Knee OA (prevalence 16%) was associated with a 7.2-points lower PCS score (95% confidence interval -9.5, -4.8). PCS score was also negatively associated with obesity and comorbidities; however, no interaction with knee OA was seen. Low fat-free mass was associated with a lower PCS score and interacted with knee OA in men. Interaction between concurring OA and low fat-free mass attributed to 64% of the decrease in PCS score, as compared with men without OA and with high fat-free mass.