Consider muscle strain, synovitis, osteoarthritis, gout and so on. 1. Muscle strain: if strenuous exercise or walking too much can cause muscle strain, localized soreness, aggravated by exertion, alleviated after rest. 2. Synovitis: Chronic inflammation of synovial membrane caused by strain, trauma or cold can lead to synovial hyperplasia, hypertrophy, secretion of fluid and pain, and some joints will be swollen. 3. Osteoarthritis: joint hyperplasia and degeneration, articular cartilage wear and tear, can cause joint pain, squatting activities are limited. X-ray film can be taken to make a clear diagnosis. 4. Gout: uric acid excretion disorders can cause gout, sudden joint pain, redness and swelling, test uric acid can be clear. If the soreness inside the knee is not alleviated by resting, go to the hospital for a checkup to clarify the cause of the disease and treatment.