Each muscle fiber is composed of long tubes called myofibrils which in turn are composed of filaments. There are two types of filaments, actin and myosin, and a muscle contraction happens when these two filaments slide past each other. alpha-Actinin-3 (FIgure 1) is expressed only in fast twitch muscle fibers and acts to stabilize actin and to bind the actin and myosin filaments together.
1. Touchette N. (2003). Gene Influences Speed in Elite Athletes. Genome News Network. http://www.genomenewsnetwork.org/articles/08_03/speed.shtml