Several groups are working on the design of a new generation of robots and haptics for the activities of daily life or rehabilitation of elderly people or patients with some pathology. Sometime their forces in power grasping are different from healthy people. This paper is focused on the study of grasp forces of these patients with different pathologies and presents a new methodology to calculate grasp forces applied by virtual human hand when grasping any object with power. The Denavit-Hartenberg method is implemented to analyze the 25-degree-of-freedom (DOF) hand model of the virtual humans. Human performance measures (joint torques and joint displacements) are criteria for the design of joysticks in the material handling machines. Finally we compare these analytical results with experiments where n = 3D 16 persons. Based on a comfortable position for the hand a new model is proposed to design joysticks for patients or the elderly.