an important difference between biological and engineering linkages is that revolving bars are rare in biology and that usually only a small range of the theoretically possible is possible due to additional mechanical constraints (especially the necessity to deliver blood). biological linkages frequently are compliant. often one or more bars are formed by ligaments, and often the linkages are three-dimensional. coupled linkage systems are known, as well as five-, six-, and even seven-bar linkages. four-bar linkages are by far the most common though.