Definition and Function of the Muscle System

The muscle system, also known as the muscular system, consists of the skeletal muscle tissue and connective tissue that make up the individual muscles that are attached to the bones of the skeleton. These muscles form the flesh, covering the bones on the outside, and helping to give the body its shape.

The muscular system has three main functions, those of causing movement, maintaining posture, and producing heat. It brings about movement by exerting a pull on tendons that move bones at joints. The pulling force is due to the contraction, or shortening, of the muscle. Parts of the body, such as the limbs, are moved in this way. When the entire body moves from one place to another, locomotion is said to occur.

Usually muscles are attached by their tendons to two articulating bones on either side of the joint. When a joint is moved, one of the two articulating bones remains stationary while the other one moves. The attachment of the muscle to the stationary bone is the muscle origin, ie the anchorage end of the muscle. The attachment to the bone that moves is the insertion, ie the pulling end of the muscle. In the limb muscles the origin is usually proximal (meaning close-by), while the insertion is distal (meaning further away).

During movement bones act as levers, hinged at the joint that acts as the fulcrum (F). The muscle provides the effort (E), while the weight of the part being moved is the load (L). The positions of the fulcrum, effort and load determine the type of lever action:

• In a first class lever, the fulcrum is placed between the effort and the load. The movement of the head on the vertebral column is an example of first class lever action. When the head is lifted, the muscles at the back of the neck provide the effort, while the weight of the facial region of the skull is the load. The joint between the skull and atlas vertebra is the fulcrum;
• In a third class lever, the fulcrum is at one end, the load is at the other end of the lever, and the effort is between them. Flexing the arm at the elbow is an example of third class lever action. The elbow joint is the fulcrum, the biceps muscle provides the effort, and the weight of the forearm and hand is the load. The body movements usually involve lever action of these two types;
• In a second class lever, the fulcrum is at one end and the effort at the other, with the load in between. An example of this type of lever action is raising the body onto the toes. The weight of the body is the load, the ball of the foot is the fulcrum, and the contraction of the calf muscles provides the effort that lifts the heel off the ground.

The mechanical advantage that lever action provides is greatest when a large weight (load) can be moved by a small muscular effort. This occurs in the second class lever action where the body is raised onto the toes. In the third class lever action at the elbow joint great muscular power develops in the biceps muscle to move the small weight of the hand, so the mechanical advantage is small.

In most cases several muscles acting in groups bring about movements. Many muscles work in antagonistic pairs, where one contracts to move the bone one way, and the other contracts to move the bone back. The calf and shin muscles form an antagonistic pair that lower and raise the foot. Isotonic contractions develop tension, and the muscle shortens (eg the raising of the arm at the elbow by the biceps muscle, which bulges outwards as it shortens). Isometric contractions develop tension, but the muscle does not shorten (eg carrying a weight on the hand with the arm extended, when the biceps muscle does not bulge).

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Read more in the next article titled: Muscle Tissue