The contractile proteins in muscles are myofibrils. Myofibrils are bundles of both thick and thin fibers that have a cylindrical appearance.
Muscle contraction is a complex process that involves a vast number of structures within the body. The muscles themselves contain muscle fibers, each of which contains cylindrical strands of contractile proteins called myofibrils. Hundreds of myofibrils exist within a cross-section of muscle fiber.
A myofibril consists of cellulose and glycoprotein. The term is often used to describe a protein fiber. Each myofibril is divided into segments called sarcomeres, which contain thick and thin filaments. The thin filaments are more numerous and consist of a protein called actin, while the thick filaments are made of a different protein called myosin.
During a muscle reaction, the thick and thin filaments within the myofibril slide through the use of a chemical called adenosine triphosphate for energy. The sliding process shortens, or contracts, the length of the sarcomere. Ultimately, when the sarcomeres contract all at once, the entire muscle strand contracts. The purpose of ATP is to transport chemical energy within a cell and is an end product of various cellular processes. Muscle cells are able to make small amounts of ATP in the event there is a decrease in oxygen in the body, which allows the muscles to work even in physically difficult situations.