I want to analysis a dynamic system by applying inertial load on only one part of the body. By default inertial loads are applied globaly on all bodies.
I think that you need to define initial loading conditions in your transient analysis. See page 49 of the manual http://orange.engr.ucdavis.edu/Documentation12.1/121/ans_bas.pdf and an initial state (chapter 4, pg 87)