This paper presents a virtual environment for conducting vehicle interior layout design. A virtual human called Santos that is biomechanically correct, has realistic musculoskeletal system, and natural motion/posture is created to live in this virtual world. One of the objectives of this virtual environment is to allow Santos to explore the interior package design such that one designs new defense and security vehicles without having to create a physical prototype to enhance safety, save time and cost. Different controls require different tasks, for example, pulling a clutch lever, pushing a button, turning a knob, and so on. Therefore, different tasks correspond to different human upper-body motions and hand loads, which in turn correspond to different displacement and torque at each joint. This is a dynamics problem for interior layout design with external loads. The formulation of dynamic equations of motion is implemented within optimization algorithm to predict joint profiles. This methodology allows Santos to help vehicle interior layout design while executing various tasks.