Objectives: To identify the factors that constitute optimal inertial properties for a lower-limb prosthesis. Design: Experimental, controlled; simulation using a subject-specific double-pendulum model based on individual kinematic data and leg inertial properties. Simulation outcomes were compared with gait analysis data. Setting: A gait laboratory. Participants: Ten transtibial amputees and 10 matched healthy controls. Interventions: Not applicable. Main Outcome Measures: Inertial properties of the lower leg; kinematics and kinetics of the swing phase; and kinematics of double-pendulum model simulations. Resuits: In all amputees, inertial properties were reduced. No between-group differences existed in kinematics; hip and knee joint torques and powers were reduced in the amputees, Deviations between the double-pendulum model and experimental data were larger in the amputees than in the control subjects. Conclusions: Although current lightweight prostheses have less optimal pendular behavior, their light weight requires smaller joint torques to influence the pendular trajectory, Therefore, optimal inertial properties, in terms of swing phase kinematics and kinetics, will be a compromise between pendular properties and efficient control. © 2003 by the American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation.