The aim of this randomized controlled trial is to evaluate the effect of Nordic walking and inspiratory muscle training (IMT) on cardiopulmonary function in patients with knee osteoarthritis (OA). Mild-to-moderate knee OA patients are randomly allocated into Experimental group (Nordic walking plus routine management) or Control group (Routine management). While End-stage knee OA patients are randomly allocated into Experimental group (IMT plus pre-operative rehabilitation exercise) or Control group (Pre-operative rehabilitation exercise). Participants are invited to undergo two assessments, including baseline assessment, post-intervention assessment. Researchers will compare two groups to see if the cardiopulmonary function can be improved after intervention.