The human tongue slows down to speak Ira Sanders MD Institute for Biomedical Research, Hackensack University Medical Center IraSanders@optonline.net The tongue is the main organ of speech yet its biomechanics is poorly understood. Most vertebrate tongues have fast contracting muscles and the ATPase in their muscle fibers reflects this as all non-primate tongue muscles tested to date are composed entirely of fast twitch muscle fibers. In this study we examined the tongue muscles of neurologically normal adult humans (adults) for specializations as reflected by presence and distribution of slow twitch muscle fibers (StiMF), muscles fibers involved in fine gradations of force. In addition we used immunochemical methods to identify slow tonic muscle fibers (StoMF) a type of muscle fiber with unique biomechamical attributes that is extremely rare in mammalian muscles. As little is known regarding the functional significance of different types of muscle fibers in tongue muscles a variety of specimens were studied whose tongue activity in vivo differs from the adults (experimentals): early human developmental stages (newborn and 2-year-old infant); neurological disease (idiopathic Parkinson^Òs disease (IPD)); and comparative specimens (macaque monkey). We found that adult human tongue muscles have a high percentage of StwiMF (54%). Among the experimentals the IPD patient (50%) as well as the 2 year old (54%) were similar to the adult whereas the human neonate and macaque had significantly fewer StiMF (31%). In both adults and experimentals the StiMF were distributed in a spatial gradient with few in the tongue blade and higher proportions posteriorly in the tongue base. The spatial gradient of StiMF is consistent with a postural role. The posterior tongue probably serves as stable platform for the more mobile tongue blade. An extraordinarily high content of StoMF were found in adult tongue muscles (31%). In contrast to the gradient of StiMF the highest amounts of StoMF were in the blade (37%) and base (34%), with significantly less in the body (29%). Moreover, StoMF concentrated in muscles that shape the superior surface of the tongue, including newly described oblique muscles that were composed of nearly 90% StoMF. Among the experimentals both the IPD (24%) and neonate (16%) had significantly fewer StoMF then the adults (the 2 year old specimen was not tested). In contrast the macaque was remarkable for a relative lack of StoMF (estimated 5%). The results suggest that tongue motor control is significantly higher in primates compared to other mammals, with adult humans having the greatest amount. In addition, the presence and distribution of StoMF in adult human tongues suggest it is related to speech, possibly the rapid changes in tongue shape that are uniquely seen during human speech. As similar specializations have recently been reported in the muscles of the human larynx and pharynx, but appear at comparatively low levels, if at all, in the same muscles of newborn humans or other mammals, it is proposed adult humans have a specialized motor control subsystem in upper airway muscles related to speech.