The heart is the only area containing cardiac muscles. These muscles make up the heart walls -- the ventricle and atria. These muscles are considered involuntary as a person does not consciously control when they contract and relax. Cardiac muscles contain striated fibers, which give the muscle a striped appearance, and specialized muscle cells help the heart maintain a consistent heart rate.
Smooth muscles are found in hollow organs, veins and the stomach. These muscles work in layers, with one layer spanning up and down and the other layer covering circumference. When one layer contracts, the other relaxes, thus allowing food or liquid to be maneuvered through the system. These muscles are controlled by the nervous system and work automatically. When these muscles contract, they can remain contracted for both short and long lengths of time.
Most skeletal muscles are attached to bone by tendons. Many skeletal muscles remain partially contracted giving the body posture and giving muscles tone. This also provides an ability to remain ready for physical activity. Accounts vary in regard to how many skeletal muscles are within the human body as muscular structure varies between people and scientists vary in their opinion as to what constitutes an independent skeletal muscle. Some scientists believe there are over 850 skeletal muscles and others believe there are around 650 skeletal muscles. The difference is that some believe certain muscles are individual and other scientists believe the same muscles are a part of larger muscles. When muscles contract, they expel heat energy. When a person shivers, the muscles contract, thus providing heat energy to a cold body. Skeletal muscles are considered voluntary muscles as most of the time you can consciously control when they contract.
There are hundreds to thousands of muscle cells in every muscle. These muscle cells are referred to as muscle fibers because each muscle cell has an elongated shape. The shape allows the fiber to stretch to the length of the muscle. In a thigh muscle, this means the muscle cell or fiber can stretch up to 16 inches. These cells also differ from other cells in the body in that they contain more than one nuclei and can have more than a hundred nuclei in one cell, or fiber.
The fibrous cells within the muscles use chemical energy to create contractions and then relax. This in turn allows the muscles to move. Proteins found in meat, dairy and beans provide the nutrients necessary for proper muscle maintenance and care.