Muscle fibers can contract and relax which enables movement in the body. Muscles can connect to bones via tendons. When a muscle contracts, since it connects to the bone, the bone will move as when the bicep in the upper arm contracts and the lower arm is raised.
One can easily recognize the appearance of muscle cells under a microscope due to the fact that they have many nuclei per cell and they are fibrous in appearance.
One type of muscle tissue is composed of smooth muscle cells and these are located in places that have involuntary muscle movements. These can be found in the lining of arteries which pump blood away from the heart, in the iris of the eye which can automatically adjust to the level of light by contracting or widening, and in the digestive tract where peristaltic muscle movements push food along the digestive system in a wavelike motion.
Striated or skeletal muscle fibers appear to have lines or striations in them. These fibers are responsible for voluntary movements of the arms and legs--for example--throwing a baseball or running in a marathon.
A third type of muscle is cardiac muscle which is found in the walls of the heart and is both striated like skeletal muscle but involuntary, like smooth muscle and allows the heart muscle to beat from before birth and throughout an individual's life.