<qa>  What is the primary cause of activity restriction and physical disability in older adults?Knee OA is a chronic, progressive, degenerative disease that affects over 20 million Americans, and is cited as the primary cause for activity restriction and physical disability in older adults. The joint damage and symptoms i.e., pain, stiffness, and fatigue accompanying symptomatic knee OA result in activity restriction, muscle atrophy, reduced quality of life and difficulty in performance of functional tasks. 