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Background: Age-related changes of human gait characteristics associated with muscle weakness have been reported in previous studies. Human gait is considered as a cyclic motion adapted to individual body-characteristics and the surrounding-environment based on motion criteria. Based on this hypothesis, elderly gait characteristics may be caused by an adaptation to muscle weakness.Research Question: What role does gait adaptation to muscle weakness play in the development of elderly gait, and what criteria are used in elderly gait adaptation?Methods: We examined the effects of gait adaptation to muscle weakness on steady gait characteristics using computational forward dynamics simulation with a two-dimensional neuro-musculo-skeletal model. For gait adaptation, we tested two motion criteria: (i) energy cost minimization, which is a widely used criterion for healthy adults; and (ii) energy rate minimization, based on existing measurements of elderly gait characteristics.Results: Progression of muscle weakness enhanced the reduction of joint angle motion and minimum toe clearance, and finally resulted in falling. Gait adaptation to muscle weakness successfully formed stable walking patterns regardless of motion criteria, even at muscle weakness of 30 %, which represents a moderate degree of elderly muscle weakness. When criterion (i) was used, the time courses of joint motion were similar to those of healthy adults and a relatively high level of muscle activation was found in the whole gait cycle to compensate for muscle weakness. When criterion (ii) was used, the muscle activity level was lower than that of criterion (i) to minimize the energy rate, and the constructed gait successfully captured the characteristics of elderly gait reported in previous studies.Significance: These findings suggest that gait adaptation to muscle weakness plays an essential role in the development of stable gait characteristics, whereas elderly people might use a different motion criterion compared with healthy adults in gait adaptation. [ABSTRACT FROM AUTHOR]