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Introduction: Osteoarthritis is one of the most common musculoskeletal disorders. Analyzing the energy expenditure in people who suffer from musculoskeletal disorders provides objective evidence to measure individual’s disability and the effect of therapeutic interventions. Present study investigated the effect of various severities of knee osteoarthritis on mechanical energy of lower extremities. Materials and Methods: 48 women aged between 40-70 years were purposefully divided into mild, moderate and sever osteoarthritis and healthy control group according to simple X-ray features of their knee joints. The participants walked at their convenient self-selected speed to collect five clean trials. The data were recorded using a motion analysis system and a 3-dimentional model of lower extremity segments was developed from motion capture data using visual 3-D software. Mechanical energy of lower extremity was calculated using this model. Results: The study groups were significantly different in age (P ≤ 0.001) and walking speed (P ≤ 0.001). One-way analysis of variances (ANOVA) and post-hoc Tukey test revealed that there was significant difference between the study groups regarding mechanical energy. However, univariate general linear models with adjustments to age and to walking speed were developed; in addition, a bivariate general linear model was adjusted to age and walking speed. All the models confirmed that the differences were not imposed by the severity of knee osteoarthritis. Conclusion: It seems that increase in mechanical energy of lower extremity in subjects with knee osteoarthritis intensifies with the progression of the disease; this is not a direct effect of the level of joint deterioration but is an indirect effect of older age and lower walking speed in subjects suffering from more advanced knee osteoarthritis.