Fusiform muscle. Definition. noun. A muscle that has a shape of spindle, which is being wider in the middle and narrowing towards both ends. Supplement. Example of fusiform muscle is the biceps brachii.Fusiform muscles are arranged in a strap-like fashion to provide the greatest degree of shortening, thus, enabling the muscle to produce quick and wide range of motion. However, compared with penniform muscle, they are not a very powerful type of muscle. muscle that has a shape of spindle, which is being wider in the middle and narrowing towards both ends. Supplement. Example of fusiform muscle is the biceps brachii.