There is a wide variety of shapes and sizes in muscles. Based on general shape and the orientation or muscle fibers in relation to the direction of pull, they can be grouped into two classes.
1. Parallel : These muscle fibers are parallel to the line of pull. The muscles may be flat, short, quadrilateral or long and strap like. The individual fibers run the entire length of the muscle.
2. Oblique : These muscle fibers are oblique to the line of pull. The muscles may be triangular or pennate (feather-like). The pennate forms may be unipennate, bipennate, multipennate or circumpennate. Some muscles have a spiral or twisted arrangement.