A computer method for simulating motion of at least a portion of a virtual body is disclosed. The method comprises the steps of: providing a surfaced body model; capturing or computing motion data for an external surface of the surfaced body; dividing the body model into at least two volumes, one of the volumes comprising at least a portion of the external surface of the surfaced body and being a deformable volume, and one of the volumes being a prescribed motion volume, the two volumes defining an interfacial surface; moving the external surface of the surfaced body according to the motion data while computing motion of the interfacial surface; and, simulating motion of the portion of a body by driving motion of the interfacial surface through the computed motion to provide an accurately positioned moving deformable surface that can be used in garment design.