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Summary: Objective: To propose an experimental protocol for using high tibial osteotomy (HTO) as a model for studying in vivo biological effects of large permanent changes in ambulatory load. Design: This study is a prospective multimodal (clinical, biomechanical, biological) data collection without randomization. The study will examine a cohort of 40 patients with medial compartment knee OA undergoing opening wedge HTO. Experimental protocol: Before planned HTO, patients will be clinically assessed (including mechanical axis measurement from radiographs) and complete questionnaires on physical function. Patients will complete a walking stress test with blood sampling (30 min walking, 5.5 h sitting), and undergo gait analysis. Six weeks after HTO (at the time of full weight bearing), the mechanical axis will be measured from radiographs. Patients will complete the questionnaires and a walking stress test with blood sampling, and undergo gait analysis 6 months after HTO. The peak external knee adduction moment, knee external knee adduction moment impulse and peak external knee flexion moment will be used as surrogates of ambulatory load. Load-induced changes in cartilage biomarkers will be used as surrogates of metabolic changes in response to ambulatory load. At the 12-month follow-up, subjects will complete the questionnaires. Conclusion: The results of this study can be considered as proof-of-concept of a potential diagnostic test (walking stress test) for cartilage degeneration and its prognostic value. A direct relationship between ambulatory load and cartilage metabolism assessed as degradation to synthesis ratio would allow developing novel load-modifying interventions and evaluating the efficacy of existing interventions.