Background: The gait mechanism requires an efficient intersegmental coordination in order to ensure the displacement of the body while simultaneously maintaining the postural stability. However, intersegmental coordination may be disrupted by neurological or orthopaedic involvement, this increasing the metabolic cost associated with excessive or prolonged muscle co-contraction. Research question: Our aim was to evaluate and to understand how hip OA affects lower limbs coordination during gait by using the kinematic segmental covariation law method and predict the energy expenditure. Methods: In order to evaluate the influence of unilateral alteration of the lower limbs on the gait mechanism, three groups namely 63 hip osteoarthritis patients, 65 chronic hemiparetic stroke patients and 72 healthy subjects performed an instrumented gait analysis. The subjects had to walk barefoot for at least 3 min at a self-selected speed on a force measuring motor-driven treadmill. The biomechanical variables (kinematic, kinetic and energetical cost) were simultaneously recorded. Results: The comparison between the three groups was tested using a repeated measure ANOVA. All biomechanical parameters show significant differences between the 3 groups highlighting the gait alteration for the patients groups. However, the energetic cost remains normal in the hip osteoarthritis group despite of the alteration of the other variables. A multivariate analysis allowed to identify the independent variables affecting more specifically their gait mechanisms. Significance: This study showed the importance of quantitative functional evaluation in order to better understand the impact of hip osteoarthritis on the gait mechanism. The biomechanical analysis provides objective evidence of the altered gait mechanism and more particularly of the intersegmental coordination in these patients. This gait analysis is therefore an interesting tool in the functional evaluation of the patient to better guide the diagnosis.