Given the task definition and input, reply with output. Given a scientific passage and an answer, generate a question for the given answer.

Passage: Muscles are the main organs of the muscular system. Muscles are composed primarily of cells called muscle fibers. A muscle fiber is a very long, thin cell, as you can see in Figure below . It contains multiple nuclei and many mitochondria, which produce ATP for energy. It also contains many organelles called myofibrils. Myofibrils allow muscles to contract, or shorten. Muscle contractions are responsible for virtually all the movements of the body, both inside and out. Answer: muscle fibers
Muscles are composed mostly of cells called what?