Purpose: Present models in the literature, predicting that prostheses should not be too lightweight, are not supported by empirical evidence. Recent studies suggest that these models are incorrectly based on the assumption that the swing phase is uninfluenced by muscle activity. The purpose of the present study was to introduce a new mathematical model to predict the effect of mass properties on the gait of transtibial amputees, based on experimental findings that subjects adapt to mass perturbations by maintaining the same joint kinematics. Method: Effect of mass perturbations on the lower leg was evaluated in terms of muscular cost and forces between stump and socket, using a linked-segment model of the swing phase. Gait analysis and anthropometric data from 10 transtibial amputees were used as model input. Results: Location of perturbation strongly influenced the muscular cost. Cost generally increased after distally adding mass but decreased after proximally adding mass to the lower leg. Stump - socket interface forces always increased after mass addition. Conclusions: A new model was introduced, predicting that the weight of distally located components (e.g. foot, ankle, shoe) strongly influence the estimated muscular cost, in contrast to proximal components. A comparison with experimental literature suggests this new model better describes the experimental data than existing models. © 2004 Taylor & Francis Ltd.