Soreness in the joints of the left leg is alert to osteoarthritis, synovitis, meniscus injury, osteoporosis, rheumatoid arthritis and so on. 1. Osteoarthritis: joint pain, restricted joint movement, inability to squat and go up and down stairs, X-ray can be a clear diagnosis. 2. Synovitis: synovial membrane hyperplasia and secretion of fluid, joint swelling, pain and limitation of flexion and extension. 3. Meniscus injury: joint pain, flexion and extension activities, there is a ringing sound or joint jamming, a knee MRI film can be a clear diagnosis. 4. Osteoporosis: pain in bone and joints, and joint deformation in serious cases. 5. Rheumatoid arthritis: symmetrical swelling of joints, pain, morning stiffness. There are many reasons for left leg joint pain, if the rest does not get better, it is recommended to go to the hospital to check the cause of the disease and treat the cause of the disease.