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Purpose: Exercise therapy with a focus on muscle strengthening has proven to be effective for the management of knee osteoarthritis (OA). Yet it is not known whether high-intensity resistance training (RT) is more effective in improving upper leg muscle strength and physical performance than low-intensity RT. Still, there is some controversy regarding the effectiveness of high-intensity RT and whether or not it is harmful, for instance by accelerating cartilage degeneration, osteophyte formation, or increasing synovitis. Any catabolic or anabolic response of musculoskeletal tissue to RT might first be visible on a biochemical level before changes in clinical symptoms are measurable. Serum biomarkers can objectively measure early biochemical changes and assess whether RT leads to a more anabolic or catabolic response. The aim of this study is to assess (i) whether high-intensity RT elicits a different response (e.g. catabolic) on systemic inflammation and musculoskeletal tissues in and surrounding the joint, including bone, cartilage, muscle, and synovial tissue compared to low-intensity RT; and (ii) whether there is an association between changes in serum levels of inflammatory and musculoskeletal tissue-derived biomarkers and improvements in clinical outcomes (performance-based tests and self-reported outcomes on pain and activity limitations).Methods: In a randomized controlled trial, 177 participants with knee OA conducted a high-intensity (70%-80% of the Repetition Maximum (1RM)) or low-intensity (40%-50% 1RM) RT program 3 times a week for 12 weeks. Measures of clinical outcomes and serum samples were collected at the start of RT (pre-intervention), after 3 months at the end of RT (post-intervention), and 6 months after RT (follow-up). As a reflection of systemic inflammation (CRP), synovitis (CRPM, C3M), bone turnover (OC, CTX-I), cartilage turnover (PRO-C2, C2M, huARGS), muscle turnover (PRO-C3, PRO-C6), and cell behaviour (col10neo) a total of eleven serum biomarkers were analysed. With the exception of CRP, which was determined with an immunoturbidimetric assay, ELISA assays were used to quantify serum levels of the other 10 serum biomarkers. The primary outcome measures are the changes in serum biomarker levels. Other outcome measures include upper leg muscle strength, performance-based tests, and self-reported outcomes on pain and activity limitations.Results: High-intensity RT resulted in greater improvements in muscle strength compared to low-intensity RT when measured by the estimated 1RM. No significant differences between groups were found for upper leg muscle strength (Nm/kg) when measured with an isokinetic dynamometer. Both groups showed similar improvements in pain and physical functioning. Although there is no difference between groups in clinical outcomes, except for the estimated 1RM, we expect that participants in the high-intensity RT group are more likely to have enhanced serum levels of catabolic biomarkers than participants in the low-intensity RT group. Since both the high-intensity RT group and low-intensity RT group improved over time, we expect that changes in serum biomarker levels are associated with overall improvements in clinical outcomes. Almost all participants had normal CRP values (Conclusions: The work to date on CRP serum levels suggests that RT did not influence CRP levels. This result may be explained by the high percentage of participants with normal CRP levels (