In order to do these different types of jobs, muscles come in three forms. Cardiac muscle found only in the heart, smooth muscle which lines our blood vessels and digestive system and skeletal muscle which makes us move. Common skeletal muscle is actually made up of thousands of individual cells, each wrapped in connective tissue. These elongated cells, referred to as fibers are arranged in bundles. Thin threadlike structures called myofibrils run through each fiber, and when a muscle contracts, these myofibrils pull past each other, shortening the cells.
Skeletal muscles create motion by pulling on tough cords of connective tissue called tendons.
These tendons in turn pull on the bone which creates motion. Since muscles can only contract, each joint must have two opposing sets of muscles to give the full range of motion. As a muscle contracts, it causes the bone to act like a lever with the joint serving as a fulcrum. While this means a great deal of force is required to pull the lever, it provides maximum distance and velocity at the lift end. Have students research the different muscle systems on the body and identify which joints and muscles control which bones.