The subsections of a myofibril that perform muscle contraction are called

Sarcomere

Explanation:

The sarcomere is the smallest contractile unit of muscle. It extends from one Z disc to the next and therefore, is the region between two consecutive Z discs. It contains an A band flanked by half an I band at each end. Contraction of a muscle fiber occurs as the sarcomeres within the myofibrils shorten. During the shortening of the sarcomere, the thin actin filaments slide past the thick myosin filaments and approach one another. The resultant shortening of the I band and complete disappearance of H zone cause contraction of muscle fiber.