Dynamic responses of under-actuated prosthetic hands determine the performance and operation safety of these systems. In order to control the gripping force and to improve the grasping characteristics of the hand, it is necessary to analyze the dynamic responses of the system. This paper presents a forward dynamic model of a linkage being used in most of commercial prosthetic hands. The finger mechanism is composed of a four-bar linkage with one degree of freedom. Constrained Lagrange method is used to derive the dynamic equations of the finger. The three joint angles of the four-bar linkage are selected as the generalized coordinates of the system while the motor torque is chosen as the input. Since the mechanism has one degree of freedom, two kinematic constraints are also considered. The results of the dynamic equations of motion are compared to the responses of a software model of the finger, developed in a dynamic simulation software. The good agreement between the results verifies the dynamic model developed for the prosthetic finger.

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