When executed correctly, gentle, weight-bearing movements like sitting, squatting and bending will stretch and strengthen thigh muscles, which support the knees. Movement increases synovial fluid in the joints, nourishing and lubricating them. The joints become more slippery and move more easily. Tai chi, with its gentle, flowing, low-impact movements—can be an essential component in a fitness regimen designed to keep knees healthy and pain-free with a full range of motion.
http://www.taichi.ca/tai-chi-for-physical-health-taiji-quan-qi-gong-biomechanics-chronic-pain-balance-posture-vital-energy-mississauga-brampton-streetsville-cooksville-port-credit-vaughan-maple-etobicoke-toronto/strengthen-and-protect-knee-joints-with-tai-chi/