Humans and other vertebrates have three typesTrusted Source of muscle: skeletal, smooth, and cardiac.
Skeletal muscles move the external parts of the body and the limbs. They cover the bones and give the body its shape.
As skeletal muscles only pull in one direction, they work in pairs. When one muscle in the pair contracts, the other expands, and this facilitates movement.
The muscles attach to strong tendons, which either attach to or directly connect with the bones.
Skeletal muscles are striated, which means that they consist of thousands of equally sized sarcomeres, or muscle units, which have transverse bands.
Smooth musclesTrusted Source are responsible for movements in the stomach, intestines, blood vessels, and hollow organs. The smooth muscles in the bowel are also called visceral muscles.
These muscles work automatically without stopping, day and night. They are similar in structure to the skeletal muscles, so doctors sometimes classify them as striated muscles.