Complex sentence: The myofibrils are composed of actin and myosin filaments repeated as a sarcomere, the basic functional unit of the muscle fiber and responsible for skeletal muscle's striated appearance and forming the basic machinery necessary for muscle contraction.
Split into simpler sentences: The myofibrils are composed of actin and myosin filaments repeated in units called a sarcomere, the basic functional unit of the muscle fiber. The sarcomere is responsible for skeletal muscle's striated appearance and forms the basic machinery necessary for muscle contraction.