Mild soreness in the knee joint can be caused by the following common reasons: 1. Cold in the knee joint. Cold causes vasoconstriction of the blood vessels around the knee joint, resulting in poor blood circulation in the knee joint, which can lead to stagnation of blood and blood stasis, resulting in soreness and pain. 2. Minor injuries and strain on the knee joint, such as strains and contusions of the soft tissues around the knee joint after strenuous exercise, and excessive impact loading on the bone, can also result in minor injuries. Chronic strain is caused by long-term cumulative overload activities, resulting in micro-injuries, which can also produce symptoms of soreness. 3. Arthritis of the knee joint, such as rheumatoid arthritis, rheumatoid arthritis, synovitis, etc. In the early stage, the symptoms often appear as soreness of the knee joint, which may be relieved quickly if the corresponding treatment and maintenance are carried out. If the symptoms are not treated and maintained, the symptoms will continue to worsen, and swelling, pain and dysfunction of the knee joint may occur.