E-Mail The human heart contracts about 70 times per minute, while that of a rat contracts over 300 times; what accounts for this difference? In a new study publishing 10th June in the open-access journal PLOS Biology, led by Michael Geeves and Mark Wass of the University of Kent and Leslie Leinwand from the University of Colorado Boulder, reveal the molecular differences in the heart muscle protein beta myosin that underly the large difference in contraction velocity between the two species. Myosin is a "molecular motor" - an intricate nanomachine that forms the dynamic core of a muscle's contractile machinery, burning cellular chemical energy in the form of ATP to rapidly and reversibly exert force against cables of actin. In so doing, it pulls the ends of the muscle cell closer together, causing muscle contraction. It has long been known that the maximal rate of contraction, called V0, varies predictably among mammals: In small mammals with their high metabolic rate, V0 is higher than in larger mammals, which have lower metabolic rates.