COMP and osteoarthritis: The higher baseline serum levels observed in marathon runners, in osteoarthritis, and rheumatoid arthritis than in healthy controls (Neidhart et al., 2000; Garnero et al., 2001; Sharif et al., 2007) and the larger lag time to return to basal levels after a marathon (Neidhart et al., 2000; Kim et al., 2009) than after a moderate physical activity (Mündermann et al., 2005) suggest a maintained increase in synthesis or diffusion due to the rapid elimination of COMP once it has reached the circulation (Andersson et al., 2006).