what is muscle fiber
1. muscle fiber-an elongated contractile cell that forms the muscles of the body. muscle cell, muscle fibre. fibre, fiber-any of several elongated, threadlike cells (especially a muscle fiber or a nerve fiber).oun. 1. muscle fiber-an elongated contractile cell that forms the muscles of the body. muscle cell, muscle fibre. fibre, fiber-any of several elongated, threadlike cells (especially a muscle fiber or a nerve fiber). muscle, musculus-one of the contractile organs of the body.