PURPOSE: Individuals with anterior cruciate ligament reconstruction (ACLR) engage in fewer steps per day (steps/day) compared to uninjured controls. While regular physical activity can improve outcomes of idiopathic knee osteoarthritis such as disability, it is unknown if physical activity influences post-traumatic knee osteoarthritis outcomes following ACLR. Cartilage oligomeric matrix protein (COMP) is a biomarker outcome associated with cartilage breakdown, and it is responsive to mechanical loading during walking. Therefore, the purpose was to determine the association between steps/day and change in COMP (∆COMP) following walking in individuals with an ACLR. METHODS: : Steps/day was assessed over 7 days using ActiGraph accelerometers worn on the right hip in 31 participants (age=22±4years, body mass index=23.9±2.9kg/m², 52±37 months post-ACLR, 55% females) with primary unilateral ACLR ≥ 6 months at the time of testing. Subjects walked at a preferred speed for 3000 steps (~30 minutes) on a treadmill to introduce cartilage loading. Blood samples were collected immediately pre- and post-walking, and serum COMP concentrations were analyzed with ELISA kits. ∆COMP was calculated with post-walking concentrations expressed as a percentage of pre-walking levels. Greater ∆COMP was interpreted as greater cartilage breakdown during the walking protocol. A univariate linear regression was conducted to determine the association of steps/day with ∆COMP.
RESULTS: Fewer steps/day (mean±SD: 9,626±2,452) associated with greater ∆COMP (+11.5±16.4%; R²=0.152, β=-0.003, p=0.030).
CONCLUSIONS: Individuals after ACLR who habitually engage in fewer steps/day demonstrated greater ∆COMP during a walking protocol. We postulate fewer steps/day in individuals with a history of ACLR may result in deleterious changes in cartilage homeostasis due to inadequate mechanical stimulation of joint tissues. Future studies should determine if increasing steps/day improves knee cartilage outcomes following ACLR.