Knee pain after prolonged standing can result from muscle strain, early arthritis, vitamin D deficiency, excess body weight, overuse, poor posture, or weakness around the knee joints. A 500 mg calcium supplement may help only if your pain is related to low calcium intake or associated with vitamin D deficiency, but calcium alone may not relieve all types of knee pain. Doctors sometimes recommend calcium along with vitamin D supplements when a deficiency, bone weakness, or low dietary intake is suspected. Along with supplements, maintaining a healthy weight, avoiding prolonged standing, using supportive footwear, and doing gentle knee-strengthening exercises may help reduce strain on the joints. Applying a warm compress and taking adequate rest after long standing may also provide relief. Please consult a doctor if the pain is persistent, associated with swelling, stiffness, redness, locking of the knee, or difficulty walking, as you may need evaluation for arthritis, ligament problems, or other joint conditions.