Energy rich fuels like glucose cant be used directly for muscle contraction. They must be converted to ATP (adenosine triphosphate) first. ATP is a substance that stores, carries, and releases energy. During contraction ATP enables myosin and actin to interact. ATP is created inside a muscle fiber by two types of cell respiration: aerobic or anaerobic. A muscle fiber contains enough ATP to power a few seconds of contraction. Afterward, contractions of ATP need to be maintained at a steady level.