At both ends of every muscle, the fascia covering the muscle tapers to form a strong, rope-like length of specialized muscle called a tendon. The tendon, in turn, is connected directly to a bone. One end, which connects to a relatively unmoving skeletal part, is the origin of the muscle. The point where it's attached to a moving bone is the insertion of the muscle.
When a muscle contracts, it pulls its origin and insertion closer together, often across a joint. Contraction of the muscle creates movement around the joint, allowing the pushing and pulling motions that make up physical movement.
Most often, this movement involves a shortening of the involved muscles such as when you lift a heavy weight off the ground. This is called a concentric contraction. If the opposing force is greater than the muscle force, however, the muscle may actually lengthen as it works to contract. For example, when you lower a heavy weight down to the floor, your bicep muscle lengthens even though it's tensing. This is called an eccentric contraction. Finally, if the muscle doesn't change length at all during the contraction when you push against a stationary wall, for instance the result is an isometric contraction.
Other types of connective tissue also help to create smooth, controlled movements. Ligaments are tough, elastic bands that connect the bones together and help stabilize a joint. The best way to think of ligaments is as tethers that hold the bones together at the joint. The ligaments help guide how the bones move in relationship to each other. Nerve receptors in the ligaments and tendons also send information to the brain, to help regulate the intensity of muscle contractions and to help avoid injury. For example, if a joint is extended to its maximum, a signal is transmitted to the brain to avoid any further extension. A fluid-filled sac, called a bursa, cushions and lubricates the tendons and other tissues sliding across the larger joints such as the hips, knees and shoulders.
Because tendons, ligaments, and bursae take longer to adapt to activity than muscle fibers, these connective tissues are particularly vulnerable to inflammation, tears or other injury, especially from any type of repeated movement. For this reason, bursitis (inflammation of a bursa), tendinitis (inflammation of a tendon) and ligament injuries are particularly common problems after prolonged, repetitive activities, such as running, racket sports, or typing at a word processor. Avoiding such overuse injuries is one of the keys to maintaining a lifelong exercise routine.