Osteoarthritis (OA) knee is one of the commonest chronic degenerative conditions. It causes disability in elderlies due to pain and stiffness. The prevalence of radiologic knee osteoarthritis increases in proportion to age, reaching an astounding 64.1% for patients who are over 60 years of age. In 2021, there were over 26,000 patients on the Hospital Authority (HA) waiting list for knee total knee replacement (TKR) and with only 4300 TKRs performed, the nominal waiting time for TKR was almost 89 months. Low vitamin D can adversely affect cartilage thickness and study suggested that low serum vitamin D is associated with increased radiographic knee OA progression. A systematic review concluded that vitamin D supplements can improve pain and function in patients with knee OA. Vitamin D has long been recognized for its effect on musculoskeletal health and increasing attention has been focused for its effect on muscle function. Vitamin D have a direct effect on muscle hypertrophy by acting on specific vitamin D receptors (VDRs) on myocytes, and sufficient levels of vitamin D in patients have been found to correlate with an increase in the size, number, and strength of muscle fibres. Vitamin D also seems to exert beneficial effects by its interplay with myokines such as myostatin and irisin. One study also showed that muscle nuclear VDR was increased by 30% and augmented muscle fibre size by 10% in elderly females (mean age of 78 years) taking vitamin D orally at a rate of 100 µg/day (4000 IU/day) for 4 months. This will be a double-blinded RCT investigating the effect of vitamin D supplements or knee muscle strength, physical function, pain symptoms and, sarcopenia status. The study will be a follow-up study with assessment at baseline, 3- ,6-and 12-months post vitamin D intervention.