Osteoarthritis (OA) is a common chronic health condition resulting in significant personal and economic burdens. In the absence of a cure, the main areas of research to-date have centred on developing treatments that can improve symptoms of pain and physical dysfunction as well as improving methods to diagnose the disease and to monitor progression. Most commonly affecting the knee, much of the current literature in these two research areas has been conducted in populations with knee OA.
Plain radiographs remain the most commonly used clinical method of assessing joint structure for the purposes of OA diagnosis and classification of severity. Magnetic resonance imaging (MRI) has also been used to assess changes in the morphology of the bone and cartilage to supplement radiographical findings . Though these approaches are reliable and well-established clinically, their use for the monitoring of changes in cartilage over time is limited by the fact that these changes take place over long periods of time and simply detail soft tissue damage that has already occurred . Indeed, a recent systematic review showed greater responsiveness of radiographic joint space width measurements in studies using follow-up periods of greater than 2 years . Given that significant changes in symptoms and joint structure can occur over the course of 2 years, coupled with the fact that most non-surgical and non-pharmacological interventions are conducted over months instead of years, improved methods of assessing changes in cartilage structure and outcomes following shorter-terms treatments over shorter periods of time are needed.
Joint tissue-related biomarkers in the blood and urine have been used to further the understanding of the pathogenesis of knee OA. Many biomarkers are produced during the synthesis or degradation of articular cartilage and are found in different concentrations based on the presence and severity of knee OA [4, 5]. For example, the biomarkers C-propeptide of type II procollagen (CPII), hyaluronic acid (HA), and cartilage oligomeric matrix protein (COMP) in serum, as well as C-telopeptide of type II collagen (CTX-II) and type II collagen cleavage neopeptide (C2C) in the urine have all been shown to be elevated based on the presence and severity of knee OA . Importantly, given that their concentrations reflect processes directly implicated in the synthesis or degradation of articular cartilage, analysis of biomarker concentrations may represent an effective method of assessing cartilage structure over shorter periods of time than conventional methods such as radiography or MRI. However, much is still unknown about these biomarkers, including factors involved in their production and responses to non-pharmacological interventions.
Excessive joint loading is a recognized risk factor for breakdown of articular cartilage based on early in vitro studies . These findings have been supported by later gait analysis studies showing a significantly higher rate of knee OA progression over six years in people with high baseline peak external knee adduction moment (KAM) values  – a valid and reliable measure of medial compartment knee joint load during walking [9–11] – as well as a relationship between knee cartilage volume loss over twelve months and the baseline KAM impulse  – the time integral of the KAM during stance . A link between joint load and biomarker concentrations has also been reported. Piscoya et al.  have shown increases in COMP production in response to dynamic mechanical load in cartilage explants, and acute bouts of moderately intense physical activity have been shown to temporarily increase the concentration of sCOMP in healthy individuals  as well as those with knee OA [16, 17]. However, the relationships between other cartilage biomarkers and measures of everyday joint loading, such as the KAM, are less well known. Further, the effects of common non-pharmacological interventions on biomarker concentrations are also not known.
Given the paucity of research on the relationships of OA biomarkers with joint loading and changes following exercise, the purpose of the present exploratory, pilot study was to address these two knowledge gaps in a sample of individuals with knee OA. Specifically, the primary objective of this study was to determine the relationships between different biomarker concentrations and knee joint load, as measured by the KAM. The secondary objective was to examine changes in biomarker concentrations following a 10-week muscle strengthening intervention.