Obesity is a significant public health concern, with implications for the health and well being of millions of people in the United States and world. The purpose of this thesis is to explore the relationship between obesity, segment biomechanics in the form of joint stress and restricted range of motion, and cardiorespiratory fitness. In three studies, anatomical modeling using three dimensional infrared motion capture system was used to calculate joint stresses and range of motion limitations. The results are applied to better understand the impact that obesity has on biomechanics, taking into consideration factors of fitness and fatigue, during walking and common rehabilitation exercises. The first study compared the biomechanical differences between ten obese and ten normal weight adults while performing squat and lunge exercises and found significant increases in lower limb moments in obese females. Increased moments suggest that obese individuals experience higher biomechanical stress than normal weight individuals and may have less flexibility in selecting movement strategies while performing basic rehabilitation exercises. The second study examined how walking, a commonly prescribed exercise to combat obesity, may impact an obese individual's joint biomechanics. While the results of this study, comparing joint stress in ten obese and ten normal weight females, did not observe significant differences between the obese and normal weight groups, stronger relationships were present between moments and VO2 max, rather than BMI, suggesting the importance of fitness in context of developing walking programs. The final study explicitly addressed the relationship between obesity, fitness and biomechanics. Walking and jogging biomechanics was reported in 30 boys and girls, aged 8-11, before and after a fatigue protocol. Hip and knee moments were moderately associated with cardiorespiratory fitness and showed significant increase after cardiorespiratory fatigue. Including measures of adiposity and strength improved the associations between fitness and moments. This thesis concludes that the relationship between biomechanics and obesity, observed in adults and children, should be understood in the larger framework of the reverse causation hypothesis. The reverse causation hypothesis describes obesity as a positive feedback loop, in which certain factors are related to obesity in a bi-directional manner: both causing, but also amplified by, the presence of excess adipose tissue. While further study is required to clarify these relationships, these associations may have implications for participation in physical activities and long-term effects on the musculoskeletal system, thus illustrating the positive feedback loop of the reverse causation hypothesis.