Amputations (i.e., limb loss) and osteoarthritis are two of the leading causes of long-term disability. Persons within these associated populations suffer from gait abnormalities and pain, which lead to a lower quality of life. Individuals with transfemoral amputation who are ambulatory don prosthetic sockets that often need frequent clinical refitting/reconfiguration and have few realistic alternatives, especially in individuals with obesity and/or vascular disease. To overcome these issues, osseointegration, or direct skeletal attachment of a prosthesis to a limb, has been proposed and shown to greatly improve functional mechanics of the lower-limb. But osseointegration surgery has only be used on patients with traumatic limb loss (representing the minority of the population) and can lead to infection of limb soft tissue and bone that requires additional amputation surgery and loss of limb length. Medial thighplasty, or excision and liposuction of adipose tissue in the residual limb, is a more conservative surgical alternative with higher eligibility than osseointegration and that may also improve prosthetic function. However, this limb “recontouring” procedure is often viewed as a surgery that improves limb cosmesis (i.e., its appearance), as opposed to its function. In fact, most medial thighplasty candidates are non- amputees who are obese. This dissertation evaluates the influence of medial thighplasty surgery in the context of transfemoral limb loss and quantifies improvements in lower-limb and center-of-mass kinematics and kinetics during straight-line walking as well as during transient changes of direction. This surgical technique—modifying only the limb adipose tissue—improved biomechanical function and maneuverability characterized by greater path efficiency, faster movement completion times, smoother turning curvatures, tighter coupling between movement velocity and curvature, and increased shear ground reaction forces. This work suggests that confidence and fluidity of movement can be improved if medial thighplasty is applied in individuals with transfemoral limb loss and provides a missing link between the underlying structure of the residual limb and the function of lower-limb prostheses. Osteoarthritis has been shown to be secondary to developmental joint abnormalities, which are often expected to lead to higher contact stress of the articulating joint surfaces. Dysplasia and femoroacetabular impingement are two common hip deformities that lead to premature joint degradation. Yet, diagnosis is complex, and the course of treatment is different depending on the pathology. A robust treatment plan for therapy or surgery is vital to preserving the hip joint before the onset of osteoarthritis. This dissertation evaluated patients with diagnosed dysplasia or impingement prior to hip preservation surgery and uncovered significant differences in their gait biomechanics. The dysplasia cohort exhibited widespread gait deviations throughout the stride, while impingement group showed localized differences in mechanics that primarily occurred during peak hip extension (i.e., last stance phase of the stride). Furthermore, this dissertation investigated the use of dynamic simulation of static medical imaging from these patients. We argue, as an alternative to dynamic imaging modalities, that imposing normative motion of these static joint images in a simulation framework can lead to further insight about the underlying root causes of each deformity and calculate geometric properties of the hip center-of-rotation, as well as inadmissible variations of the relative motion between femur and acetabulum during normative level and sloped walking. Finally, continuous classification schemes were evaluated (linear and nonlinear discriminant analyses) to probe the separation between time-varying features of these multiple pathologies that occur throughout the stride. These final studies pertaining to hip preservation also deepen our understanding of the functional link between lower-limb structure and its function during widely varying ambulation scenarios and provide a potentially powerful tool to improve the diagnosis and treatment of a given patient.