A linkage mechanism for an adjustable arm assembly. The linkage assembly may be used to support an object, such as a headrest for use in connection with an examination table. The linkage mechanism provides structure to interlock or disengage two adjoining arm sections. The linkage mechanism controls relative movement of two adjoining arm sections without affecting the movement of non-adjoined sections or the headrest. The linkage mechanism further provides friction retention means so that the adjoining arms will retain their relative position to one another even if they are not in an interlocked position.