Fibers expressing MHC I have a slow contractile speed, whereas those expressing MHC IIx are much faster (Bottinelli, 2001). A third isoform, MHC IIa gives rise to fibers with a contractile speed leaning towards type IIx. The very fast MHC IIb isoform, commonly found in limb muscle from small animals such as rats and mice, are rarely found in the limbs of larger animals, but may be expressed in specialized muscles, such as the larynx or eye