Skeletal muscle fibre. Coloured scanning electron micrograph (SEM) of a freeze-fractured skeletal (or striated) muscle fibre. The fracturing of the fibre has revealed that it consists of a bundle of smaller fibres called myofibrils. These are around 1-2 micrometres (millionths of a metre) in diameter. Skeletal muscle is so-called because the skeletal bones are moved with this muscle. Unlike smooth muscle, it is under voluntary control.