each end of the muscle, the tissue transitions into a tendon, a tough cord of connective tissue that anchors firmly to bone. When the muscle contracts, the force travels through the tendon and pulls on the bone, creating motion at a joint.
This division of labor matters because each structure is built for a different job. Muscle tissue is soft, elastic, and packed with contractile proteins that let it shorten and lengthen rapidly. Tendon tissue is stiff and incredibly strong, composed of about 75% collagen by dry weight, arranged in tightly ordered parallel fibers designed to resist pulling forces. A muscle alone would tear away from bone. A tendon alone can’t generate force. The two structures need each other.
Inside a Skeletal Muscle
Skeletal muscle makes up roughly 40% of total body weight and is responsible for all voluntary movement, from blinking to sprinting. Under a microscope, it has a striped (striated) appearance because of the way its internal proteins are arranged.
The organization follows a nesting pattern. The whole muscle is wrapped in a tough outer layer called the epimysium. Inside, bundles of muscle fibers called fascicles are each wrapped in a middle layer called the perimysium. Within each fascicle, individual muscle fibers are surrounded by the endomysium. Each single fiber contains hundreds to thousands of even smaller threads called myofibrils, and these are where contraction actually happens.
Contraction works through a sliding mechanism. Inside each myofibril, two types of protein filaments overlap. When your nervous system sends a signal, tiny molecular bridges form between the filaments and ratchet them past each other, shortening the fiber. Multiply that across millions of fibers firing in coordination, and you get enough force to lift a heavy bag or launch yourself off the ground.
How the Nervous System Controls Force
A single nerve cell branches out to contact many muscle fibers scattered across the muscle. That nerve cell plus all the fibers it controls is called a motor unit, and it’s the smallest amount of force your body can activate at once. Small motor units with just a few fibers handle delicate, sustained tasks like standing upright. Large motor units with many fibers kick in only when you need explosive power, like jumping. Your brain recruits these units in order, from small to large, which is why you can thread a needle and throw a ball with the same muscles.
Inside a Tendon
Tendons are dense, rope-like bands of connective tissue. Their collagen molecules are organized in a hierarchy that mirrors muscle: individual fibrils bundle into fibers, fibers bundle into fascicles, and fascicles form the complete tendon. This layered architecture gives tendons enormous tensile strength, meaning they resist being pulled apart even under heavy loads.
Tendons vary widely in size depending on the forces they handle. The Achilles tendon at the back of your ankle is the largest and strongest in the body because it transmits the force of the entire calf muscle group during walking, running, and jumping. By contrast, the tendons in your fingers are thin and flexible enough to allow precise, independent finger movements.
Where Muscle Meets Tendon
The spot where muscle tissue transitions into tendon tissue is called the myotendinous junction, and it’s one of the most injury-prone areas in the body. This zone has a distinctive design: the tendon’s collagen fibers fold into deep finger-like projections that interlock with the muscle cell membrane. These folds dramatically increase the contact area between the two tissues, spreading the force of contraction over a larger surface so no single point bears too much stress.
Most muscle strains occur right at this junction. During high-effort activities, especially explosive movements like sprinting or jumping, the forces passing through this transition zone can exceed what the tissue tolerates. Periods of inactivity can shrink the contact area between muscle and tendon, weakening the junction and making it more vulnerable when you return to exercise. This is one reason gradual training progressions matter for injury prevention.
Injuries to Each Structure
Because muscles and tendons are distinct tissues, they get injured in different ways. A strain is damage to muscle fibers or tendons (or both), ranging from minor overstretching to a complete tear. Symptoms typically include pain, swelling, bruising, weakness, and reduced range of motion. Mild strains involve microscopic damage at the fiber level. Severe strains can mean a full rupture that requires surgical repair.
Tendon injuries often develop gradually. Repeated stress without adequate recovery leads to breakdown of collagen fibers faster than the body can repair them, eventually producing chronic pain and stiffness. Muscle injuries, on the other hand, more commonly result from a single forceful event, like a sudden sprint or an awkward landing. Both types of injury heal, but tendons recover more slowly because they have a much lower blood supply than muscle tissue.
Three Types of Muscle Tissue, One Muscular System
The body contains three types of muscle tissue: skeletal, cardiac, and smooth. When textbooks refer to “the muscular system,” they mean the skeletal muscles and their tendons, the structures responsible for voluntary movement and posture. Cardiac muscle forms the walls of the heart and beats automatically without conscious input. Smooth muscle lines blood vessels, the digestive tract, airways, and other internal organs, contracting involuntarily to move blood, food, and air through the body. Both cardiac and smooth muscle belong to other organ systems (cardiovascular, digestive, respiratory) rather than the muscular system itself.
Skeletal muscle is the only type you can consciously control. It’s also the only type that attaches to bone through tendons, which is why these two structures, muscles and tendons, define the muscular system as a functional whole.