Skeletal muscle fibres. Light micrograph of a longitudinal section through skeletal muscle. The muscle is composed of many fibres (several seen here), containing contractile proteins arranged in smaller structures called myofibrils. The myofibrils lie in parallel along the fibres, and the regularly arranged proteins cause the fibres to have this cross-striated (banded) appearance. The light (I) bands and the dark (A) bands can be seen here as alternating light and dark vertical stripes. Skeletal muscle is responsible for voluntary muscle movement in the body, including the leg and arm muscles.