This research seeks a new insight into the design of controlled mechanical systems. Most of conventional mechanical designs are based on requirements involving weights, stiffness, strength, and so forth, which are not directly related to controlled performance. The objective of this research is to identify mechanical (structural properties that are critical for guaranteeing good controlled performance). Our preliminary result shows that the control bandwidth of a servo-tracking system, achievable by a dynamic output feedback controller with reasonable amount of control effort, is closely related to the frequency range where the open-loop transfer function exhibits positive-realness. A finite frequency positive-real property is exactly characterized in terms of linear matrix inequalities and structural design methods are proposed using the characterization.