Soreness at the bend of the leg may be caused by knee joint disease, lumbar spine disease, muscle strain and other reasons. 1. Knee diseases: such as knee synovitis, meniscus injury of the back side of the knee, osteoarthritis of the knee, rheumatoid arthritis, etc., can lead to symptoms of soreness and swelling at the bend of the leg. 2. Lumbar spine diseases: such as lumbar disc herniation, lumbar spinal stenosis, etc., when the lumbar nerves are compressed, the symptoms of soreness and swelling at the bend of the leg may also appear. 3. Muscle strain: When the muscles in the crook of the leg are strained for various reasons, it can also cause localized soreness and pain. When the soreness and swelling at the bend of the leg continue to be unrelieved or progressively aggravated, you should consult a doctor in time, have a relevant examination to clarify the cause of the disease and standardize the treatment, so as to avoid serious consequences.